Low frequencies on Spectrum Analyzers - Page 1
Forum section: electronics / testgear

author: jgalak
author_rank: Regular Contributor
timestamp: March 27, 2018, 01:59:03 pm
content: My next major test gear purchase is going to be a Spectrum Analyzer. Probably either a Rigol DSA815-TG or a Siglent SSA3021X. Both are priced similair, and both fit my budget (let's please not start a fight over which one to pick, that'll be a different thread).My use is mostly for VHF and UHF radio use, 140 and 440 MHz, so both have adequate bandwidth.However, I also occasionally need to look at low frequency audio signals (like, say APRS tones at 1.2 and 2.2 kHz) and I've noticed that both SpecAns have a range that only goes down to 9kHz.Is this normal? Are there any SpecAns in this price bracket (sub-$2k) that go down lower? Or is this inherent in this sort of SpecAn design?I think I can use my Analog Discovery 2 in SpecAn mode for the low frequencies (though a quick google search didn't find a range specification), but that seems annoying - I was hoping a single device could do both.

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 27, 2018, 02:20:22 pm
content: Most of the "RF" spectrum analyzers have to worry about local oscillator feedthrough and can't get too close to DC. If you are willing to go old, theHP 3561A or HP 3562A are very nice 0-100 kHz analyzers.Or in a pinch, I've always found that using the sound card of a computer and a freeware FFT program gives very reasonable results.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 27, 2018, 02:44:23 pm
content: While most SAs only go down to 9kHz or so, there are a number of models that have extended range, down to 100 or 20Hz. A good example is some of the R&S FSE series analyzers, some of the current Keysight models, some Advantest models, etc. They are out there, but they seem to be less common - at least if you're buying new, though, I don't know of too many options that go that low. That said, if you don't need it all on one instrument, there are many options for audio band FFT measurements and many are inexpensive.

author: nctnico
author_rank: Super Contributor
timestamp: March 27, 2018, 04:11:51 pm
content: For low frequency stuff I'd get a DSO with good FFT unless you need a large dynamic range. Even if a spectrum analyser supports really low frequencies the performance is not very good. For example: the DANL for my (higher end) spectrum analyser is specified at -125dBm at 10kHz and -156dBm at 1Ghz.

author: jpb
author_rank: Super Contributor
timestamp: March 27, 2018, 05:24:13 pm
content: As people have said, audio solutions are relatively cheap - much cheaper than a spectrum analyser and will work a lot better, e.g.https://quantasylum.com/products/qa401-audio-analyzer(I don't own the above but there has been quite a lot of discussions on these forums).If you're spending $2k on an analyser, spending <$500 on an audio solution (probably more like < $200 if you go for a usb audio interface or sound card) is only 10 -20% of the cost.

author: jgalak
author_rank: Regular Contributor
timestamp: March 27, 2018, 05:25:02 pm
content: Quote from: nctnico on March 27, 2018, 04:11:51 pmI have a DSO (Rigol DS1054Z) but I've been very unhappy with the FFT on it for this sort of thing - I can't seem to zoom in far enough to be useful. The minimum scale seems to be in the multiple MHz range, so seeing things in the sub-10kHz range is impossible. Unless I'm using it wrong, of course...

author: srce
author_rank: Regular Contributor
timestamp: March 27, 2018, 07:55:19 pm
content: Oscilloscopes don't have great dynamic range.I'm using a Lynx E22 soundcard in to MatLab.

author: SWR
author_rank: Regular Contributor
timestamp: March 27, 2018, 08:23:09 pm
content: I recently had to make the exact same decisions as you are facing with very similar applications.I chose the Siglent SSA3021X based on better specs and solid reviews, and use it for RF data communication (LoRa 170/433/868Mhz, WiFi/GPS (2,4GHz: easy mod to 3,2GHz) and EMC pre-compliance.For the low frequency stuff and general debug I chose a GW Instek MDO-2204EX multi instrument with 4ch 200MHz oscilloscope, DC-500MHz spectrum analyzer, dual arbitrary waveform generator, digital multimeter and dual power supply. The SA has >80dB dynamic range which is about 40dB less than the Siglent SSA at low frequencies.Those two instruments alone cover about 95% of my measurement needs.

