Two Tone Test with Scope and SA - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: July 29, 2020, 06:12:05 pm
content: Hello,I've been looking at various scopes from the usual sources and wanting a scope that could give high Dynamic Range for the standard 2 tone IMD tests using an FFT. The moderately priced scopes mostly seem to have 8 bit ADCs with exception of the R&S which has a 10 bit ADC. Has anyone done any FFT based 2 tone IMD tests at low (~10KHz & 100KHz) and medium (1MHz) and higher frequencies (>10MHz)?I know this test is more in line with a Spectrum Analyzer, has anyone done these test and frequency range with a modest SA?I think the new dual channel 16 bit AWGs from Rigol and Siglent might be a good source for the 2 tones, using a resistive combiner at the outputs.Anyway, interested if folks have done any of these test and any comments, recommendations are welcome.Seems this would be an interesting comparison for the various scopes, SA and AWGs now available.Thanks in advance.Best,

author: xmo
author_rank: Regular Contributor
timestamp: July 29, 2020, 07:55:25 pm
content: "... standard 2 tone IMD tests..."--------------------------------------Standard? What standard?IMD tetsts? Testing what? Ferrites? Antennas? Coax cables? Receivers? Amplifiers? Transmitters?

author: mawyatt
author_rank: Super Contributor
timestamp: July 29, 2020, 09:33:02 pm
content: To help evaluate the instruments, especially the scopes front end circuitry and ADCs. Given quality 2 tone AWG outputs (or other quality signal sources), how do the various scopes compare with each other and with a modest SA.Are the newer 16 bit AWGs good enough sources for these 2 tone IMD tests for typical scopes?Best,

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2020, 07:37:11 am
content: After some emails chatting about other stuff I got the rundown on what mawyatt considers a Two Tone Test...easy it is too using a single output from both channels of a SDG2042X and its Wave Combine feature.FYI mawyatt word for word instructions so anyone can join the fun:The test are very simple, they use two close spaced tones of equal amplitude and show the result of an FFT or direct SA output. Example create a tone of 10KHz in one AWG channel, another tone of 10.1KHz is the other channel. Resistively sum these together and set amplitudes to 1V and look at the FFT result, or SA result. You will see the 2 tones at 1V (or +10dBm(10mw) in 50 ohm system) at 10KHz and 10.1KHz and two smaller tones at 9.9KHz (10KHz -Difference frequency) and 10.2KHz (10.1KHZ + Difference Frequency), these side tones are due to Intermodulation Distortion and tell how good the DUT is, in this case the Scope or SA (assuming tones are really good to start with from AWG). Now do this at 100KHZ and 101KHz, then 1MHZ and 1.0MHz and 1.01MHz and so on. You can use different levels and tone spacing to revel things about the system under test.The math behind this involves the 3rd Order non-linearity which creates these side tones. Very important test for just about any system, audio, RF or Microwave.Behind the scenes mawyatt asked me for FFT 2 tone test results using SDS2000X Plus and now I have the 100 MHz model in stock again, here they are and there should be more than enough info in each screenshot to establish the frequencies used that BTW are based on 10 KHz, 100 KHz, 1 MHz and 10 MHz.Signal sourceSDG2042X 50 Ohm output mode 2 active channels combined. 500mV p-p.Siglent BNC cableSDS2104X Plus in FFT split screen mode.And as some sanity check the 10 MHz 2 tone into a SVA1032X.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 12:51:56 pm
content: Rob,Thanks for the tests.These tests show the difference between the two tones and the 3rd Order products is over 50dB indicating the SDS2104 has a respectable front end and ADC performance. Was this with the 2104 in 10 bit or 8 bit mode?The SA displays shows the AWG output is good. Since the two tones were combined within the AWG in the digital domain, this shows that the AWG DAC and output amplifier have good linearity. BTW, what model SA did you use?All in all nice performance of the SDS2104 & SDG2042. Hopefully others can do this simple test with other similar priced and featured scopes and we can see how they compare.Just a note, the individual tones need to set at 1V peak loaded with 50 ohms (+10dBm into 50 ohms, for 0dBm reference 0.316V peak), however this is fine as shown.Thanks again for running these 2 tone tests!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 03:51:24 pm
content: Quote from: PeDre on July 31, 2020, 02:44:24 pmSimple, if the front end doesn't degrade the ADC too much then it's a respectable front end IMO!! The scope is a system and uses an ADC and has a limit based upon the type and number of bits, so you don't want the front end to degrade the ADC performance too much. You can do this simple test at a number of levels & frequencies to "see" how the scope preforms. If you want to consolidate this into a meaningful number, then the classic IIP3 comes into play.This is a simple way to compare instruments of similar price and feature points, and for me a decision on what scope to purchase.TI has an nice article on ADCs for calculating NF and 3rd Order Intercepts. I'm sure there's lots more information available regarding this on line.https://www.ti.com/lit/an/slyt090/slyt090.pdfBest,

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2020, 06:15:48 pm
content: Quote from: mawyatt on July 31, 2020, 12:51:56 pm8 bit.QuoteThis is the graphic of how it is done:QuoteSVA1032X in SA mode.QuoteYeah sorry I stuffed up as when I put the SDG in 50 ohm output mode I forgot the selected amplitude was halved and got half way through the tests before it dawned on me why the FFT peaks were lower than they should be.Anyways I could've selected a dB output for things to line up even better.The little SDS1104X-E is next.....when I get a chance.....shame member hendorog has my SDG6022X as with that I can push 2 tone tests to 200 MHz or 500 MHz when 'improved'.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2020, 06:26:01 pm
content: The results are a couple dB better than I would expect but consistent with the dynamic range of an 8-bit digitizer at about 51 dB. The SFDR is usually lower but modern integrated ADCs are self calibrating so their linearity should be pretty good.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 06:45:25 pm
content: Quote from: tautech on July 31, 2020, 06:15:48 pmRob,Could I impose on you to do a couple of these tests with the 10 bit ADC mode? Like David mentioned below, these are a little better than one would expect in an 8 bit Data Acquisition System, which is after all what these scopes are!! Maybe the 10 bit performance will be even better!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 07:12:26 pm
content: Quote from: David Hess on July 31, 2020, 06:26:01 pmAgree these results are a little better than I expected with the 8 bit ADC, speaks well for the input circuit linearity. And since we don't know how much the ADC input range is filled, maybe even a little more possible performance! Wonder what ADC Siglent is using, but the popular 8 bit 1GSPS TI ADC81000 specs a ENOB of 7.5 bits at Nyquist and IMD of -51dB with SFDR of 58.5dB.The 10 bit results may be even better!!Best,

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2020, 07:24:10 pm
content: The results are actually so good that I wondered if non-linearity in the source was cancelling non-linearity in the DSO but the SA results rule that out.8 bit linearity is not particularly demanding of an oscilloscope front end (1 part in 200 or 0.5%, see below) and other errors tend to limit performance to below 8 bits anyway. (1) Older DSOs tend to include over-ranging so the display only encompasses a range of say 200 counts or 50 points per division for 8 vertical divisions. This leaves some leeway to align the waveform with the position control and do math on a displayed waveform which is slightly larger than the display without truncation.(1) Linearity gets worse at higher frequencies, but this particular case is almost ideal.

author: egonotto
author_rank: Super Contributor
timestamp: July 31, 2020, 07:55:29 pm
content: Hallo,several measures:RTA4004 with 1MOhm (I forget switch to 50Ohm)Picoscope 4262 with 50 OhmPicoscope 5243A with 50 OhmBest regardsegonotto

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2020, 08:02:12 pm
content: Quote from: mawyatt on July 31, 2020, 06:45:25 pm10 bit mode with 10 KHz and 10 MHz 2 tone tests. AWG output now 0 dB.Envelopes are always fun to get solid triggering on so holdoff added as/if required and done previously.Again, SDS2104X Plus and SDG2042X.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 31, 2020, 08:59:16 pm
content: Here's a SDG6022X into a RTO1024 oscilloscope and FSP spectrum analyzer.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 09:32:48 pm
content: Quote from: PeDre on July 31, 2020, 07:52:46 pmThink you want to resistively combine the two sources for the tones unless you can combine them internally like the Siglent. A resistive "T" should work, think this has 16.7 series ohms on each leg to maintain 50 ohms on all three ports.Don't think you need to use the FFT max hold feature.Best,

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2020, 09:40:14 pm
content: Quote from: mawyatt on July 31, 2020, 09:32:48 pmYou certainly don't if you don't mind looking at all the noise at noise floor levels.I don't as I'm only interested in the fundamental and its associated peaks therefore I mostly use some form of peak/max hold.All screenshots done thus far here are with max/peak hold.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 09:40:36 pm
content: Quote from: jjoonathan on July 31, 2020, 08:59:16 pmThis is superb performance, what you would expect from a 16 bit ADC based scope!! However it's well out of the price range of these other scopes, and I see it's been replaced with the RTO2000 starting at only $14,500!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 09:45:33 pm
content: Quote from: tautech on July 31, 2020, 08:02:12 pmThanks for running these tests again. Appears the 10 bit doesn't improve over the 8 bit performance which makes me wonder if the 10 bit is achieved with some sort of signal averaging rather than a true direct 10bit digitized result?Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 09:50:19 pm
content: Quote from: tautech on July 31, 2020, 09:40:14 pmI prefer to see the noise floor, as sometimes there might be some information buried within. Everyone has a preference!!BTW the 10KHz Time Domain plot doesn't look like the envelope is extending down to zero, is this just an artifact of the display & time base or is the waveform envelope not getting to zero?Best,

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2020, 10:00:00 pm
content: Quote from: mawyatt on July 31, 2020, 09:45:33 pm10 bit explanation here:https://www.eevblog.com/forum/testgear/siglent-sds2000x-plus-coming/msg2783928/#msg2783928Quote from: mawyatt on July 31, 2020, 09:50:19 pmEach to their own.QuoteYeah I spotted that too so will investigate..........Did it also happen in 8 bit mode ? Haven't looked yet.

author: mawyatt
author_rank: Super Contributor
timestamp: July 31, 2020, 10:10:48 pm
content: Quote from: tautech on July 31, 2020, 10:00:00 pm

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2020, 10:41:00 pm
content: Quote from: mawyatt on July 31, 2020, 10:10:48 pmSDS2000X Plus also has ERES of normally up to an additional 3 bits.Check out those posts more for further info on this.

author: tggzzz
author_rank: Super Contributor
timestamp: July 31, 2020, 11:01:24 pm
content: If you can still get one, the Digilent Analog Discovery 2 has 14bit ADCs, AD9648https://reference.digilentinc.com/reference/instrumentation/analog-discovery-2/reference-manual#scope_adc

author: David Hess
author_rank: Super Contributor
timestamp: August 01, 2020, 02:58:13 am
content: Quote from: mawyatt on July 31, 2020, 09:32:48 pmYou have to be very careful combining the signals because the output amplifiers of the signal generators will produce the same distortion products that you are testing for if they are driven with the other signal.Ideally a combiner which provides isolation between the input ports is required but it is usually sufficient to pad each output with an attenuator and use minimum loss resistive divider.Update: I meant to type "minimum loss resistive combiner".

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 03:12:58 am
content: Quote from: David Hess on August 01, 2020, 02:58:13 amThat's why you want to look at this with a SA, to see if the AWG is causing any self-generated IMD issues when resistively combined outputs, if so then add some pads. The Siglent AWG has this nice feature where the two tone combining is done internally (digitally I suspect).Best,

author: TheSteve
author_rank: Unknown
timestamp: August 01, 2020, 06:35:29 am
content: Keysight 33522B generating 10 kHz and 10.1 kHz @ 1 VPP, combined via resistive network, 50 ohms everywhere.FFT on Keysight MSOX3024T and SA on Keysight N9918A(20 dB external atten used here, also at the very bottom of its freq range).Same test repeated at 10 MHz and 10.1 MHz:

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 01:49:39 pm
content: Quote from: PeDre on August 01, 2020, 09:07:48 amThis shows off the R&S true 10 bit ADC, nice results!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 02:24:18 pm
content: Quote from: TheSteve on August 01, 2020, 06:35:29 amThe scope results are interesting, they don't show the 2F1-F2 & 2F2-F1 3rd Order IMD products, or they are buried in the noise floor. Seems Keysight is getting more performance out of the 8 bit 5GSPS ADC than I would expect, maybe some sort of manipulation to extend the apparent ADC resolution? Anyway, very nice result for a very nice scope!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 03:29:00 pm
content: Quote from: PeDre on August 01, 2020, 02:07:56 pmYou need a longer time span to separate these two tones.Best,

author: 2N3055
author_rank: Super Contributor
timestamp: August 01, 2020, 04:26:31 pm
content: 10 kHz and 10.1 kHz 0 dBm from SDG6052X with digital WaveCombine.Keysight MSOX3104TPicoscope 3406D MSO (8bit)Picoscope 4262 (16bit)SDG6000X WaveCombine works quite well for low frequencies.. And Picos are great for this kind of work...

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2020, 07:41:41 pm
content: The $499 SDS1104X-E.......Signal source as previous, SDG2042X + Wave Combine, 0dB into 50 ohms and max everything...mem depth, 1 Mpts FFT......Averages engaged this time, just 4.

author: 2N3055
author_rank: Super Contributor
timestamp: August 01, 2020, 07:48:15 pm
content: Quote from: tautech on August 01, 2020, 07:41:41 pmThat looks good!

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2020, 08:00:54 pm
content: Quote from: 2N3055 on August 01, 2020, 07:48:15 pmThey're as good as you drive them.Rejected for posting as I didn't take enough time with the settings and particularly not a slow enough timebase:Compare with last above.The $379 SDS1202X-E will be next....when I get a chance.