author: nctnico
author_rank: Super Contributor
timestamp: March 27, 2018, 08:34:20 pm
content: Quote from: jgalak on March 27, 2018, 05:25:02 pmThe DS1054Z has useless FFT. Get a scope with 1MPts FFT and it will be usefull. Even for closely spaced frequencies. The problem with an oscilloscope is that the FFT frequency bins depend on the samplerate. With longer FFT you'll get a decent frequency resolution without needing to be very specific about the samplerate. With short FFT you'll need to set a very low samplerate to look at low frequencies.

author: SWR
author_rank: Regular Contributor
timestamp: March 27, 2018, 08:35:38 pm
content: Quote from: jgalak on March 27, 2018, 05:25:02 pmI don't think you're using it wrong. The FFT on these units leaves a lot to be desired.You really need a fast updating higher resolution FFT like a million points to get closer to the feel of working with a real SA. The user interface on a spectrum analyzer is much easier to work with than a FFT math function. The newer GW Instek scopes have this interface, and maybe others do as well?

author: SWR
author_rank: Regular Contributor
timestamp: March 27, 2018, 08:48:14 pm
content: Quote from: srce on March 27, 2018, 07:55:19 pmThat's not a bad idea if you don't mind dragging along a stationary PC.You get 117dB dynamic range but with inflexible attenuation, so you might loose some of it depending on how well your signal maches the line level input. You're propably not going to loose 30dB though.

author: jgalak
author_rank: Regular Contributor
timestamp: March 27, 2018, 08:58:18 pm
content: I do have the AD2, which has a SpecAn mode. I've been unable to get good spec on it, but it seems to go down all the way to DC. How good it is I can't tell. Gotta be better than a soundcard, right? Or not...

author: technogeeky
author_rank: Frequent Contributor
timestamp: March 27, 2018, 09:29:06 pm
content: Quote from: jgalak on March 27, 2018, 05:25:02 pmI'll have to check later, but the best you can get for low-ish frequency FFTs is to start the FFT, go into the 2nd page of options, and go to longer and longer timebases until the optionTracegreys out. Then, go back one setting to a shorter timebase, and selectMemory. This will give you your best chance of seeing low frequency signals.I also want to add: As exemplified by Shahriar @ The Signal Path, some (older for sure, but some newer) spectrum analyzers claim to have some lowest frequency range (for example, 9kHz). Some of them are restricted (in software) to not go below this. However,some of them(in this example the DSA1030A)willactually display tones well below that limit with the caveat that the amplitude accuracy is no longer guaranteed. This video shows tones as low as 100Hz.

author: technogeeky
author_rank: Frequent Contributor
timestamp: March 27, 2018, 09:36:22 pm
content: Quote from: nctnico on March 27, 2018, 08:34:20 pmIt's still not great, but I think with Memory mode things aren't as useless as they once were. However, searching for the depth just now reveals this might only be 16k points which is pretty atrocious. In any case, whenever I use the Memory option on my 1054z I am able to get results -- even when down at power line frequencies like 60Hz (buried in other audio frequency content).You can, of course, save and transfer waveforms to a computer and perform FFT which will give you arbitrarily good results up to the poor resolution of the 8-bit ADC.I do agree that other oscilloscopes (perhaps the newer Siglents, but surely the Instek) provide pretty damn good FFT options for a oscilloscope.I am not nearly as expert as you are, so please feel free to correct anything I say.

author: G0HZU
author_rank: Super Contributor
timestamp: March 27, 2018, 10:08:13 pm
content: All of my big old classic (boatanchor) lab RF (GHz+) spectrum analysers go down close to DC. I think they all go down to about 50-100Hz but because they are swept analysers with analogue RBW filters the sweep time will be slow on something like a 10kHz span with a low RBW.They also maintain a low noise floor down to a few kHz (especially the HP8568B) but I still wouldn't recommend using them like this for audio work because of the limited distortion performance of the analyser (compared to a soundcard) and the slow sweep speed. It quickly becomes tedious to use the analyser like this.Probably the best RF spectrum analyser I have here for LF work is my old 8GHz Tek RSA3408A as this has a 14bit ADC and can cover from a few Hz up to about 40MHz in its 'direct to ADC' mode. So it's a bit like having a 14 bit soundcard that can cover up to 40MHz rather than just 20kHz. But it does have a few low level spurious signals in this range because of the limitations of the ADC etc.