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 08:08:40 pm
content: Agree, this SDS1104X-E looks really good even slightly better performance than the SDS2104X!! Amazing considering the price!!Best,

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2020, 08:21:35 pm
content: Quote from: mawyatt on August 01, 2020, 08:08:40 pmSiglent's do like a lot of waveforms displayed for FFT so the limits with low frequencies are at a firm 50ms/div when anything slower drops into Auto Roll mode.FFT Horizontal menu settings (Hz/div, Span) are also limited by timebase settings in SDS1104X-E so some small juggling act ensues to obtain best settings. Practice makes perfect.Maybe I'll do a comparison between a few models at just one frequency squeezing the best I can outta each model.

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 08:24:51 pm
content: Quote from: tautech on August 01, 2020, 08:00:54 pmRob,The 1 and 4 markers should be on the two small tones on each side of the larger tones, they are spaced the delta frequency of the larger tones on each side. Since the two large tones are at 10.00MHz and 10.01MHz, delta F is then 0.01MHz (10KHz), then the 3rd Order IMD products are at 9.99MHz (2F1 -F2) and 10.01MHz (2F2-F1). This looks like these 3rd Order IMD products are over 65dB down (don't know how you squeeze this from an 8 bit converter, but your result show such!!), which is quite good indeedNice workBTW can these results be brought into an app for a better resolution FFT display?Best,

author: tautech
author_rank: Super Contributor
timestamp: August 01, 2020, 08:46:39 pm
content: Quote from: mawyatt on August 01, 2020, 08:24:51 pmFYI nothing special was done with Markers in these shots other than select Peak which is why they are where they are.We can select Frequency which will place them on the nearer tones or manually position them and select how many markers we wish to display.To date I've just let the Markers do their thing with just a Peak setting.This ^ applies to all Siglent models shown thus far.QuoteProbably but beyond my pay grade and understanding. Most stuff can be exported as a CSV Save.We can however instead of using Split screen FFT use an Exclusive FFT mode and maybe I need drop a couple of shots in to show how different it looks.With models shown thus far we can also port to any size display we like using the Webserver feature via a LAN link to a PC.The LAN speeds on these are pretty quick however there is a wee latency when porting to another device.

author: mawyatt
author_rank: Super Contributor
timestamp: August 01, 2020, 09:59:43 pm
content: Quote from: tautech on August 01, 2020, 08:46:39 pmOn most systems the closer small 3rd Order IMD tones are more dominate than the ones father out as things begin to roll off as you move away from the dominate 2 tones. This appears to be a little different, either case it's really good performance!!Yes, a single screen image of the FFT would show better. The TD display doesn't shown much as we've seen (all look almost exactly alike) and why this 2 tone test shows more on what's going on within the scope under test regarding DR.I'm already sold but others may benefit form this.Best,

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 12:01:37 am
content: Quote from: mawyatt on August 01, 2020, 09:59:43 pmAnother point of reference when you asked about better resolution.Well of course if we know the fundamentals are where they should be we can go chasing more resolution with a different Ref position and increased vertical scaling.Here I've selected a full screen display that still shows the source waveform yet if that's not required we can select exclusive FFT, neither of which are hamstrung for resolution like the SDS1104X-E 50/50 split display.

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2020, 12:32:55 am
content: That's a nice result and display.I'm sold, are discount coupons available for the SDS1104X-E & SDG1032X?Thanks,Best,

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 01:51:37 am
content: Here's another shot from SDS2104X Plus diving deeper towards the noise floor in full screen FFT mode.

author: bicycleguy
author_rank: Frequent Contributor
timestamp: August 02, 2020, 05:02:18 am
content: Since you seem to be comparing FFT implementations wouldn't it be better to ask for something that can be compared like:What are the closest 2 tones that can be separated with 3dB of dip between peeks and what's the highest and lowest frequency they can be done at.Unfortunately the limits are set by the weakest link in the tone generator or the scope.With Siglent SDG2042x and Keysight DSOX2014A, both enhanced.Example: 119.99MHz and 120MHz high freq. limited by SDG and resolution by DSO FFTExample 10 Hz and 10.05 Hz and low, resolution limited by taking too much time to scan

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 09:19:15 am
content: Quote from: PeDre on August 02, 2020, 09:15:30 amTry a faster timebase as AFAIK Rigols FFT works better with lesser waveforms on the display.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2020, 09:30:02 am
content: Quote from: PeDre on August 02, 2020, 09:25:45 amYes similar problems exist with other entry level DSO's.Lower frequencies allow for better results.Have a try with 10 or 100KHz and see if that makes any difference.You won't find the limits unless you go looking for them.

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2020, 12:54:18 pm
content: Quote from: bicycleguy on August 02, 2020, 05:02:18 amIn my case I was interested in the linearity performance of the scope front end and ADC. Also looking at the AWG output with a SA provides a view into how good the AWG signal is. The 2 tone tests was a simple means to show such and why I asked. If you are interested in the limits of the scopes FFT capability, then other tests like you've suggested might be considered.Honestly I'm impressed by all the scopes behavior, even the low cost "entry' level devices seem to be performing better than I expected.Now that I've retired (chip design) my plan is to purchase a scope and AWG, then a modest SA for starting a home lab. Have a good initial start with a couple repaired Tek 2465 analog scopes, a couple repaired 34401A DVMs and a new Keysight 34465A DVM (superb DVM).I realize a scope can't compete with a SA regarding linearity & dynamic range, thus including the SA in the desired equipment list.Anyway, this has created lots of responses and I wish to thank everyone for jumping in on the funBTW the two images show exactly the same screen capture??Best,

author: bicycleguy
author_rank: Frequent Contributor
timestamp: August 02, 2020, 03:53:55 pm
content: Quote from: mawyatt on August 02, 2020, 12:54:18 pmI was modifying the post. Maybe try reloading the page.

author: gf
author_rank: Super Contributor
timestamp: August 02, 2020, 04:34:32 pm
content: Quote from: PeDre on August 01, 2020, 02:07:56 pmIf you can't increase the number of FFT points, then you can alternatively reduce the sampling rate in order to improve the resolution (i.e. choose a timebase which happens to lead to a lower sampling rate). For 10MHz sine waves you don't need a sampling rate of 1GSa/s, but (say) 25 MSa/s should basically suffice. Reducing the sampling rate from 1 GSa/s to 25 MSa/s is supposed to improve the FFT frequency resolution by a factor of 40 then (if the number of FFT points remains the same).Quote from: bicycleguy on August 02, 2020, 03:53:55 pmUnfortunately still the same. The two preview icons look indeed different, but the large images are still the same. Maybe a bug in the forum softwareI have noticed this effect a couple of times at other posts, too.EDIT: The image in this postinghttps://www.eevblog.com/forum/testgear/two-tone-test-with-scope-and-sa/msg3168886/#msg3168886seems to be broken as well.

author: _Wim_
author_rank: Super Contributor
timestamp: August 02, 2020, 06:39:45 pm
content: Quote from: mawyatt on July 31, 2020, 09:40:36 pmI think the RTO1024 is an 8-bit scope, but increases its ENOB by using massive oversampling at these low frequencies.

author: nctnico
author_rank: Super Contributor
timestamp: August 02, 2020, 07:13:36 pm
content: Quote from: mawyatt on August 02, 2020, 12:54:18 pmAFAIK the two-tone test is primarily a way to see how good the mixer is in a spectrum analyser. A DSO doesn't have a mixer so it is not a surprise this effect doesn't occur.

author: _Wim_
author_rank: Super Contributor
timestamp: August 02, 2020, 07:26:21 pm
content: Quote from: nctnico on August 02, 2020, 07:13:36 pmThe scope's ADC also has non neglectable IMD distortion, but probably only significant at higher frequencies than the 10kHz typically tested here.For example the datasheet of TI ADC08D500 (8-bit 1GSPS) states an IMD of -50db at 121Mhz.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: August 02, 2020, 07:36:08 pm
content: QuoteYep, it's 8 bit! The "16 bit HD Mode" marketing wank is one of the more egregious examples out there. It was good sport to watch Lecroy and Keysight take shots at R&S for it. Of course, in the scheme of things, it's not *that* big of an issue. Obviously it didn't stop me from buying one (for unrelated reasons).Still, there actually is a kernel of truth at the center of the marketing turd: R&S did a better than usual job at averaging. Many implementations are stuck at a pre-Signal-Processing-101 level. They use boxcar filters before downsampling and don't calibrate their PGA settings to ensure good dithering across the instrument bandwidth. R&S brought things up to Signal-Processing-101 spec. That's nice. It plays especially well with the fully digital trigger, which still works great when you've got de-embedding and filtering on. The third timebase knob (record length) also shines in these situations.I was hoping to provoke a response from someone with an Infiniium S. There was a big split in ADC philosophy this generation, with Keysight taking the "farm out to a bunch of slow CMOS cores, individually calibrated" strategy and R&S (Lecroy too, I think) taking the "one core and step on the gas" strategy. I wanted to see what that looked like in the low frequency "average to the moon" regime. There's still time, I suppose

author: egonotto
author_rank: Super Contributor
timestamp: August 02, 2020, 07:53:02 pm
content: Hello,jjoonathan wrote: "I was hoping to provoke a response from someone with an Infiniium S"perhaps you can ask Shahriar. He can make test with Infiniium S and Infiniium MXR and Tek MSO 5 and MSO 6.Best regardsegonotto

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2020, 08:26:21 pm
content: Quote from: nctnico on August 02, 2020, 07:13:36 pmDon't know about others, but we often used the 2 Tone Test as a verification of the custom chips we developed (for example to compare against simulations) and the linear systems involved (LNA, active filters, attenuators, VGAs and so on) well before any intended non-linear functions (mixers) were encountered, we used the 2 Tone Test on these (mixers) as well. Also used the 2 Tone Test to characterize the high resolution DACs and ADCs we were involved with. In fact this 2 Tone Test was one of the main tests involved in a new type ADC developed (non-uniform sampling ADC) that utilized simultaneous time and amplitude quantization. This ADC has the unique property on not requiring a pre anti-aliasing filter as the input signal itself imposes the time & amplitude quantization that allows post conversion anti-aliasing filtering. Someday these may end up in our test equipmentIn the area I recently retired from (chip design), the 2 tone test was utilized for just about any "analogish" circuit or system, linear or non-linear, to help characterize performance. This test is also a very valuable tool for calculating the classic IIP3 and OIP3 points without having to sweep the input power levels assuming a well behaved system.Anyway, don't think the 2 Tone Test should only be relegated to systems involving mixers (like SA), but can be highly beneficial in all sorts of "linear" and "non-linear" systems, including the data acquisition systems we call Digital OscilloScopesBest,

author: jjoonathan
author_rank: Frequent Contributor
timestamp: August 02, 2020, 09:49:07 pm
content: > perhaps you can ask Shahriar. He can make test with Infiniium S and Infiniium MXR and Tek MSO 5 and MSO 6.He's already gearing up for a shootout. That's what the preselector was for. Can't wait

author: David Hess
author_rank: Super Contributor
timestamp: August 02, 2020, 11:53:17 pm
content: Quote from: nctnico on August 02, 2020, 07:13:36 pmThe sampling process before conversion *is* mixing, and the same types of circuits are used. (1) The difference is that an impulse replaces the local oscillator signal so while a mixer has fine resolution in frequency, a sampler has fine resolution in time.Aliasing in a sampling analog-to-digital converter is identical to downconversion, and subsampling receivers take advantage of this. The impulse being used in place of the continuous local oscillator signal contains harmonics which mix with higher frequencies to allow this.(1) Microwave mixers are a good source for microwave samplers when the transformers are removed from the RF and IF ports.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 03, 2020, 12:07:00 am
content: Just for fun, here is what the tone test looked like in 1975!I fired up an ancient HP 3571A Spectrum Analyzer - a 100% analog spectrum analyzer full of gold, crystal filters, and engineering love.The two tones are generated by HP3325A x2 summed into a 50 ohm load.The analog output from the 3571A was tracked on the scope in roll mode for display.It seems to me the performance is not embarrassing at all, considering this thing is 45 years old. The noise floor is a bit high, but the 3325A signal generators are not exactly low noise... The 3571A is capable of -130dB, perhaps a little better, performance on a clean signal. The ancients knew how to build stuff! (It probably didn't cost $450)

author: mawyatt
author_rank: Super Contributor
timestamp: August 03, 2020, 02:14:05 am
content: Quote from: SilverSolder on August 03, 2020, 12:07:00 amThose old instruments can still hold their ownBrings back fond memories, along with the superb Tek analog scopes of that eraBest,

author: rf-loop
author_rank: Super Contributor
timestamp: August 03, 2020, 02:21:40 am
content: Quote from: nctnico on August 02, 2020, 07:13:36 pmThey are not named as Mixer but other way, how many "mixer" you find in one oscilloscope... think about it. Do you know what all circuits can produce mix. Sorry... I feel it was necessary..but not so seriously.Is it good to know even basic fundamentals...Basic: When two frequency signals, f1 and f2, are input toany nonlinear device.... etc... do you know any other nonlinear devices than these what are commercially named as Mixer. Ok lets start from bottom, do you know diode... how about transistor etc etc.

author: tggzzz
author_rank: Super Contributor
timestamp: August 03, 2020, 07:34:41 am
content: Quote from: rf-loop on August 03, 2020, 02:21:40 amPrecisely.Don't forget to add "capacitor" to that list.

author: jjoonathan
author_rank: Frequent Contributor
timestamp: August 03, 2020, 01:43:15 pm
content: I've seen a rusty cable mix surprisingly well even at low power.

author: 2N3055
author_rank: Super Contributor
timestamp: August 03, 2020, 01:49:07 pm
content: Quote from: jjoonathan on August 03, 2020, 01:43:15 pmMany chemical compounds are semiconductors..

author: SilverSolder
author_rank: Super Contributor
timestamp: August 03, 2020, 01:58:21 pm
content: Quote from: 2N3055 on August 01, 2020, 04:26:31 pmThat looks like pretty impressive performance!

author: 2N3055
author_rank: Super Contributor
timestamp: August 03, 2020, 02:09:55 pm
content: Quote from: SilverSolder on August 03, 2020, 01:58:21 pmThat is a real 16bit scope. Fantastic little beast.Even 8 bit one (3406D) is excellent.