author: llkiwi2006
author_rank: Regular Contributor
timestamp: March 27, 2018, 11:07:51 pm
content: Quote from: jgalak on March 27, 2018, 05:25:02 pmThe FFT on the 1054z sucks, but you can see down into the kHz range. You just need to fiddle around with the memory depth and timebase to get a low enough sample rate (maybe around a few hundred kSPS). Also set FFT to process memory instead of trace. But ultimately the dynamic range is terrible, so you might get better results just using your pc soundcard.

author: egonotto
author_rank: Super Contributor
timestamp: March 28, 2018, 12:23:28 am
content: Quote from: jgalak on March 27, 2018, 08:58:18 pmHello,I don't understand this. Analog discovery has a good SA mode. What have you and what expect you?Best regards

author: CalMachine
author_rank: Frequent Contributor
timestamp: March 28, 2018, 12:27:36 am
content: You want an SR780!http://www.thinksrs.com/downloads/PDFs/Manuals/SR780m.pdf

author: jgalak
author_rank: Regular Contributor
timestamp: March 28, 2018, 06:04:03 am
content: Quote from: egonotto on March 28, 2018, 12:23:28 amWhat I mean is that I can't find any detailed specifications on the SA mode of the AD2. I am assuming it's better than a sound card, but don't know that for sure.

author: tecman
author_rank: Frequent Contributor
timestamp: March 28, 2018, 05:10:16 pm
content: I have an Advantest R9211C. You can occasionally find one for a good price on ebay. Nearly identical specs to the SRS or HP 35770. Only complaint is that the interface is not as intuitive as I would like. Same problem on the RF spectrum analyzers. But performance to price ratio (on used) is very good.paul

author: thm_w
author_rank: Super Contributor
timestamp: March 28, 2018, 09:35:08 pm
content: Quote from: jgalak on March 28, 2018, 06:04:03 amIts better in terms of sample rate/bandwidth/input range, but worse in terms of SNR and THD.AD: Two-channel oscilloscope (1M?, ±25V, differential, 14 bit, 100Msample/sec, 5MHz bandwidth). SNR = 75dBfsDecent sound card: 24-bit 192kHz, probably >100dB SNR.https://reference.digilentinc.com/_media/analog_discovery:analog_discovery_rm.pdf

author: jpb
author_rank: Super Contributor
timestamp: March 29, 2018, 09:18:18 pm
content: Quote from: CalMachine on March 28, 2018, 12:27:36 amThere is a SR760 for sale on ebay UK, in fact it has been for sale for quite a while which isn't surprising as they want a lot of money for it:https://www.ebay.co.uk/itm/Stanford-Research-Systems-SR760-FFT-Spectrum-Analyzer-100-khz/272851949264?hash=item3f873e56d0:g:xrwAAOSwgjFZukt~

author: Co6aka
author_rank: Unknown
timestamp: March 29, 2018, 11:55:07 pm
content: Quote from: jpb on March 27, 2018, 05:24:13 pmFWIW, I have a QA401 and am quite pleased with it.

author: David Hess
author_rank: Super Contributor
timestamp: March 30, 2018, 06:18:26 pm
content: Quote from: jgalak on March 27, 2018, 01:59:03 pmThere are two big reasons:1. The first mixer is usually AC coupled through the transformer. Some spectrum analyzers have an option to swap the mixer inputs for operation down to DC but this risks damage to the mixer if DC is applied.2. Close in phase noise from the first local oscillator limits dynamic range at low frequencies anyway. Early DDS shortwave receivers had lower frequency limits much higher than 9kHz because this lowered their sensitivity so much.

author: joeqsmith
author_rank: Super Contributor
timestamp: March 30, 2018, 08:17:57 pm
content: Seems like you should use this as justification to buy more test gear later on.For low frequency, I have an old HP3589A spectrum/network analyzer that will is spec'ed for 10Hz to 150MHz. It's big, weighs a lot and is nothing fancy. I would think these things are fairly cheap now days.