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: August 03, 2020, 02:55:15 pm
content: I made some quick tests on my MSO7014. source is a Siglent SDG2082X :

author: SilverSolder
author_rank: Super Contributor
timestamp: August 03, 2020, 03:15:45 pm
content: Quote from: mawyatt on August 03, 2020, 02:14:05 amSometimes I just buy weird stuff on eBay because it looks interesting and doesn't cost much. It gives the same feeling as starting an old muscle car and taking it for a drive - giving a hard time to the occasional new car!I have managed to avoid going down the Tek scope rabbit hole, even though I can see the admirable engineering that is going on with them. There is only so much space in the man cave!

author: tautech
author_rank: Super Contributor
timestamp: August 03, 2020, 07:09:12 pm
content: Quote from: jemangedeslolos on August 03, 2020, 02:55:15 pmIn Dot mode ?

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: August 03, 2020, 07:26:46 pm
content: Quote from: tautech on August 03, 2020, 07:09:12 pmNo, vector mode on channel 1 but color grading option enabled in FFT settings.I left the memory depth management in auto.I hid the channel on most on my screenshot but I can redo the measurements without hiding the channels and without activating color grading option.In the end, the results are the same.We miss max hold on FFT settings in Rigol scope so I can't replicate your screenshots.

author: tautech
author_rank: Super Contributor
timestamp: August 03, 2020, 07:58:04 pm
content: Quote from: jemangedeslolos on August 03, 2020, 07:26:46 pmThen why is there just dots below the 2 highest markers ?QuoteCan you engage any averaging ?Enough and it cleans up FFT much like Peak hold albeit at a lower value.

author: rf-loop
author_rank: Super Contributor
timestamp: August 04, 2020, 03:31:30 am
content: This whole two tone test can be just fun playing or it can be serious tests for analyze equipment.Separate peoples do separate tests with different settings and ... all different. So results are some times just nonsense and or not at all comparable.First need do some definitions and this also depends what are we testing. if there is under testing oscilloscope front end it is really not 10kHz thing. After then also signal level is more than important. After then signal source is very important. Example two tones need really be well isolated from each others ... and list continue... endless untiol peoples do some deal and define test parameters what all use. Other way... like old sentence. Garbage in - garbage out.Different oscilloscopes have different analog front end. Input levels are divided to some bands and inside bands some small steps.One perhaps important thing is try first find where is front end 1dB compression point. Why. Because more linearity less mixing. So different scopes may be tested with very different front end working point related to its best linearity. Result may be totally different if test same scope with different level.So, it need define what is right level for two tone tests so that it is any kind of useful and comparaple or is it better to keep just for fun playing. And now one question is, is it same power.... ups voltage level for different scopes. Or is it same proportional level from one V/div setting full scale.I am thousands of km far from my test equipments so I can not demonstrate how important this is specify so that results are comparable and give some real useful information.Used test level.Two tones sources isolation. Is it isolated when one function generator digitally combine two tones - really.Oscilloscope BW is mostly least 100MHz. Often 200... 300.... 500 etc etc. Testing with 10kHz and then compare scopes is like fun joke.Last time I characterize one noname SA using two tone test I have two fully separate identical high quality RF generators and well isolating combine with test lab grade components. Even if I need test only up to 100MHz I have not even think to use function generator what can internally "somehow" combine two carriers if I do not want just keeping some fun. Yes they can combine and this is suitable and ok for many kind of purposes but imho fully questionable for serious IP3 tests.https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ma71/1MA71_2e_amplifier_nonlin_meas.pdfSpecially chapter 3 beginning and then there bit later 3.4.Of course now SA used is now inside DUT, oscilloscope.

author: SilverSolder
author_rank: Super Contributor
timestamp: August 04, 2020, 04:22:17 am
content: Quote from: rf-loop on August 04, 2020, 03:31:30 amAre you saying, we might really be measuring the distortion of the function generator itself if we do that?

author: gf
author_rank: Super Contributor
timestamp: August 04, 2020, 06:16:19 am
content: Quote from: SilverSolder on August 04, 2020, 04:22:17 amThat's why you need to take care, cross-checking the signal on a SA, in order to verify that the a priori distortion of the signal is significantly lower than the distortion of the scope under test. Note that the SA isnotsupposed to be the DUT for the test which is subject of this thread. Testing a high dynamic range SA is much more demanding.

author: 2N3055
author_rank: Super Contributor
timestamp: August 04, 2020, 06:50:12 am
content: Quote from: SilverSolder on August 04, 2020, 04:22:17 amOf course you will measure distortion of the function generator if it is worse than scope or SA noise floor..I agree with Rf-loop that this is not serious testing... but...We did get some interesting and real data from it despite not being done by rigorous method.Not accurate data, but more of a glimpse of order of magnitude of some behaviour.First thing I discovered is that digital WaveCombine on my SDG6000X does respectable job on low frequencies at 0dBm . Also most of the scopes shown here, inexpensive and expensive, has shown that pretty much none of them have any appreciable intermod distortion above noise level and resolution of scope A/D, at frequencies and levels tested..This makes me curious to actually sit down and do more comprehensive test, first to figure out IMD of SDG6000X across frequencies and output levels using digital WaveCombine, to figure out how well it works when D/A isn't full scale, and when switching attenuators, at lower levels when S/N ratio gets low or distortion when I get to higher levels and amplifiers start distorting...Also here we could see difference in FFT implementation on different scopes, and where are the limitations...So not a waste of time.But it is all, so far a discovery journey to learn something about your equipment (Thanks to Mawyatt who brought it up as a topic). This type of testing is very common in industry for much more than just mixers and RF, like he said, but his bringing it up here made it visible to many who didn't use it before for this purpose. And it sparked some very good posts, so all very good and well...If we would like to make standardized test, we would have to define it. Which would need some sort of goal or target result expectations (what do we want to accomplish), some sort of technical measurement equipment spec (what equipment and what spec we would need to do it), and measurement method (what needs to be measured and how exactly).Problem is, that could be specified, but it would vary for different purposes (IMD distortion of audio front end, or 100MHz scope or 1Ghz scope) and would soon evolve into IEC type of standard, that not many would be able to follow, if not for any other reason than lack of equipment necessary.Regards,

author: mawyatt
author_rank: Super Contributor
timestamp: August 04, 2020, 01:17:27 pm
content: As the OP this request was to help show how good or bad the scope front ends are and and how well they preserve the ADC performance. Having AWGs available at reasonable cost, and modest SAs allows the Two Tone IMD Test to be implemented by just about anyone that's seriously interested, and many folks certainly did contribute...so thanksThere's no need for a complex expensive setup as long as the input signal source IMD is at least 10~20dB better than what you are measuring. The AWGs have shown very good performance even when the two tones are combined digitally (I assume this for the Siglent AWG, very nice feature).My goal was to help decide on which Digital Scope to purchase for my developing home lab now that I'm retired and this cost is out of my own pocketHopefully it may help others interested in new instruments, or just evaluating the instruments they already have on hand.So many thanks to all that have contributed and helped me spend my retirement savingsBTW just purchased the Siglent SDS2102X Plus, wanted the 2104X Plus but they are not in stock here is US. Soon will be the AWG and SA, and maybe a programmable PS.Best,

author: jjoonathan
author_rank: Frequent Contributor
timestamp: August 04, 2020, 07:09:40 pm
content: Is SDG6000X WaveCombine digital? Mine clicks a relay when I engage it. I suppose it's probably range related. In any case, the resulting signals are quite nice!

author: mawyatt
author_rank: Super Contributor
timestamp: August 04, 2020, 11:37:21 pm
content: Quote from: jjoonathan on August 04, 2020, 07:09:40 pmDon't have one so just speculating and why I said "think" it might be. Analog combing would have the advantage of better resolution. The results certainly look good, so either way it's a nice performing AWG.Best,

author: tautech
author_rank: Super Contributor
timestamp: August 04, 2020, 11:49:15 pm
content: Quote from: mawyatt on August 04, 2020, 11:37:21 pmAll the SDG X models offer the Wave Combine feature including the 14 bit SDG1000X models.I checked the SDG2042X I've been using and indeed if channels are ON and Wave Combine is selected a relay click is heard however until someone RE's these we don't yet know where in the signal path this occurs.Still don't have my SDG6022X back from a mate but will have over the weekend so to push 2 tone tests to much higher frequencies than thus far...... but it might need improving a bit first.Will also do 2 tones at different amplitudes like rf-loop suggested and all for shits and giggles although we might just learn a bit more.

author: mawyatt
author_rank: Super Contributor
timestamp: August 05, 2020, 12:42:19 am
content: Quote from: tautech on August 04, 2020, 11:49:15 pmWell if you and JJ are hearing a relay click that's gotta hint at the combining is taking place in the analog section, not digital. Since a relay is used this also hints that this is at a higher analog level where a relay is preferred for linearity over a SS switch, maybe even at the outputs which are resistive summed??I'll be happy to RE if someone will lend me oneBest,

author: mawyatt
author_rank: Super Contributor
timestamp: June 09, 2022, 03:58:08 pm
content: Hey, this is old but think it's the best place to start with a continuation with the new Siglent SDS2000X HD which has true 12 bit ADCs. Nice to compare how well the 12 bit core ADC improves the 2 Tone IMD tests.Know a couple of you folks out there have these new MSOs and hoping you'll be kind enough to do some additional 2 Tone IMD testing with the basic 12 bit and any additional resolution enhancements available.Best,

author: rf-loop
author_rank: Super Contributor
timestamp: June 11, 2022, 04:59:52 pm
content: Generator SDG1032X. It need need tightly understand that this with splitter is not enough for serious dual tone test. Who knows what it itself generate...Separate channels externally resistive combined (resistive 50Ω Δ type splitter)So, SDG outputs are 6dB higher. To SDS2504X HD input ~0dBm signals.9.9kHz and 10.1kHz (distance 200Hz)f1 = 9.9kHz f2 = 10.1kHz2*f2 - f1 = 10.3 Peak Marker 4 (-85dBc)2*f1 - f2 = 9.7 Peak Marker 1 (-83dBc)99.9 and 100.1kHz (distance 200Hz)0.9999MHz and 1.0001MHz (distance 200Hz)(note FFT samplerate change)10.999MHz and 11.001MHz (distance 2kHz diff)(note FFT samplerate change)

author: mawyatt
author_rank: Super Contributor
timestamp: June 11, 2022, 06:05:32 pm
content: Quote from: rf-loop on June 11, 2022, 04:59:52 pmWell we may not know just how good the signal from the SDG1032X is (could be verified with a quality SA tho), but the result you've shown is quite good indeed!! If we "assume" the IMD result is root sum square of the AWG and the DSO, then the DSO worst case IMD is as shown (assuming AWG IMD is zero)!!!Siglent evidently got this right just like they did with the 8 bit SDS2000X+Thanks for taking the time to show thisBest,

author: David Hess
author_rank: Super Contributor
timestamp: June 12, 2022, 10:05:01 am
content: Run the same test with a low distortion summing amplifier and see if the results agree.

author: G0HZU
author_rank: Super Contributor
timestamp: June 12, 2022, 04:59:02 pm
content: For comparison, a fairly modern lab spectrum analyser should achieve about 95dB SFDR in a two tone test with a 100Hz RBW selected. Greater SFDR than this is possible with a narrower RBW but the measurement time will obviously be longer.

author: Performa01
author_rank: Super Contributor
timestamp: June 13, 2022, 05:46:23 pm
content: Since we have seen some LF results so far, I thought I’ll add some measurement at the upper end of the spectrum, specifically at 90% of the specified bandwidth of 500 MHz.So, this IMD test is about 450 MHz and 200 kHz frequency offset.It has already been brought up several times before: for comparable results, we need a bunch of conventions and preconditions. I cannot meet all of them, for instance I cannot guarantee that the dual tone test signal has any lower than -74 dBc IMD.As has been already discovered, the results also depend on the scope settings. Like a “real” spectrum analyzer, we won’t get impressive results if we feed the scope input with full scale signals (in SA terms: right below the compression level). In general, the lower the input signal(s), the lower the distortion. For testing the third order dynamic range of an SA, we usually have signals in the realm of at least 30 dB below the 1 dB compression point. Consequently, I think it is only fair to use signal levels at least about 10 dB below full scale for the scope as well.Now let’s calculate the settings for 0 dBm test signals (sine waves assumed throughout these calculations).0 dBm into 50 ohms means 223,61 mVrms or 632,46 mVpp.With two 0 dBm signals combined, the peak output voltage doubles to 1.265 Vpp.The SDS2000X HD ADC input range is roughly 9.6 divisions, so we need at least 132 mV/div to have the dual tone signal within full scale. If we want to keep the signal at or below -10 dBFS, this makes for 4 Vpp full scale.In the end I chose 500 mV/div, which is equivalent to 4.8 Vpp full scale.We can get ~74 dBc IMD3 in this scenario, this is equivalent to a TOI (third order input intercept point) of +36.7 dBm.See attached screenshot:SDS2504X HD_IMD_500mV_C450MHz_O200kHz_0dBm_LPlease note that this is not a bad result at all. There were (and still are) plenty of low(er) end spectrum analyzers that might not even reach 70 dB third order dynamic range, whereas top instruments can provide up to 110 dB.For example, the ancient HP8590A has a third order dynamic range of 70 dB at -30 dBm input.