author: G0HZU
author_rank: Super Contributor
timestamp: March 30, 2018, 10:15:32 pm
content: Quote from: David Hess on March 30, 2018, 06:18:26 pmMost strange... What RF spectrum analysers have an input like this with a transformer and why would you put the DC block at the mixer anyway?The normal place for the switchable DC block is right at the analyser input ahead of the step attenuator. Often there will be a diode limiter between the attenuator and the mixer. I've not seen a regular lab analyser with a transformer at the mixer input either. I've not looked inside that many analysers but it would be odd to put a transformer at the RF input to the mixer. By transformer I means something with wires wound around a toroid or similar.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2018, 02:49:56 am
content: Quote from: G0HZU on March 30, 2018, 10:15:32 pmThe transformer is at the input in the form of one of the two transformers in the first double balanced mixer. Below shows an example where the normal IF and RF port on the first mixer are swapped to extend the input range down to DC. When the diode ring is directly exposed to DC at the input like this, an external AC coupling adapter is recommended to prevent damage because the mixer is awfully easy to burn out in this configuration.

author: Performa01
author_rank: Super Contributor
timestamp: March 31, 2018, 10:00:45 am
content: Quote from: David Hess on March 31, 2018, 02:49:56 amYou seem to think this is something special, but it's just the standard arrangement for wideband upconversion receivers (what an SA essentially is) regardless of the lower input frequency limit - and for good reasons.For an SA to work down to DC the very first and obvious requirement is to have the DC block switchable (or no internal DC block at all).The performance will be usually poor because upconverting very low input frequencies of a few Hertz into the GHz Range of the 1st IF is equivalent to zooming into the close-in phase noise of the LO. Any error of just 1ppb in the LO will translate to >10% at 10Hz. Image rejection also gets rather difficult under such conditions.Because of this, Analyzers that start at DC (or very low frequencies like 10Hz) usually have a dedicated direct processing path with no frequency conversion at all up to e.g. a few hundred kHz.

author: G0HZU
author_rank: Super Contributor
timestamp: March 31, 2018, 01:00:49 pm
content: QuoteOk, what has thrown me was your use of mixer inputs (plural) as in the quote below. This conjured up an image of a pair of inputs to a transformer. That would be very strange at this port of the mixer. Usually there is a single feed point. Often this is direct to a pair of diodes and then onto a balun transformer.QuoteNote: I think your block diagram is for a spectrum analyser plug in module for a old Tek oscilloscope? I'm not sure I'd want to try using that at low frequencies for various reasons.See below for an image of the range 1 mixer for my old HP8566B analyser for example. This doesn't have a DC block anywhere but it does have a diode limiter ahead of this mixer to try and protect it from overload. This analyser works down to about 100Hz although I wouldn't really recommend using it down at AF frequencies. There are so many other options that will be better nowadays.

author: David Hess
author_rank: Super Contributor
timestamp: March 31, 2018, 01:07:29 pm
content: Quote from: Performa01 on March 31, 2018, 10:00:45 amIt is only special in the sense that spectrum analyzers which wire the first mixer as it is commonly shown cannot operate down to DC and may have a factory option to swap the IF and RF ports.QuoteThe local oscillator's phase noise limiting the performance close to DC was my second point. Did you even read what I posted?With the phase noise limitation, it makes sense to wire the first mixer with the RF going into one of the transformer ports for better overload protection and I assume that is why it is done. For applications which require operation down to DC and that can live with low dynamic range, the mixer has to be wired unconventionally and other provisions for overload protection made. As you point out, a standard spectrum analyser is the wrong instrument for these applications but some people want to do it anyway.The question about rewiring the first mixer for operation down to DC shows up periodically on various technical email lists. I get the feeling that most of the old spectrum analyzers with blown first mixers died because of DC applied to the RF input and damaged diodes rather than excessive RF or a damaged transformer. Maybe newer ones have it but I have never run across one which included a switchable DC block.Quote from: G0HZU on March 31, 2018, 01:00:49 pmIt is but was just the best example I had immediately available for illustrating the point. Tektronix advertised these for operation down to DC although I am not sure what application they had in mind.

author: G0HZU
author_rank: Super Contributor
timestamp: March 31, 2018, 01:39:27 pm
content: QuoteMy old Advantest TR4172 has a switchable DC block at the input. This design dates back to about 1981. With the DC block inline it works from 1.8GHz down to 10kHz but with it switched out it says it works down to 50Hz on the front panel. It takes a quick button press to select the DC block. The DC block is right at the input of the analyser ahead of the step attenuator rather than at the mixer itself.I think my 1.5GHz HP8568B (dates back to the 1970s) works down to 100Hz on one of its inputs. It uses a simple internal RF relay switch to select an input with a DC block or another without a DC block. The default is the N connector with the DC block inline. I rarely used the alternative input without the DC block and left it on the default N connection.

author: TAMHAN
author_rank: Frequent Contributor
timestamp: March 31, 2018, 03:46:44 pm
content: IF (big IF) you can get one cheap, the HP 4195A goes down to 10Hz. I already drove it there, and it worked quite well.Of course, it is a heterodyne sweep unit, and is not the fastest to sweep.Tam

author: G0HZU
author_rank: Super Contributor
timestamp: March 31, 2018, 05:21:41 pm
content: We used to have an HP4195A at work. It's a very versatile instrument. However, it is very slow to sweep. For a bit of fun at work I think we managed to get ours to display a sweep time of 25 years by setting it to some silly settings. I recall it actually gave the sweep time in years but I can't be certain. It was about 25 years ago so maybe it would have finished by now if we had left it alone all this time

author: Performa01
author_rank: Super Contributor
timestamp: March 31, 2018, 05:31:13 pm
content: Quote from: G0HZU on March 31, 2018, 05:21:41 pmPity you didn't do that. Now we'll never know how accurate the sweep time estimation of this instrument has been!

author: G0HZU
author_rank: Super Contributor
timestamp: March 31, 2018, 05:44:23 pm
content: I miss having the HP4195A at work because it worked so well as a VNA or a spectrum analyser at low frequencies. But it was costing too much in yearly calibration to justify us keeping it. It often went unused for a year or more so it got disposed of.My 1.5GHz HP8568B doesn't really suffer from the LO noise limitation at low frequencies. It typically manages a noise floor of about -149dBm/Hz up at VHF. It only degrades by about 6dB or so down at audio frequencies and this is with the correct 100Hz+ input selected. So it can show about -143dBm/Hz typical noise floor across a span of 0-10kHz.Other analysers will usually show a steep rise in noise floor below about 200kHz but my HP8568B is very good here. Much better than the datasheet limits. But I still don't use it down at these frequencies unless I'm looking for very tiny signals and can afford to wait for the slow sweep time.

author: Performa01
author_rank: Super Contributor
timestamp: March 31, 2018, 05:57:45 pm
content: Do you happen to know how the 8568B manages to provide such a decent low frequency performance?Separate band with much lower 1st IF?I don't know this oldie, but I guess it's a safe bet that it doesn't have any means for digital signal processing?

author: G0HZU
author_rank: Super Contributor
timestamp: March 31, 2018, 06:31:21 pm
content: It was an extremely expensive analyser that was designed to try and maintain as much performance as possible over its full frequency range. It has a synthesised LO scheme with a decent internal 10MHz OCXO and also has very low close to carrier phase noise on the first LO and so this also means it has an advantage over other analysers here. So my HP8568B can achieve maybe -143dBm/Hz noise floor even down at 1kHz. However, there will probably be quite a spread in this level across various examples of this analyser. This level of performance is much better than the 'limit' figures in the datasheet and maybe not all HP8568B analysers will be as good as mine. Generally, the typical performance for the HP8568B analyser is a lot better than the datasheet limits in a lot of areas.It achieves this performance without needing the separate direct path to the IF. The AF signals get upconverted to a 2GHz IF just like all other signals.It can do some DSP tricks (eg post detection FFT with a choice of the classic Hanning or Flat top windowing) but not in the regular analyser mode.

author: G0HZU
author_rank: Super Contributor
timestamp: March 31, 2018, 09:33:21 pm
content: Here is a plot of my HP8568B with a span of 0-2kHz with a RBW of 30Hz. It can go down to 10Hz RBW but it would takes ages to sweep.Sorry for the gloomy image as my camera doesn't take pictures of CRTs very well and also the CRT is getting a bit dim in this analyser. It hasn't been used for ages and I think the CRT might benefit from me leaving it on for a while. Because the noise level is so low, some people might think the analyser is faulty in some way but I can send a known test signal to it and it shows up correctly in terms of level. This was a very special instrument in its day. The display is digitised even though it is a CRT and it has 1000x1000 datapoints in the grid area. This is still good even today.