author: mawyatt
author_rank: Super Contributor
timestamp: June 13, 2022, 06:37:13 pm
content: Quote from: Performa01 on June 13, 2022, 05:46:23 pmHowever you can guarantee that the DSO (or the AWG) is better than this....which is impressive for the DSO, and not so bad for the AWG either IMOQuoteFor the DSO case the lower signal level may not be the "Sweet Spot" of IMD, this is because you need to partially fill up the ADC range to capture as many "bits" as possible and this conflicts with the smaller the signal the better the amplifiers IMD performance. So the overall DSO "Sweet Spot" is more likely somewhere in the middle DSO range where you've positioned your input signal.QuoteAgree, this is very good indeed and indicative that the Siglent engineers got this front end design right just like the X+ version, of course they had the advantage of that excellent design to start with!!This speaks very well for the new HD version, and shows that Siglent took the time and energy to not just replace the 8 bit ADC with a 12 bit version, but to reengineer the front end to support the new 12 bits ADC and not degrade it's performance!!Makes one wonder just how good this DSO is, we know it's better than -74dBc IMD at 450MHz from your tests. Maybe someone with a really good SA and a couple quality signal sources with much better than -74dBc IMD when resistive attenuator summed can duplicate your tests. Also especially interested if the lower frequency Two Tone Test can be done with the ERES function (if possible within FFT) which I understand is hardware based in the HD version.Anyway, thanks so much for the testing and reports!!Edit: Maybe this front end design is from the 6000 series that they did for LeCroy with the 12 Bit ADC??Best,

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 09:44:12 am
content: QuoteNote that the displayed result on the scope or analyser screen can appear artificially higher or lower than the real result due to the linearity limitations of the scope/analyser.Normally, the distortion performance of the analyser will be fairly well known and understood beforehand and the usual aim is to ensure that the IMD level in the analyser is >20dB lower than the IMD caused by the device under test. This keeps the overall uncertainty of the displayed result below about +/- 1dB.So to measure -74dBc within +/- 1dB the scope or analyser needs to be capable of -94dBc IMD. To measure -74dBc within about +/- 3dB the scope or analyser needs to be capable of -85dBc IMD.

author: gf
author_rank: Super Contributor
timestamp: June 14, 2022, 02:13:21 pm
content: Quote from: G0HZU on June 14, 2022, 09:44:12 amBut if A+B = -74dBc, can't we still conclude that A <= -74dBc and B <= -74dBc?[ Btw, this leads to the question: How do IMD products from two sources actually add up? ]

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 02:34:30 pm
content: Quote from: G0HZU on June 14, 2022, 09:44:12 amThe scope or any instrument will always show a result for the Two Tone IMD that is the Root Sum Square (RSS) of the signal source and measuring instrument. Thus, the shown result is the worst case of such, either source or measuring instrument. If the instrument were an older analog display types then there could be a display uncertainty in reading, however these DSOs are actually digital data acquisition systems which utilize ADCs and digital techniques so this isn't an issue.Can't think of a case were the instrument linearity issues would cause an "artificially lower" IMD result (read better), if you have such a case please provide details.QuoteHere the device under testISthe analyzer, so the source needs to be better than the analyzer. Again the result is always the RSS of both the source and the analyzer.QuoteSee above, the scopeISthe DUT.Normally when measuring a device the actual measured IMD result is square root of {(Source IMD)^2 + (DUT IMD)^2 + (Measurement Instrument IMD)^2}, so if the Source and Instrument IMD is << than DUT IMD, then the result becomes the DUT IMD.In your example of a -74dBc IMD measurement, if the source has no IMD then the Scope IMD is obviously -74dBc. If the source has -77dBc IMD (3dB difference), then the Scope has -77dBc IMD, if the source has -80dBc (6dB difference) then the Scope IMD is -75.3dBc, if the source has -84dBc (10dB difference) then the Scope IMD is -74.5dBc, and if the source has -94dBc (20dB difference), then the Scope has 74.04dBc. Of course you can do this analysis with a true DUT IMD and include the Source and Scope/Analyzer IMD and find the 10dB separation is sufficient for the 1dB uncertainty margin. This tends to confirm the usual 10dB minimum separation in measurements for a reliable resultant, however because the measurement instrumentISthe DUT the 10dB margin actually confirms a 1/2dB uncertainty.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 02:38:18 pm
content: Quote from: gf on June 14, 2022, 02:13:21 pmYes, the result is the Worst Case of -74dBc IMD for all things involved. See directly above for more details.Best,

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 03:12:11 pm
content: QuoteIt isn't that simple, you have to think in terms of voltage level and phase rather than power.

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 03:59:22 pm
content: Quote from: G0HZU on June 14, 2022, 03:12:11 pmThese were all actually computed based upon the RSS method as outlined above. Here one must convert to actual levels (voltage in the case of the DSO) in linear terms not dB, do the proper calculations, then revert back to dB scales. Since you can't add or subtract linear terms in dB (actually a exponential/logarithmic scale, so adding in dB is multiplying, and subtracting in dB is dividing), you can't do the RSS computations without converting to linear terms, do the squares, then summation, then square root, then convert back to dB scales ....elementary signal processing!!BTW phase has nothing to do with 3rd order IMD as outlined in this thread about Two Tone IMD, it's all about the mathematical relationship between odd order system non-linearity and a method to represent the DUT in a simple way to convey such. This 2 Tone IMD is utilized in all sorts of various fields and why we wanted to see how well these new DSOs behave. It's also useful in extrapolating the well known RF metric of 3rd Order Intercept as was shown earlier (without sweeping the input), however this extrapolation does have a baseline assumption that the DUT has a well behaved non-linearity and exhibits gain compression as signal levels increase. There is a little know amplifier where this metric is not applicable, where the gain actually expands over a range before going into compression, but this is out of the scope for discussion here (will discuss if folks want, but better in another separate thread).Anyway, hope this helps clear up any confusion regarding the Two Tone IMD test and how they infer the performance of the instruments and DUT involved.Best

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 04:15:47 pm
content: QuoteWhen you combine two (same amplitude) IMD tones in phase the voltages sum in phase so you get twice the voltage. This means the level seen on a spectrum analyser goes up 6dB relative to each tone. If the tones are out of phase they cancel so the IMD level will tend to cancel. So I think phase can be important.Normally with a narrowband test setup the IMD terms tend to be in phase. However, in theory at least, the phase shift can be sufficient to cause significant cancellation of the IMD terms when two non linear stages are connected in series. So you have to consider phase in any analysis.

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 04:40:16 pm
content: QuoteIf the source and the scope both truly had -77dBc IMD, the resultant IMD would typically appear at -71dBc on the analyser (a 6dB difference and not the 3dB you imply). This assumes the IMD terms are in phase. In reality there might be a small phase shift. If the test was done with wide tone spacings the phase cancellation effects could be more pronounced (in theory at least).Your other calculations above are wrong as well.

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 04:41:16 pm
content: Quote from: G0HZU on June 14, 2022, 04:15:47 pmDon't think you understand the Two Tone IMD, what you are missing here is that the Two Tones are not the same frequency, but separated by a small fraction of the absolute frequency of either. In the last IMD test graph shown the Two Tones were at ~450MHz, with a 200KHz frequency difference, and both Tones at equal amplitude. So the Two Tones wander in and out of phase and the amplitude varies between twice the individual Tone amplitude (in phase) to zero (out of phase) at the difference frequency rate. Note how the composite amplitude varies between the maximum of twice the single tone level to identically zero, this exercises the total amplitude range of the DUT between these extremes and why this simple 2 tone test is so reveling of the DUT non-linearities.Best,

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 04:48:39 pm
content: What matters here is the relative phase of a small IMD term generated in stage 1 (and this IMD tone is then amplified by stage 2) vs the phase of the IMD term actually generated in stage 2 from the two main test tones.If the phase of these is not the same then there will be some cancellation in stage 2. Do you understand it now?

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 05:08:29 pm
content: QuoteLike I said, your calculations above are totally incorrect. I think you should have used this equation for the case where the terms are in phase.Amplitude Error = 20*log( 1+ 10^(d/20)) where d is a negative number that represents the difference between the two IMD levels.In your analysis a 10dB separation delivers <1dB uncertainty. This is obviously wrong and the correct answer is just under 2.4dB.

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 05:29:01 pm
content: Quote from: G0HZU on June 14, 2022, 04:40:16 pmWell let's just go over those very calculations that you infer are wrong.Scope IMD is -77dBc and Source IMD is -77dBc,then -77dBc = 10^ (-77/20) or 10^-3.85 or 141.25 E-6 or 19.953 E-9 squaredSo 19.953 E-9 + 19.953 E-9 = 39.905 E-9Square root of 39.905 E-9 = 199.76 E-610 base LOG of 199.76 E-6 = -3.699520*(-3.6995) = -73.99 or ~ -74dBcBTW you can perform the above using dBc as power ratio, i.e. 10*Log rather than voltage ratio which is 20*Log and the result is the same as it should be, since dBc is a ratio of like terms!!Care to point out where this is wrong!!! How about you post your calculations here and shown how you come to the incorrect conclusion that this is all wrong!!Best,

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 05:40:17 pm
content: What you are overlooking is that the IMD terms are coherent. If you combine two coherent terms in phase the level on the spectrum analyser goes up 6dB with respect to the power of each tone and not 3dB.I'm not sure I need equations to prove this as this should be common knowledge.You have made the common mistake of just doubling the power. The power goes up by 6dB in the in-phase/coherent case which is a multiplication of four not two.Put the numbers in to my earlier equation for the case where the IMD terms are at the same level. This is when d = 0. You should get 6.02dB as the answer (not 3.01dB).

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 05:45:44 pm
content: A good practical example would be to take two identical 12dB gain amplifiers and put them in series with an attenuator in between them that has 12dB attenuation. So you end up with the same 12dB gain of a single amplifier. Then compare the narrowband IMD level of this arrangement to the IMD seen on a single 12dB gain amplifier with no attenuator. Note that this analysis assumes a perfect measuring tool. Or you could consider the second amplifier stage to be the limiting factor of the measurement tool if you like.For a narrowband system the IMD of the first and and the second amplifier usually sums in phase so the IMD levels for the two amplifiers in series will usually be about 6dB worse (higher in level) compared to the case where you just measure a single amplifier on its own with no attenuation.This is because the two IMD terms sum together in phase so you get twice the voltage (four times the power) hence a 6dB increase in IMD level seen with the dual amplifier setup.

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 05:46:01 pm
content: Quote from: G0HZU on June 14, 2022, 05:08:29 pmThink you are talking about your very own calculations being "totally incorrect"!! And your thinking istotally incorrect!!The simple straightforward analysis we have shown above shows that the result we arrived at is correct as stated, best review how and where to use the Amplitude Error equation you proposed.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 05:51:36 pm
content: Quote from: G0HZU on June 14, 2022, 05:40:17 pmNothing overlooked, the two signal generators areNOTcoherent, as shown in just about every app note anywhere these source are completely independent sources. The IMD products are computed a 2F1-F2 and 2F2-F1, where F1 and F2 are completely independent sources, so the common mistake is yoursBest,

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 05:56:13 pm
content: Wow, just wow... this is taking much longer than it should...Have a look at page 13 of this white paper from Rohde and Schwarzhttps://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ma219/1MA219_2e_IM_Distortion.pdfIf you think they are as clueless as me you could also look at page 21 of this article from Keysight.https://www.testunlimited.com/pdf/an/5980-3079EN.pdfThey use the same equation as me. The same equation I've been using for decades.QuoteYou simply aren't listening to what I'm saying. The generated distortion terms are coherent in both stages. That's the point I'm trying to make.

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 06:16:50 pm
content: Quote from: G0HZU on June 14, 2022, 05:45:44 pmWell this won't work as you state either!! The two identical 12dB gain amplifiers the second "sees" the IMD of the 1st attenuated by the 12dB attenuator, however the source signal level will be the same for the 2nd, so the net result will NOT be twice IMD level as you claim, but slightly more IMD than a single amp.Please note that typical amplifiers and most circuits the IMD grows faster than the input signal level grows, so in your example the 1st and 2nd amplifier would have the same IMD if the second one had the 12dB attenuator input, but the 1st would have the IMD also attenuated by 12dB, so the 2nd would see an input with the same signal level as the 1st and the 1st IMD attenuated by 12dB, thus likely not much additional overall IMD.Anyway, this is why noise is usually determined by the first stages and linearity by the later stages in well designed systems, and from the posts shown here the new SDS2000X HD is a very well designed data acquisition system disguised as a scopeBest,

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 06:53:17 pm
content: Quote from: mawyatt on June 14, 2022, 06:16:50 pmIf you won't take my word for it then read the app notes I linked to in post #109 written by R&S and Keysight. There is a graph there if it helps. The graph shows that the IMD level rises 6dB when the IMD level in stage 1 is the same as the IMD in stage 2. The two identical amplifiers in series (with an attenuator between them) is a neat way to demonstrate this. It can be demonstrated on a decent RF simulator or you can do the tests on a pair of real amplifiers and use a decent spectrum analyser. I've demonstrated both methods in the past. The result is usually very close to a 6dB increase. This assumes the IMD terms generated in each stage are in phase so they sum and this gives a 6dB boost.See the blue curve below. It shows a 6dB boost just as I told you it would.

author: SilverSolder
author_rank: Super Contributor
timestamp: June 14, 2022, 07:03:07 pm
content: Are you saying that if we have two signal generators running, and we are able to change the phase between them, we should see a difference in the result?

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 07:13:58 pm
content: No, what matters is the relative phase of the distortion term from stage 1 delivered to stage 2 with respect to the phase of the distortion term generated in stage 2. The theory and equations are at their best when the distortion terms are very low in amplitude compared to the amplitude of the test tones.Note that mawyatt could maybe take a look at the graph in my earlier post and see that his calculation of <1dB uncertainty when there is a 10dB difference in IMD is wrong and my calculation of 2.4dB was correct. The blue curve shows about 2.4dB error at a 10dB difference.

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 07:38:21 pm
content: Quote from: G0HZU on June 14, 2022, 05:56:13 pmAgree it's taking too long, and I'm listening very well thank you!!These are good references for a swept SA and assumes a separate DUT is being evaluated and measured by the SA. Whereas the DSO under discussion here are FFT based and the actual DUT is themself being measured, so effectively DSO measuring themselves. These references also state the signal sources are coherent, whereas we've stated all along they are not coherent, and as such can use the RSS approach which assumes incoherent signals. With coherent vs. incoherent signals this is likely the source of the 3dB difference we are discussing, and maybe even the swept SA vs FFT based analysis. Also suspect if the 2 Tone IMD beat waveform were analyzed exactly over a integer number of complete cycles which might be difficult with a conventional SA, then the results might converge to a similar value.Here's a couple papers on IMD for ADCs, and this is actually what's being discussed regarding the Two Tone Test, that and the scope input.https://www.analog.com/media/en/training-seminars/tutorials/mt-012.pdfhttps://www.ti.com/lit/an/slyt090/slyt090.pdf?ts=1655216827186This one discusses placement of the two tones so the IMD products fall within the FFT frequency bins.https://pdfserv.maximintegrated.com/en/an/AN728.pdfAnyway, interesting discussions.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 14, 2022, 07:57:15 pm
content: Quote from: G0HZU on June 14, 2022, 06:53:17 pmA) With the attenuator the total IMD should be just slightly more than the IMD of the second amp since this distortion products of the 1st are attenuated by 12 dB.B) But what would the IMD be if the single amp had the same gain (read twice) as both amps in series wo the attenuator, 6dB worse maybe??QuoteYes, saw the graph and it clearly states coherent input sources, not incoherent sources as we've stated all along as this is the basis for utilizing RSS as stated.Edit: Added hereFigure 2–17Amplitude Uncertaintydue to Two Coherent 8.00 CW Tone Adding 6.00 TogetherBest

author: nctnico
author_rank: Super Contributor
timestamp: June 14, 2022, 08:57:56 pm
content: Quote from: G0HZU on June 14, 2022, 04:48:39 pmAfter reading your other posts: what you are saying is that the measurement uncertainty depends on whether the intermodulation products of various devices in the chain are in phase or not? For two identical amplifiers the assumption is that the intermodulation products are in phase and thus -worst case- add up (no matter whether the input signal is a single tone or multiple tones).Edit: better wording

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 09:39:32 pm
content: In the case of a DSO I guess a lot depends on the signal handling performance of every stage of the analogue front end before it gets into the digital domain. All these stages can generate IMD ahead of the ADC.The same applies to a spectrum analyser. Often it's the front end that defines the IMD performance.QuoteI'm confused why you come to this conclusion. This is for a classic two tone IMD test. The 12dB attenuation is offset by the 12dB gain of the first stage. So the second stage is being driven by a signal that now has (say) -60dBc IMD. The second stage will amplify these IMD terms by about 12dB but it will also generate coherent -60dBc IMD terms of its own at a similar level at the output. These two mechanisms are usually in phase so the IMD terms will appear 6dB worse for the dual amplifier compared to the single amplifier case.The single amplifier is being fed the same amplitude drive level but there are no IMD terms in the drive signal. So it just generates IMD3 terms close to that predicted by the datasheet for that drive level. The dual amplifier will have IMD terms 6dB higher.

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 10:22:04 pm
content: See the attached video showing a dual stage amplifier vs a single stage amp. Each amp has 13.1dB gain and there is a 13.1dB attenuator between them. The IMD3 terms are -57dBc for the dual amplifier.When the second stage and the attenuator is removed then the IMD levels drop to about -63dBc. This is only a simulation so not exactly conclusive but I have done similar tests on a pair of 'real' and identical ERA-1SM test amplifiers. These produce about 12.2dB gain so I did the test with a 12.2dB attenuator in between them for the dual amplifier scenario. The IMD change was very close to 6dB when I changed to a single amplifier with no attenuator. It was within about 0.3dB of 6dB.

author: 2N3055
author_rank: Super Contributor
timestamp: June 14, 2022, 10:30:29 pm
content: Quote from: G0HZU on June 14, 2022, 09:39:32 pmYou keep repeating same things and mentioning first stage and second stage and spectrum analysers.What does it have to do with two tone performance of oscilloscope in question?How do you map your statements to that?So plainly, if you feed two-tone into scope input like Mike did (noncoherent two tones) and get some IM distortion (whatever the number) isn't that IM distortion of the scope.He is not trying to measure any external DUT. He is simply trying to characterize the scope internal IM distortion.What am I missing here?

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 10:50:03 pm
content: You can think of the two tone source itself as the first stage. I got the impression the IMD performance of this was unknown.Normally, a high performance two tone source will use two sources separated by a high isolation (linear) combiner and there will be lowpass filters in place as well.

author: G0HZU
author_rank: Super Contributor
timestamp: June 14, 2022, 11:06:20 pm
content: You can also experiment with the type of resistive combiner. It's often risky to use a classic delta or star resistive combiner as these have limited port to port isolation. I've built and used alternative resistive combiners that offer higher port isolation at the expense of some loss in the combiner. The alternative is to add attenuators before the combiner.

author: tautech
author_rank: Super Contributor
timestamp: June 15, 2022, 01:21:40 am
content: Quote from: G0HZU on June 14, 2022, 10:50:03 pmMostly the 2 tone sources in this thread have been created in SDG models with their 2ch wave combine feature although I don't remember now if the very early one in this thread I did for Mike had locked phasing however I don't think that was needed or even a requirement from Mike.Anyways, riveting discussion that we can all learn something from.

author: mawyatt
author_rank: Super Contributor
timestamp: June 15, 2022, 01:39:11 am
content: Think the issue boils down to whether the various contributors of Two Tone IMD in an DSO & signal sources can be determined by the RSS method which assumes incoherent signals, or not, which assumes coherent signals. This is where the 3dB difference arrises I believe.Anyway, as long as the methods for DSO Two Tone IMD evaluations are the same, then the results can at least be compared wrt each other, and in either case the actual 2 Tone IMD of the new SDS2000X HD is impressive as has been shown.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 15, 2022, 01:53:33 am
content: Quote from: tautech on June 15, 2022, 01:21:40 amDon't recall details either, was this plot from the SDG2042X AWG? Recall something about hearing AWG relays click when the wave combine function is active and brings up the question as to how and where this waveform combining takes place?Anyway, speaks highly of the SDS2000X HD. Do you know if the front end section of the SDS2000X HD & ADC are the same as the SDS6000 series 12bit ADC version sold only in China, or the one for LeCroy?Agree, very interesting discussions!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 15, 2022, 02:09:13 am
content: Quote from: 2N3055 on June 14, 2022, 10:30:29 pmDon't think you are missing anything, spot on to what reviving this thread is about!!Best,

author: tautech
author_rank: Super Contributor
timestamp: June 15, 2022, 02:12:58 am
content: Quote from: mawyatt on June 15, 2022, 01:53:33 amYes, SDG2042X it was and don't you too have one now ?QuoteHave no idea Mike, maybe 2N3055 or Performa01 can help you with this one....that's if their NDA lets them.I suppose I should do a 2 tone with my SDS6204A however with what I have here 200 MHz is the current limit of toys available.

author: mawyatt
author_rank: Super Contributor
timestamp: June 15, 2022, 02:26:50 am
content: Quote from: tautech on June 15, 2022, 02:12:58 amYes, have a SDG6022X also (needed the higher frequency waveforms).QuoteWould be interesting to see how good this SDS6204A is, I'm sure others are interested as well.Best,

author: rf-loop
author_rank: Super Contributor
timestamp: June 15, 2022, 08:25:43 am
content: Quote from: tautech on June 15, 2022, 02:12:58 amdue to NDA and so on...But why even need for answer this just anything more than read from public data sheets and rest can know if think even bit. Answer to this question is obvious.SDS6204 H12 Pro and SDS6204 H12 Pro what is up to 2GHz/1GHz front end vs SDS2000X HD 500MHz front end... no, they are not equal aka same.And even more, then in question there was "& ADC". How it can even think if they are same.Example 6k H12 Pro models have 5GSa/s 12bit ADC's2GHz models have one for every channel. 1GHz models and below have shared ADC's, one for two cannel so that these have 5GSa / 2,5GSa/sThen very different animal...2kXHD have two 2GSa/s 12bit ADC's shared one for two channels so 2GSa / 1GSa/s.No need any deep knowledge. It can read just from data sheet.And these SDS2kXHD ADC's are "perhaps" normal industrial models what who ever can buy from several different vendors.No need Sherlock Holmes to quess what they are.Naturally front ends are more or less close relatives. No one is so stupid that start every time from empty board. Development and design of something new is a continuous and cumulative, but also self-correcting learning process

author: G0HZU
author_rank: Super Contributor
timestamp: June 15, 2022, 08:30:16 am
content: If it helps I have an early/old Tek spectrum analyser here that has a digital IF. When used down at below 40MHz it is effectively a 40MHz baseband scope sampling at 102.4MHz but the display is trapped in the frequency domain. This is not a swept analyser, it samples the input and generates an FFT just like a scope.If I inject two clean tones I can generate IMD in this analyser at about -83dBc if I drive it towards FSD of the ADC. If I then deliberately sum in a 'pre-distortion' tone at the source input at -83dBc at one of the IMD3 frequencies and then rotate the phase of this the analyser shows the expected behaviour where the displayed IMD3 term rises 6dB to -77dBc when the IMD3 terms are in phase and the IMD3 term vanishes on the FFT display when the phases cancel. This shows that the equal IMD of the source can cause summing at up to 6dB on the display and it can also cause cancelling although significant cancelling is highly unlikely to happen as I'd expect the source IMD terms to normally be in phase.

author: Performa01
author_rank: Super Contributor
timestamp: June 15, 2022, 08:54:28 am
content: Everything said by our RF-guru G0HZU should be taken seriously. Up to now, he mainly talked about the (close to) +6 dB boost for the distortion products in case of two clearly (and inevitably) corelated distortion sources,but of course this assumes zero phase shift, hence zero runtime – or at least test signals whose wavelengths are long compared to the physical dimensions of the test setup. Once the distance between distortion sources happens to be half the wavelength, we get phase inversion and instead of a boost, the distortion components will cancel out.EDIT: Doh! Nobody notified me of my mistake. Of course the runtime is almost irrelevant, since the distortion products in question are almost the same frequency as the test signals!In my test, the frequency was 450 MHz and half its wavelength is about 33 cm.While it is highly unlikely that such massive distances will occur between potential distortion sources within the DSO frontend, it would be perfectly possible that an odd multiple of this distance exists (as electrical length) between the external generator and any non-linear stage within the DSO. As a consequence, there is indeed a chance of cancelling out distortion products in this scenario.My claim that I cannot guarantee the IMD performance of the test tones to be better than -74 dBc came from the fact, that I’ve measured this with my SA. It’s pure coincidence that the measurement result with the DSO was about the same. Since I don’t happen to own a R&S FSEA30 boat anchor with 110 dB third order dynamic range, I could not know whether I hit the limit of the generator or the analyzer. With two 10 dB pads for the external power combiner, hence a total isolation of 26 dB, any IMD products should be killed reliably. Now I could indeed measure an IMD of at least -85 dBc and can be confident that my source will be vastly better than this, when it already measured as -74 dBc IMD with only 6 dB isolation.With the 10 dB pads in place, I cannot maintain a 0 dBm signal anymore. Even though I could have used about -3 dBm, I voted for repeating the test with a signal level of -10 dBm. This totally changes the scope settings and its internal nonlinear transfer curves.Originally, I had 0 dBm signals at 500 mV/div. This means that there is an internal 20 dB attenuator active, and the PGA (Programmable Gain Amplifier) might have to deliver about 6 dB of gain. For an equivalent setting at -10 dBm, I would have to set the channel gain to 160 mV/div. With this, the attenuator is still in place, but the PGA now is at 16 dB gain, which might cause more distortion. In general, I think that the PGA might be the main source of nonlinearity in a wide bandwidth scope frontend, whereas the unity gain input buffer as well as the ADC should be rather benign in this regard.Look at the attached screenshot for -10 dBm at 160 mV/div. The IMD isn’t quite as good, but it still happens to be another sweet spot:SDS2504X HD_IMD_160mV_C450MHz_O200kHz_-10dBmWe can also try to replicate our former sweet spot without the internal 20 dB attenuator. For this, we need a -20 dBm test signal at 50 mV/div, see third screenshot:SDS2504X HD_IMD_50mV_C450MHz_O200kHz_-20dBmWith this setup, we can be confident that the generator signal is way better than that, so we need not speculate where the distortion comes from.

author: tautech
author_rank: Super Contributor
timestamp: June 15, 2022, 09:15:32 am
content: Quote from: mawyatt on June 15, 2022, 02:26:50 amOkay so I had a little play, SDS6204A running 500 Mpts fixed Mem depth with 189 & 190 MHz 0dB tones from SDG6022X.All except the last were at 4 Mpts and last was at 8 Mpts FFT with some averaging.Ref levels +5dB and -40dB.....most of which is visible in menus or OSD.

author: mawyatt
author_rank: Super Contributor
timestamp: June 15, 2022, 01:06:02 pm
content: Quote from: G0HZU on June 15, 2022, 08:30:16 amThink your test show we can't use the RSS method to evaluate/estimate/separate DSO sources of IMD as it can potentially introduce a 3dB optimistic result because of correlation. Thanks for taking the effort for this testing.Best,

author: G0HZU
author_rank: Super Contributor
timestamp: June 15, 2022, 02:25:08 pm
content: It's always good to revisit stuff like this. As you can tell my experience is really with spectrum analysers, either old school swept types or modern versions that can do FFT or both swept and FFT. So I'm used to using the classic equations for stuff like this.I've never had much joy using scopes to measure IMD but then again I've only ever tried this using fairly old scopes and the results were usually quite grim. Some of the screenshots of the modern scopes on this thread look to be very impressive in terms of what the FFT mode can display.I'm not sure where the significant IMD gets generated in modern scopes. In a typical scope there is usually an impedance converter stage followed by a selectable preamp and then on to the ADC. I think some scopes also split the signal path into low frequency and high frequency paths so it's difficult to know how much the front end affects the IMD generated by the scope.