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 09:17:49 am
content: I love challenges!Anyway, here comes my answer to the question what single device can “do both”, i.e. works for very low as well as high frequencies. When I saw the noise floor of the ancient HP8568B, I thought “oh boy, this would be hard to beat” – but I decided to give it a try anyway.My contender is the Signal Hound SA44, which works in the range of 1Hz to 4.4GHz (Tracking generator TG44 from 10Hz to 4.4GHz).Now the question is … is it fair to place a modern low cost SA against an ancient high end boat anchor (the datasheet says 45kg!) that has probably cost its first owner an arm and both legs?The answer can only be a resounding “Yes”!Both are affordable options today, even though for the HP8568B one has to calculate the additional costs for a crane truck as well as another (reinforced) workbench – but I guess these are minor considerations for a true test gear aficionado…First I tried to resemble the scene shown by G0HZU as close as possible, just had to use 25Hz RBW because 30Hz isn’t available on a SA44. No input signal, I’ve just terminated the input with 50 ohms.SA44_DANL_2kHz_FVoilà, David has scored against Goliath. A DANL of -149dBm/Hz should certainly be good enough for the majority of practical applications in this frequency range.However, a low noise floor alone does not make us happy – we are more interested in the signal to noise ratio, and for that we need a signal. When I tried this, I immediately realized why G0HZU preferred to show the noise floor without a signal: at frequencies that low, we can get all kinds of interferences by connecting the cable alone, even when the signal source is still off. In fact I’ve verified that the ground connection to any device (that isn’t even powered on) quite obviously causes a ground loop with lots of unwanted signals related to mains hum and its harmonics at levels up to -110dBm below 1kHz, so I had to measure the noise floor at 1500Hz this time:SA44_DANL_2kHz_1000Hz_-40dBm_FNow I would love to see how Goliath behaves in this test scenario, and I have the sneaky feeling this could already be a decisive battle in this contest…The noise floor is a bit higher now by about 4dB and without more tests we cannot be absolutely sure where it comes from – the signal generator, the ground loop or the analyzer itself when it sees a strong signal.So here’s the exact same measurement with the signal generator output off and powered off on top of that. As can be seen, the noise floor and the interferences are pretty much the same, so it’s all coming from the ground loop:SA44_DANL_2kHz_GroundLoop_FIn any case the signal to noise ratio is close to 90dB in this scenario, but for sensitive measurements one would really have to find a cure for the mains hum and other interferences caused by ground loops. Shielded audio transformers might be an option, but are usually not made for 50 ohms and the frequency response might become problematic – apart from the fact that this obviously cannot work down to DC anyway.Now finally I need to prove that this analyzer can not only work at very low frequencies, but also higher ones. Since the HP8568B works up to 1.5GHz, I’ve made another measurement at that frequency. Of course I’ve taken advantage on the built-in preamplifier, that can be engaged for frequencies above 100kHz and that’s also the reason why the noise is now even lower at -164dBm/Hz:SA44_DANL_2kHz_1500MHz_-100dBmWARNING:The SA44 does not have an internal DC block – of course not. During normal use, I always have an external one attached to its input and one really needs to be careful when working without. The input cannot tolerate DC at all and any voltage exceeding +/-100mV might damage it permanently.

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 11:06:50 am
content: I think your SH44 gives good performance at low frequencies because it uses a very low first IF of something like 10.7MHz. So the first LO runs at just a few MHz and it's easy to get low phase noise and good mixer balance at a few MHz. I'm going to guess your SH44 uses something like an ADF4350 for the first LO and this can get very good phase noise when the GHz VCO in the ADF4350 is divided down inside the chip to just a few MHz.For the designers of the HP8568 back in the mid 1970s they had to do this with an IF of 2GHz. Much harder because the LO runs at >2GHz! Normally, you can kind of predict the very close to carrier phase noise of a well designed LO at 2GHz with a few guesstimates and a simple sum.If we assume the 10MHz reference OCXO inside the analyser is a good one with maybe -150dBc/Hz phase noise at 500Hz offset then the phase noise at 500Hz offset at the 2GHz first LO could normally be as good as -150 + 20*log(2000/10) = -104dBc/Hz.But the HP8568B manages about -112dBc/Hz here and it does it with a clever comb generation scheme in the first LO which is able to dodge the 20*logN part of the equation at lowish analyser frequencies. i.e. at analyser centre frequencies up to maybe 200MHz or so. This means the HP8568B phase noise is very 'flat' at -112dBc/Hz from carrier offsets of a few 100Hz to maybe 80kHz offset across analyser frequencies of LF/HF/VHF. In order to see a -143dBm/Hz noise floor at a 500Hz input frequency the first mixer will need to have good balance to prevent bleedthrough of the first LO.So I assume that the mixer must have about (143-112) + 13 = 44dB isolation to the mixer IF port if the first LO runs at maybe +13dBm. The conversion loss of the mixer (and the front end attenuation) should also be factored in but this -143dBm/Hz noise floor at sub 1kHz is an impressive result with an LO running at 2GHz.QuoteMy HP8568B is stored downstairs so I can't easily connect any decent test gear to it. I don't use it much anymore. I just unpacked it and turned it on and took a picture. I did try testing it yesterday at 10kHz with a little Marconi 2022 sig gen at a known -80dBm to prove the analyser was healthy. The little 2022 generator was easy to carry downstairs but it can't go below 10kHz. But I've been using this same HP8568B analyser since 1990 at work and now here at home and I know its performance well.