author: 2N3055
author_rank: Super Contributor
timestamp: June 15, 2022, 03:39:59 pm
content: Quote from: G0HZU on June 15, 2022, 02:25:08 pmScopes definitely have DC-low frequency/high frequency dual path, you're right about that.And Mike's intention wasn't to really use (though you could, within limits, of course) scope for measuring IMD of devices.Two tone technique is routinely used to test linearity of ADCs (that is something Mike knows "a bit" about), and he really wanted to test how linear is front end and ADC in new Siglent scopes. This has been ongoing discussion. Results show that they are, in fact, very good...Thank you for your comments.. You reallyareRF guru like Performa said, and your contributions are very valued. Even if we get lost a bit sometimes..Best,

author: David Hess
author_rank: Super Contributor
timestamp: June 15, 2022, 06:41:58 pm
content: Quote from: G0HZU on June 14, 2022, 09:39:32 pmThe impedance buffer and first stage contribute most of the distortion because they have to operate over the widest range. There are circuit techniques like cascodes and bootstrapping to improve this considerably but they increase noise and limit bandwidth. Integrated front ends have the additional problem of more parasitic elements and coupling; see below about dual integrated JFETs.Quote from: G0HZU on June 15, 2022, 02:25:08 pmThe impedance converter is what limits performance. One reason DSOs for a long time were limited to 8-bits, and largely still are, is that the demands on the analog section limit performance to that level or lower anyway without a heroic design, and maybe even then. Settling time tests of 12-bit DSOs show that they do *not* have 12-bit performance in all respects. Usually they are not even close, but 12-bit operation is useful in other respects like noise, dynamic range, and marketing. The performance of some modern DSOs operating as a spectrum analyzer is amazing except for low frequency noise.One thing modern DSOs can do, and have done for quite some time at the high end, is apply digital correction but this brings up calibration issues.Quote from: 2N3055 on June 15, 2022, 03:39:59 pmAll budget oscilloscopes have used a two path design going back to at least the first integrated JFET operational amplifiers. It removes the expensive requirement for grading JFETs (1) to get good DC performance and later had the advantage of providing AC/DC coupling without a relay. Some modern instruments might still use a single path design to support faster overload recovery.(1) An integrated dual matched JFET cannot be used because of cross-coupling.

author: Performa01
author_rank: Super Contributor
timestamp: June 16, 2022, 04:32:32 am
content: Quote from: David Hess on June 15, 2022, 06:41:58 pmWell, at least in case of the Siglent frontends discussed here (which have of course been designed with the assistance from LeCroy), all my tests strongly indicate that the input buffer / impedance converter is not the limiting factor. At least not the HF path, which becomes effective above a few kHz...It is the PGA that is following the buffer stage. And how could we be surprised? Itisa challenge to have a fully integrated variable gain amplifier with up to >2 GHz bandwidth (in case of the SDS6000), programmable in e.g. 2 dB gain steps over a range of 40 dB.Like you said, true 12-bit performance in a wideband general purpose oscilloscope isn't going to happen anytime soon. But as I demonstrated, there can be sweet spots (read: PGA settings), where the distortion is at its minimum. Alter the gain by just one step and the distortion might rise by several dB immediately.EDIT: look at the attached datasheet of a typical 900 MHz PGA like it my have been used for the current SDS2000X series. Look specifically at the distortion figures published on page 6. This proves that we actually need to look for sweet spots whenever we want a 12-bit like distortion performance near -70 dBc.

author: Performa01
author_rank: Super Contributor
timestamp: June 16, 2022, 06:49:05 am
content: Quote from: G0HZU on June 15, 2022, 08:30:16 amThis performance got me curious. Of course, general purpose oscilloscopes cannot compete with dedicated higher end spectrum analyzers, even if they’re old. Yet I wanted to know how close a SDS2000X HD can get.We want “near full scale” signals now, so the signals are 10 MHz with 20 kHz frequency offset and -10 dBm = 70.71 mVrms = 200 mVpp each, which results in a total amplitude of 400 mVpp.First test at 50 mV/div. Visible screen range is exactly 400 mVpp. Yet the DSO already indicates possible overrange in its automatic measurements: >100.080001mV(↨)See 1stscreenshot.SDS2504X HD_IMD_10MHz_-10dBm_50mVIMD is about -70 dBc, so it is still worthy of a 12-bit oscilloscope. Even though it cannot touch this particular Tek instrument, there are quite some SA out there which would struggle to achieve a result like this at an input level as high as this.The point is, that the SA is measured way below the 1 dB compression point of its input amplifier and/or mixer, whereas the SDS2000X HD has the maximum input at 100 mV/div (above that, the first attenuator kicks in). Consequently, 50 mV/div is just 6 dB below the designed “full scale” of the input buffer and PGA input. If we look at the datasheet I’ve attached in my previous post, we can see that the maximum permissible input of the typical PGA is 1V. Consequently, 400 mVpp (after the unity gain buffer) is already close to the permissible input voltage of that stage – certainly not 30 dB below.Let’s try to get rid of the overrange warning and lower the input gain to 55 mV/div. Now we’ve found a sweet spot at IMD -77 dBc, which is not that far off when compared to the -83 dBc of the dedicated Tek SA.See 2ndattached screenshot:SDS2504X HD_IMD_10MHz_-10dBm_55mV

author: RoGeorge
author_rank: Super Contributor
timestamp: June 16, 2022, 07:24:55 pm
content: Quote from: G0HZU on June 15, 2022, 02:25:08 pmDistortions can only appear when a circuit is not linear. A perfectly linear circuit (an ideal LTI - Linear Time Invariant system) can not have any distortions. In frequency domain, an ideal LTI system will show at its output exactly the same spectrum as it was fed at its input. The output can only differ in amplitude (the ideal LTI system can amplify or attenuate, but it can not produce new frequencies). Or else said, an ideal linear system won't have any intermodulation products.In this topic, intermodulation is only a technique that can evidentiate very small non-linearities that might happen inside the oscilloscope, distortions so small that otherwise would be close to impossible to notice by just looking at the waveform in the time domain.Intermodulation products get generated anywhere there is a non-linearity. In the case of an oscilloscope for example, the frontend amplifier is not ideal, it will have some small nonlinearity, therefore some intermodulation products will appear. Later, the signal get sampled by the ADC then displayed as a spectrum, where we notice the intermodualtion products. Or, it can be some other components than the input amplifier, where the signal is distorted, even the ADC itself will add some distortions. It's a test for the whole oscilloscope (assuming the input signal is perfectly clean, with only 2 frequencies).The TL;DR is, new spectral components will appear anywhere there is a non-linear response. Non LTI system means the circuit will distort the signal, and we will notice that as intermodulation. No intermodulation can appear in an ideal, linear amplifier.I like this video about "Linear and Non-linear Circuits" from "The Signal Path" channel because it tells about intermodulation both in theory and in practice, with experimental measurements to build an intuition:

author: G0HZU
author_rank: Super Contributor
timestamp: June 16, 2022, 07:30:09 pm
content: That's very impressive from the Siglent scope. Below 40MHz the Tek analyser runs the input direct to the ADC via an attenuator and presumably a preamp when needed. If I were to try for sweet spot drive levels I think I could get better than -90dBc IMD but I'm not sure I'd trust what is displayed. The main niggle with the Tek analyser is that it has a few low level internal spurious signals even with no input so it has to be used with some care and sympathy for its limitations.I had a go at retesting the single/dual ERA1 MMIC amplifiers this evening and this time I did it quite formally by measuring each one for gain with a VNA and also I checked they were the same for IMD. I also used a precision 0.1dB step attenuator to set up the correct attenuation between the two amplifiers. See the images below. On the VNA the small signal gain was about 12.3dB for both amplifiers and the IMD performance seemed to be identical. The spectrum analyser plot below shows the IMD for both ERA1 amplifiers when tested standalone and trace 3 is when they are in series with the 12.3dB attenuator in between them. The marker shows the IMD change was very close to 6.0dB when the two amps were in series.I've used a decent Agilent lab VNA for the gain tests and I set the drive level correctly to capture the correct small signal gain for the IMD tests. I also used the VNA to set the step attenuator within 0.1dB of 12.3dB loss.So I think this is a good experiment and demonstration of the equations provided by Agilent and R&S. It shows the expected 6dB increase in IMD.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 19, 2022, 01:58:09 am
content: This thread got me curious. To see where I'm at, I first tried two ways to combine the outputs of two signal generators (a R&S SML and a SMU200A, both with ALC switched to sample-and-hold to avoid any interference between them): A Mini-Circuits 3 dB combiner/splitter, model ZFSC-2-6-N+, with a 10 dB pad at each input, and a 6 dB resistive combiner, also with 10 dB pads at the inputs. I tested at 10 MHz with a frequency difference of 1 kHz, and with 10 dBm input power for the 3 dB combiner, and 13 dBm for the resistive combiner.With the 3 dB combiner, I measure the 3rd order products at 75 dBc, and with the resistive combiner, they are more than 10 dB higher on a high dynamic range spectrum analyzer (R&S FSIQ 26).I'm pretty confident that in the case of the 3 dB combiner, the 3rd order products are for the most part not generated by the analyzer, because I choose the input attenuator setting high enough that they are no longer moving when I vary it. Here the 1 dB attenuator option comes in handy.I haven't investigated this any further, but I suspect that the higher IMD with the resistive combiner is due to the lack of isolation (only the 10 dB pad plus the 6 dB insertion loss of the combiner) between the two signal generators, and the IMD is actually produced in the generators.Then I tried two tones 20 kHz apart at 1 MHz on a R&S RTM 2054 oscilloscope. That is an older model with an 8 bit ADC. Also, its built-in FFT is a bit compromised, so that for more comprehensive tests I would have to pull the samples on a PC and do the FFT there. However, at that frequency with 100 mV/div I measure the 3rd order products at about -60 dBc (see attachment).I also verified with the analyzer that they are at least 80 dB down at the scope input.

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2022, 04:18:59 am
content: How would this measurement be made while avoiding the limits of the analyzer? Notch out the fundamentals?

author: RoGeorge
author_rank: Super Contributor
timestamp: June 19, 2022, 07:20:45 am
content: The two tone testing is particularly useful when the bandwidth is limited, because this technique is shifting the harmonics close to the two main tones, so their amplitude is preserved. Otherwise (with a single tone) the harmonics will fall out of the DUT's band and will be attenuated by the DUT's frequency response, appearing much smaller than they really are, or not visible at all.In the case of a 10MHz test signal a combiner and an SA, there is no bandwidth limitation (I expect the passive combiner to work up to many hundreds of MHz, same for the SA) so the SA can observe the harmonics of a single tone directly, at 2f, 3f, etc. without the need of a second frequency.It might worth measuring the harmonics of each tone alone, one by one (observed at 20, 30, 40MHz, etc), with the second generator disconnected, in the hope that this measurement will show only the purity of each signal, and will exclude any intermodulation it might happen in a second generator.An alternate way to know if the intermodulation is produced by the outputs of the generators (acting as mixers) might be to add a directional coupler between each generator and the combiner, so to reduce any signal from outside that is trying to travel into the generator.In theory, it should work. In practice, I have close to zero experience or equipment to try this.

author: G0HZU
author_rank: Super Contributor
timestamp: June 19, 2022, 11:47:58 am
content: Usually the way to get good results is to choose a combiner with high isolation and to try and maintain that isolation through having a good port match at the sum port of the combiner. Also, try and use a lowpass filter on the sig gen outputs. Try and use a spacing frequency that is higher than the ALC bandwidth of the sig gens as this can minimise crosstalk effects between the sig gens.Another thing to try is to use the attenuator lock/hold feature on the sig gen (if it has this feature) as this can keep some internal attenuation inline when trying for higher output levels from each sig gen.I have lots of RF combiners here and for use below 200MHz my favourite one has 6dB through loss but over 48dB isolation from LF through to about 200MHz. If I have to use a resistive combiner for wideband stuff I have one here that has about 9.5dB through loss and just under 20dB port isolation.Don't overlook that fact that a typical microwave spectrum analyser will typically have compromised distortion performance below about 20-50MHz. This is because of the image and LO breakthrough that will degrade the distortion performance of the analyser below about 20-50MHz. So the analyser might become the weakest link unless care is taken with the attenuator settings within the analyser.It should be possible to get >100dB SFDR using the above techniques if care is taken with the setup.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 19, 2022, 04:30:38 pm
content: Quote from: G0HZU on June 19, 2022, 11:47:58 amThe datasheet of the ZFSC-2-6+ I was using states about 30 dB isolation at 10 MHz typical. Plus there was a 10 dB pad at each input, so I would expect about 50 dB of isolation. No idea if that is enough. I haven't gotten the VNA out to actually measure it though. The ALC was turned off on both generators, so that it would not cause any trouble.Quote from: G0HZU on June 19, 2022, 11:47:58 amTrue, but I have set the attenuator of the analyzer high enough that the IMD products would not move with a variation of the attenuator. Therefore I suppose that what I'm seeing is not generated within the analyzer.Quote from: G0HZU on June 19, 2022, 11:47:58 amI wonder if the IMD products are generated in the splitter. It is a core and wire model, rated from 2 kHz to 60 MHz, and the core might introduce some distortion. I should check that by varying the drive level at the inputs.Quote from: David Hess on June 19, 2022, 04:18:59 amIf you can make a filter that is steep enough to notch them out without disturbing the 3rd order products.Quote from: RoGeorge on June 19, 2022, 07:20:45 amThis is true, at least in theory. If you model the output voltage of the two-port DUT as a nonlinear function and expand it as a Taylor series, i.e.,$$v_{\rm out}(v_{\rm in})=\sum_{k=0}^\infty a_kv_{\rm in}^k,$$and consider it up to order 3, then in a single tone excitation the products at \(3f_0\) are proportional to \(a_3\), and the same is true for the third order products at \(2f_1-f_2\) and \(2f_2-f_1\) in a two-tone test. So the harmonics contain information about IMD products. However, this assumes that the Taylor series is an accurate model, and there are no frequency dependent effects. In practice, I suppose there is no way around a two-tone measurement.Quote from: RoGeorge on June 19, 2022, 07:20:45 amTrue, but I guess that a highly isolating combiner is more promising.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 19, 2022, 06:25:30 pm
content: Okay, I had another go at it.Quote from: rf-messkopf on June 19, 2022, 04:30:38 pmI measured the isolation of the ZFSC-2-6+, see the attachment. I get about 33 dB at 10 MHz with the sum port terminated in a high quality load (the orange Trc3 trace), and about -18 dB isolation with 12 dB return loss at the sum port. provided by a 6 dB attenuator with one port left open (the blue Mem4 trace). So I can confirm the 50 dB of isolation with the two 10 dB pads.I also measured the IMD again, with two 10 dB pads at the inputs of the ZFSC-2-6+, plus a 6 dB pad at the sum port to improve termination. The generators were set to a level of 15 dBm, and the RF attenuation of the analyzer was set to 40 dB. Again, I observe no dependence of level of the 3rd order products when I very the attenuator around 40 dB.I also tried two analyzers, a R&S FSIQ26 (20 Hz to 26.5 GHz) and a FSEA30 (20 Hz to 3.5 GHz), with consistent results, see the attachments.I'm still not sure though if the 3rd order products might to some extent be generated at the IF stage of the analyzer, or if it is all due to the splitter. When I set the FSIQ26 to FFT mode, I get the same results (only much faster, with 400 ms sweep time).

author: G0HZU
author_rank: Super Contributor
timestamp: June 19, 2022, 08:03:23 pm
content: I can only really offer general advice to the forum on stuff like this as I've not used the R&S gear you have there. Trying for a narrow tone spacing like 1kHz is quite challenging if you want to try for (say) -80dBc IMD. This is well inside the normal ALC bandwidth of a sig gen so this means the sig gens will be very prone to crosstalk issues. One option is to turn off the ALC or to select a lower ALC bandwidth. It looks like your sig gen can change the ALC in other ways as well.I would try and avoid doing IMD tests at such a narrow tone spacing but if it has to be done then I'd recommend a high isolation combiner, try using the attenuator hold option in the sig gen and turn down the ALC bandwidth if the sig gen permits. The attenuator hold option can typically introduce an extra 10dB isolation per signal generator if used correctly.See below for a plot from my old Tek analyser. This analyser doesn't sweep the LO and the plot below is taken using an FFT. To get about -90dBc IMD the tone levels have to be 10dB below the reference level on the display. This is why the tones are so low compared to the reference level. I don't really trust this analyser when exploring IMD levels as low as this. I'd normally use something else.I've added a second plot below where I've turned up the sig gens such that I test with two tones at 0dBm each into the analyser. This gives similar results.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 19, 2022, 09:47:25 pm
content: Quote from: G0HZU on June 19, 2022, 08:03:23 pmI used the sample-and-hold function of the ALC, i.e., it measures the output level once, adjusts, and then holds that level, with the control loop turned off.Quote from: G0HZU on June 19, 2022, 08:03:23 pmOne option might be to use a 4-way splitter, because it has higher isolation between opposite ports than adjacent ones.With this splitterone could achieve more than 50 dB of isolation between opposite ports at 10 MHz, which could be further increased with attenuators. Unfortunately I don't have a suitable 4-way splitter here. And most datasheets don't quote the IMD performance of splitters. I've only seen that for high power splitters intended for cellular radio applications.Quote from: G0HZU on June 19, 2022, 08:03:23 pmThanks for running it. I also didn't see much difference with different drive levels.But I don't mean to hijack the thread with this discussion. Maybe the additional data point for the R&S RTM2054 is of some interest.

author: Performa01
author_rank: Super Contributor
timestamp: June 20, 2022, 02:59:27 am
content: I’ve never tried to use RF-signal generators for dual tone tests, even though I do have two of them available. A single modern dual channel AWG is just so much handier for this and up to now I’ve not had any desire for such tests at frequencies higher than 500 MHz.I would expect a R&S FSEA30 measurement to provide much better results than what has been shown here. Since I’ve never experienced any problems and always assumed that the analyzer is the weakest spot in tests like this, it becomes obvious that the output stages of a modern AWG must be much less prone to intermodulation distortion from injected signals. Most likely there is no ALC either, even though the accuracy of the output levels can easily compete with the best RF-signal generators, like a specified accuracy of 1 % and +/-0.3 dB flatness over the entire bandwidth.As usual, actual performance is much better than this and there are sweet spots, like 100 kHz - 500 MHz +0.06 / -0.04 dB absolute error at -30 dBm. See this old reply #162 in the thread linked below, showing the response of my (original) SDG6052X and comparing it to an older high performance signal generator.https://www.eevblog.com/forum/testgear/siglent-sdg6000-series-awg_s/msg2621457/#msg2621457Long story short, the signal quality, stability and accuracy can easily compete – and the insensitive (to intermodulation from outside) output stages are a big plus.The first attached screenshot shows the IMD at 450 MHz with a resistive wideband power combiner (DC – 12.4 GHz) and two additional 10 dB inline attenuators at its inputs, for a total of 26 dB of isolation.Ref_450MHz_O200kHz_-10dBm_Iso20dBWe get better than -85 dBc IMD – and mind you, this is not an SA with 110 dB third order dynamic range like the R&S FSEA30. Base line is, that I expect the actual signal to be much better than this. Even without additional isolation, i.e. just the 6 dB of the power combiner alone, I could measure -74 dBc.I can verify that the signal is better than this with another instrument, but only up to 5 MHz:Pico4262_IMD_4MHz_O100kHz_0dBmAt 4 MHz I can easily demonstrate the IMD to be better than -97.5 dBc – and once again I hit the limits of the analyzer, not the test signal.The whole point of this posting is to demonstrate, that with the right tools the quality of the test signal needs not be an issue at all.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 20, 2022, 08:23:04 am
content: Quote from: Performa01 on June 20, 2022, 02:59:27 amOkay, but you are testing at 200 kHz and 100 kHz spacing. I was testing at 1 kHz. That's a bit more challenging.But I'll re-run my measurement at >100 kHz and see what I get.I guess the main problem I'm having with the IMD level is due to the lack of isolation (about 50 dB in my case) between the signal generators. A better splitter (e.g. a 4-way one) should make an improvement. Still not 100% sure but I don't think that the analyzer is the weakest link in my setup.

author: Performa01
author_rank: Super Contributor
timestamp: June 20, 2022, 09:27:40 am
content: Quote from: rf-messkopf on June 20, 2022, 08:23:04 amWell, if you think the frequency spacing is supposed to change the distortion characteristics of a nonlinear device - and that's about the only thing we're interested in – then please find attached the equivalent measurements at 1 kHz spacing.First attachment is the measurement at 10 MHz with the SA, which should be comparable to yours.Ref_10MHz_O1kHz_-30dBm_Iso20dBThanks to your complaint! Because of this I finally found out (after many years of usage) that my SA performs much better at narrow spans – probably because it uses FFT exclusively instead of swept mode then. Consequently, I now can finally measure an IMD of -92 dBc even at high frequencies!To get a truly comparable result with my earlier measurements, the second attachment shows the 4 MHz test again:Pico4262_IMD_4MHz_O1kHz_0dBmNothing changed. It is still -97 dBc, hence right at the edge of what this 16-bit DSO is specified for.I know, there are suspicions that it might have something to do with the ALC in the levelled signal generators, but I was talking about an AWG (which apparently doesn’t have nor need that). And then, if I wanted an appropriate test signal to measure distortion products of a DSO, then I would certainly stay away from conditions (like narrow frequency spacing) that might challenge the signal generators and prevent me from getting a clean dual tone test signal.Yes, your FSEA30 certainly is not a weak link, yet it should be easy to fully exploit its phenomenal intermodulation free dynamic range with proper signal sources and distortion-free power combiners. I wish I had one here to demonstrate just that…

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 20, 2022, 10:07:56 am
content: Quote from: Performa01 on June 20, 2022, 09:27:40 amThe frequency spacing would certainly not change the characteristics of the DUT, but it gets harder to measure levels of 3rd order product reliably at narrow spacing.Anyway, thanks for testing at 1 kHz.Quote from: Performa01 on June 20, 2022, 09:27:40 amNot all signal generators have an ALC. Some of the old Marconi ones always run open loop, I think the 2024 is an example. I'll repeat the test with two function generators and see if that makes any difference.And it does not necessarily have something to do with the ALC (which was disabled anyway), but the IMD seen can also be due to nonlinearities in the generator output stage. Also, the two signal generators (R&S SML and SMU200A) both have electronic attenuators. Maybe that has an influence as well.Quote from: Performa01 on June 20, 2022, 09:27:40 amNot so easy with the stuff I have in the drawer it seems.

author: Performa01
author_rank: Super Contributor
timestamp: June 20, 2022, 10:56:45 am
content: Quote from: rf-messkopf on June 20, 2022, 10:07:56 amI only had (and still have them) 3 RF signal generators in my whole life. So my practical experience with these is limited. Since none of them appears to have any settings for the ALC, I cannot know whether they have it or not. But at least my first generator, a Wavetek 3000, settles its output amplitude in a way that it's quite obvious that there must be some ill-designed control loop for itWithout the ALC, it's hard to see where there is a difference. If we ignore modulation, there should not be any substantial differences between the output stages of a signal generator and an AWG. We have an output amplifier, which is of course prone to intermodulation when fed with external signals and a step attenuator (which usually covers a much wider range in the signal generator, but nothing that cannot be rectified by using an external device for additional attenuation).My Siglent AWGs are somewhat old school because we hear several relays click when dialing the output amplitude within its range of -56 dBm and +25.5 dBm (at frequencies up to 40 MHz). Yet I suspect that there will be some additional electronic attenuator to manage the fine steps in between. Since this is just a switch, i.e. a negligible serial resistance within the signal path, I do hope that it won’t cause any significant distortion.BTW, for all these recent measurements, I've not used the SDG6052X where I demonstrated the amplitude accuracy, but the new SDG7102A, which covers the frequency range up to 1 GHz and can still maintain +13 dBm at that frequency. Normally irrelevant, I want to make clear that I do not compare apples to bananas, by using a 20 MHz AWG, rather than something with a proper high power HF output stage. Of course, in this context it is totally irrelevant what’s been used, as long as it results in a distortion-free dual tone signal that's fit for the purpose.

author: G0HZU
author_rank: Super Contributor
timestamp: June 20, 2022, 11:32:24 am
content: If it helps, I've been doing critical IMD tests like this all my career. In my case I would normally testing an RF downconverter. I designed lots of RF downconverters from about 1990 onwards. These were for early versions of SDR receivers for gov/military use back then. The ADC/DSP back end was in a huge rack system in those days. Very different to today. The usual spec for IMD back then varied from -72dBc to -80dBc for the downconverter but obviously some margin was required beyond this. The aim of the overall system was to digitise the RF spectrum up to several GHz as quickly and cleanly as possible. Often a high performance spectrum analyser was used at the IF in place of the ADC/DSP during dev work.If IMD tests are done at narrow spacing then this can be inside the ALC bandwidth of the sig gen and this will degrade the performance unless extra isolation is included somewhere. Also, a conventional spectrum analyser will usually have degraded IMD performance below about 50MHz and also the IMD of the analyser can be worse on narrow spans as both test tones travel further together down the signal path of the analyser. In those days the top spectrum analysers for IMD performance were the HP 8568B, Advantest TR4172 and the Marconi 2382 analyser.It's fairly standard to have ALC in a classic RF lab sig gen because the ALC system is often also used for the AM modulation. This usually means the ALC bandwidth is at least 20kHz although some sig gens can turn the ALC bandwidth right down if AM isn't needed. I'm not aware of any mainstream lab RF sig gens that run open loop unless the ALC is deliberately disabled. I guess sig gens like this do exist but I've not used one unless you include really old or low cost RF signal generators or RF capable function generators.I've got quite a few RF sig gens here and I've used many different types at work over the years. With care it's possible to achieve -100dBc IMD at the RF combiner output. More is possible but it becomes much more challenging to prove it. When testing really high performance HF receivers the IMD3 SFDR can easily be >110dB so this requires a very good test setup indeed.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 20, 2022, 02:39:24 pm
content: Quote from: rf-messkopf on June 20, 2022, 10:07:56 amCorrection: From the block diagram, the Marconi 2024 does have an ALC. But it cannot be switched off, I think, I've never seen such an option in the menu of mine.Quote from: Performa01 on June 20, 2022, 10:56:45 amUsually, rf signal signal generators have a class A output stage, whereas the classical function generator has a class AB complementary output stage with a separate DC path. No idea how the output amplifier in one of these modern x GSa/S arbitrary function generators looks like. The electronic attenuator/PGA for fine control of the output level usually is further up in the signal path, before the output amplifier, and likely will not see much of an externally applied signal.Quote from: G0HZU on June 20, 2022, 11:32:24 amRight. The 115 dB intermodulation free dynamic range of the FSEA30 is specified for inputs from 50 MHz onwards. But as I said above, I would expect that the intermodulation products I have seen would depend on the attenuator setting if they were generated in the first mixer. And if they were generated further down the signal path at an IF, then they would also be a problem for input frequencies above 50 MHz. Perhaps this could be checked by varying the reference level.Anyway, some further tests are necessary. Currently I suspect the signal generators and/or the splitter.

author: mawyatt
author_rank: Super Contributor
timestamp: June 20, 2022, 03:28:58 pm
content: Quote from: rf-messkopf on June 20, 2022, 02:39:24 pmInteresting discussions!!The AWG output amps are usually wide-band higher performance op-amp types with very little introduced non-linearity. TI and AD have created op-amps especially for this type of use.The mentioned "relay clicking" when using built-in two tone signals from some of the modern AWGs (For example Siglent's SDG2042X and 6022X) hint that this is combining the separate tones from each channel signal path in the analog domain and not the digital domain, which may provide better results than a simple digital combine.An interesting IMD test might employ a couple wide-band quality linear Power Amps buffering each individual tone source before combining into a single two tone signal. Since the required two tone signal is low relative to the linear PA output capability, maybe this PA wouldn't introduce much additional non-linearity and it's output wouldn't be effected as much by the "other" signal, thus reduced IMD. We've utilized high power wide-band low noise linear PA types as receiver front ends for high input dynamic range in "special" type receiver applications, and might prove useful here.Best,

author: 2N3055
author_rank: Super Contributor
timestamp: June 20, 2022, 04:06:17 pm
content: Quote from: mawyatt on June 20, 2022, 03:28:58 pmMike,SDG series from Siglent definitelly combine in digital domain...Relay clicking is from mundane reason. If amplitude of single channel is right there on the edge where it would go between output ranges, once you combine amplitudes it will be large enough for output relays to "switch to higher gear"..Best,

author: mawyatt
author_rank: Super Contributor
timestamp: June 20, 2022, 04:12:39 pm
content: Quote from: 2N3055 on June 20, 2022, 04:06:17 pmThanks, that's why I said "hint" since didn't know for sure. Suspect Siglent had done the proper evaluation for the Digital vs. Analog combining of signals from each channel and weighted the pros and cons. Starting with 16 bit DACs likely helps as well, which gives good resolution.Best,

author: Performa01
author_rank: Super Contributor
timestamp: June 20, 2022, 04:33:59 pm
content: I love challenges.Quite obviously, we need a trustworthy low distortion dual tone signal in order to perform meaningful IMD tests. To generate this seems to be a challenge in some cases already – all the more so to confirm it without a high-dynamic range SA like the R&S FSEA30.In my case it was a matter of finding the sweet spot on the existing average SA. By this I mean a combination of input signal level, input attenuation and IF-gain that adds as little distortion as possible.My tests didn’t reveal any hints where the distortion actually comes from. Changing the input level, by internal and/or external attenuation didn’t show the expected behavior. If 1 dB lower drive level doesn’t make the 3rd order IMD products drop by more than 1 dB (theoretically 3 dB), then the analyzer frontend shouldn’t be the source of the distortion. If on the other hand, additional attenuation at the generator outputs, i.e. additional isolation, doesn’t change anything, then the generator shouldn’t be the culprit either.That leaves the power combiner/splitter, which might introduce non-linearities even when it’s purely resistive – at least when we’re approaching distortion levels in the realm of -100 dBc. But my earlier tests got -92 dBc IMD, with a generator output of only +6 dBm and the isolation pads to the power combiner were 10 dB each, with a 20 dB attenuator at its output for a total of 36 dB of attenuation.The actual test setup is the dual channel AWG with +10 dBm output levels for each tone, i.e. 4 dB higher than before. These signals are connected to the power combiner via 20 dB inline attenuators, but the output of the resistive wideband combiner is now terminated by only a 10 dB inline attenuator. Consequently, there is 36 dB total attenuation again. The resultant -26 dBm signal is then fed into the SA.The attached screenshot shows what I have achieved when taking advantage on the sweet spot of the SA.Ref_10MHz_O1kHz_-26dBm_Iso40dBFinally I see distortion figures better than -100 dBc 😉

author: Performa01
author_rank: Super Contributor
timestamp: June 20, 2022, 04:46:34 pm
content: The question about the internal combiner in the Siglent AWGs has been answered in reply #238 here:https://www.eevblog.com/forum/testgear/how-much-noise-floor-and-other-things-matter-in-oscilloscope-usability/msg3906974/#msg3906974QuoteOf course, the -45 dBc are a worst-case figure and at lower frequencies and/or levels it performs a lot better than this, yet it cannot even come close to a proper external power combiner.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 20, 2022, 05:22:25 pm
content: Okay, I've re-run my measurement with the ZFSC-2-6-n+ splitter, but now with 20 dB pads at each input, plus a 6 dB pad at the sum port. That gives me 70 dB of isolation between the two signal generators. Using the same generators (R&S SML and SMU200A at 10 dBm each) I get exactly the same results as before, i.e., 3rd order products at about -75 dBc.I've also increased the reference level so that the fundamentals are now approx. 20 dB below in case there is intermodulation at an IF stage in the analyzer.Next I tested the same setup with two function generators (Keysight 33521B and an old HP 3325A). Again, exactly the same result. Except that the function generators spit out some spurs (I think it's mainly the 3325A) -- see attachment.I'm stating to believe that it is the darned splitter itself that is causing the IMD.

author: mawyatt
author_rank: Super Contributor
timestamp: June 20, 2022, 05:40:39 pm
content: Quote from: Performa01 on June 20, 2022, 04:46:34 pmYeah, forgot about that thread since it's 1/2 year old...thanks for pointing there!!My short-mid term memory is getting worse with age, but long term seems to holding up well.Need to get a much of those long term experiences documented before they disappear!!Not surprised the analog combine outperforms the digital combine in some cases. Siglent likely chose the easier digital path since the source of all these non-linearities are quite involved as has been shown.Anyway, very interesting discussions.Best,

author: G0HZU
author_rank: Super Contributor
timestamp: June 20, 2022, 06:46:41 pm
content: To explore the limits of a decent modern swept spectrum analyser (swept LO with a wideband digital IF) it's best to use wide tone spacing and then select a very narrow span to zoom in and just look at the IMD tone itself. This assumes the analyser has a suite of analogue pre-filters ahead of the ADC and this is generally the case for a high end spectrum analyser. These pre-BPFs will typically be 2 or 3 times wider than the digital RBW filter and the analyser should select the optimum pre-BPF automatically. The BPF protects the wideband ADC from having to cope with the main test tones if you set the span much narrower than the tone spacing.It should be possible to achieve the SFDR on the datasheet for the analyser using this method assuming the signal generators and the combiner are not limiting the performance. With a RBW of 1Hz some analysers should manage 115dB IMD3 SFDR. Older classic analysers like the HP 8568B use analogue RBW filters and this analyser can typically achieve just over 100dB IMD3 SFDR using the smallest RBW of 10Hz. This classic old analyser typically has a mixer IP3 of +13dBm and a DANL of -140dBm in a 10Hz RBW. So the IMD3 SFDR will typically be about 102dB.

author: RoGeorge
author_rank: Super Contributor
timestamp: June 20, 2022, 06:47:43 pm
content: Quote from: rf-messkopf on June 20, 2022, 05:22:25 pmMight be, especially if that splitter has some magnetics core, but I don't have enough practice to know what IMD levels to expect.From the datasheet it's not clear if that model is a resistive splitter. Since the specs for it specify a minimum frequency, that might be a hint that the splitter has some ferrite inside. Ferrites have hysteresis and saturation curves, they are not linear devices. Therefore, ferrites materials are expected to introduce distortions (and thus to produce IMD).Would be interesting to measure the intermodulations using a pure resistive splitter, or any other kind of splitter that doesn't have magnetics. I suspect there is close to no intermodulation produced in the output stage of the generators (because those stages are usually an amplifier with a very low output impedance in series with a 50 ohms resistor, therefore any backfed signal will appear much smaller to the output amplifier).

author: G0HZU
author_rank: Super Contributor
timestamp: June 20, 2022, 07:04:40 pm
content: I think what normally happens within an ALC based sig gen is that the tone from the other sig gen will cause a very small low frequency error term at the ALC detector diode. The ALC error amplifier within the sig gen will then amplitude modulate the sig gen to try to cancel the error term assuming it all happens within the loop bandwidth of the ALC system. The net result is the sig gen modulates itself to keep the ALC happy and this usually means you see what look to be classic IMD3 terms on the signal generator output.This is why it can be useful to use a sig gen that can turn the ALC bandwidth right down. If the tone spacing is much wider than the ALC loop bandwidth then the ALC system will be too slow to correct the error at the ALC detector diode. So it doesn't add any undesirable 'corrective' AM modulation to the output.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 20, 2022, 07:51:30 pm
content: So I've been playing around a bit. I've also tested a Mini-Circuits ZFSC-2-1 (5 MHz to 500 MHz) and an Anaren 10014-3 quadrature hybrid (500 MHz to 1 GHz) at various frequencies and tone spacing. It seems that at very narrow tone spacing like 1 kHz I'm indeed hitting a limitation of the analyzer. I currently have the ZFSC-2-1 hooked up and I'm getting 3rd order products lower than -97 dBc with 3 kHz spacing using the method described by G0HZU, i.e., zooming in on the 3rd order product. This is a bit challenging because sweeps tend to get slow, and one has to determine the right amount of input attenuation to keep the noise floor at reasonable levels without introducing IMD in the first mixer of the analyzer.Not entirely sure, but maybe with some more effort one could make better measurements also at 1 kHz spacing. I think this contribution summarizes it very well:Quote from: G0HZU on June 20, 2022, 06:46:41 pmFor the FSIQ the IMD performance is quoted for tones at –30 dBm with ∆f >5 x RBW or 10 kHz, whichever is greater, and 3rd order products at < -74 dBc (TOI 22 dBm typically). So it seems I'm asking for too much here when I try to measure everything in one sweep.Edit:After thinking a bit and reading some application from the manufacturers, the excessive IMD I have seen was likely due to the ADC being used to implement the digital RBW filters. Quoting fromthis application note:QuoteThis is the reason why the IMD products did not depend on the reference level setting. Also, it is good to keep in mind that the ADC is not included in the TOI specification. Easy to forget when you sit in front of the instrument.The FSIQ and FSEA only use digital RBW filters for RBW settings < 1 kHz; the larger RBW filters including 1 kHz are analog (LC and crystal). When digital filters are used, the 3rd IF at 21.4 MHz is downconverted to 25 kHz and the sampled by an 18 bit ADC at a rate of 200kHz, with the analog filter section still in the signal path. Unfortunately I cannot find anywhere in the manuals which analog filter is used with which RBW or span setting. But since there is an 1 kHz crystal filter, there is a chance that it is switched in front of the ADC with narrow spans or RBWs. So it may be possible to measure 3rd order products with these instruments even when the two tones are closely spaced./EditQuote from: RoGeorge on June 20, 2022, 06:47:43 pmIt's a splitter with 3 dB loss in both arms, with 30 dB of isolation, so it cannot be resistive. And I just popped the lid to confirm that.

author: G0HZU
author_rank: Super Contributor
timestamp: June 20, 2022, 10:40:56 pm
content: After a quick scan of the datasheet and the glossy brochure for the FSIQ series it looks like this analyser dates back to the late 1990s. It appears to be a crossover type of analyser that still retains a classic 95dB logamp and a full suite of analogue RBW filters. However, it also has a digital IF that can function as a vector signal analyser. It may well be the case that the digital IF has a 12 bit ADC but I'm just guessing based on the date shown on the brochure. In basic spectrum analyser mode I'd expect it to outperform the classic HP 8568B analyser as the phase noise and IP3 specs are better than the 8568B. This must have been a very expensive analyser when it was new. I think 14 bit digital IFs appeared two or three years later, at least that's when I first saw analysers with a 14 bit digital IF at work. Maybe some slightly newer FSIQ analysers also have a 14 bit digital IF?

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 20, 2022, 11:00:33 pm
content: Quote from: G0HZU on June 20, 2022, 10:40:56 pmI just edited my post above, see there: The FSEA/FSIQ do use digital RBW filters below 1 kHz RBW; the ADC is an 18 bit one, sampling at 200 kHz. And yes, they were very expensive.

author: G0HZU
author_rank: Super Contributor
timestamp: June 21, 2022, 09:43:40 am
content: Thanks! I was kind of expecting a digital IF with fewer bits but with a higher sampling frequency and wider bandwidth. If it has a narrowband digital IF with 18 bits I would have expected it to perform really well even with close tone spacing. I've not played with an R&S FSIQ analyser so I can't really be sure what to expect from it.QuoteI agree. It might be worth doing a few more experiments to explore its limits.

author: rf-messkopf
author_rank: Regular Contributor
timestamp: June 21, 2022, 12:00:08 pm
content: Quote from: G0HZU on June 21, 2022, 09:43:40 amAt least that's what I gather from the service manual I have. I would have to check if the assembly part numbers match, maybe there were changes down the line. Also there may be differences between the FSEA/FSE and FSIQ units in this regard. R&S was never generous in terms of service manuals, and full schematics were not released. If I remember correctly, the digital IQ demodulator is a completely separate unit with its own ADC and DSP, but offhand I'm not sure.I'm also not sure about the effective analog bandwidth of the digital IF. According to the assembly block diagrams and the wiring diagram, the analog RBW filter section is ahead of the digital IF assembly and the ADC. No idea what the actual bandwidth it uses is. Unless someone can dig out some documents that describe it, one would have to do some reverse engineering.Quote from: G0HZU on June 21, 2022, 09:43:40 amI'll see what I can do.

author: gf
author_rank: Super Contributor
timestamp: June 21, 2022, 12:12:27 pm
content: I stumbled across this paper which I found very interesting:https://www.researchgate.net/publication/261321090_Low-IMD_two-tone_signal_generation_for_ADC_testing