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 11:34:52 am
content: QuoteIt's easy to get a low noise floor if the analyser has a preamplifier built in. My old Advantest TR4172 has an internal switchable preamp but I rarely use the preamp as it compromises the analyser performance in too many ways.The preamp has a lot of gain and I think the noise floor is typically -166dBm/Hz with the preamp in but I've never bothered to really investigate the preamp

author: rf-loop
author_rank: Super Contributor
timestamp: April 01, 2018, 11:57:34 am
content: Just for... (image is self explanating - no comments)

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 01:14:37 pm
content: @G0HZUI do hope you didn't get me wrong in any way. I fully respect the HP8568B and its remarkable performance with regard to the architecture used. And because I usually don't use the SA44 down below some 10kHz (which requires to remove the DC block) I had no idea if it was able to keep up or even beat the HP.Of course the architecture of the SA44 is vastly different, that's the reason why I thought it might be able to compete despite the low cost. Unsurprisingly, the concept has its limitations, but still works surprisingly well for many applications, especially narrowband ones. And at the end of the day most users are interested in the results only...I also hope you can forgive what I said about you not showing a signal - do not take it too seriously pleaseWith regard to the preamplifier, you seem to be one of those purists who say that a good receiver frontend has to have a passive high level double balanced mixer, with nothing but a switchable attenuator and a pre-selector in front of it, but by no means a preamplifier. If so, then we're not far apartYet in the rare cases where we deal with very low signals exclusively, what's wrong with the preamp then?In case of the SA44, I have only once measured it at 30MHz. The CP changes from +0.5dBm to some -18dBm with the preamp on.I've just measured the DANL at 1.5GHz again. Without preamp it is about -151dBm/Hz - marginally better than at 1kHz, where no preamp is available from the outset.

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 01:20:12 pm
content: Quote from: rf-loop on April 01, 2018, 11:57:34 amNice!Guess I'll need to have a look into that eventually.Yet I think you're cheating a bit ... might it be that the input signal is actually a bit higher than -40dBm?

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 01:29:23 pm
content: Just in case any Goliath is still in combat mode, here are a few more low frequency tests taking advantage on the narrow resolution bandwidths of the SA44 down to 0.1Hz.First let’s have a look at the DANL at really low frequencies from 0-60Hz at 0.1Hz RBW. We can see the mains signal at 50Hz together with two spurs at 40 and 60Hz that look like sidebands from some 10Hz modulation. The Noise level is significantly higher down here, yet -142dBm/Hz should not cause any troubles in real applications.SA44_DANL_60Hz_MFBelow are a couple of experiments that take advantage of the high frequency resolution. Sweep is slow at about 37.5s, but that’s just the price for very narrow resolution bandwidths according to the laws of physics (and digital signal processing).We start with a 20Hz carrier, 100% amplitude modulated at 1Hz. Regular markers have been placed on the carrier, the upper sideband and the mains hum. There are many high order harmonics of the modulation frequency visible, but at relatively low levels.SA44_DANL_60Hz_20Hz_AM100%_1Hz_MFThe next screenshot shows a 3Hz square wave at -41dBm. All the odd harmonics are clearly visible, whereas the even harmonics are missing, just as it has been written in the textbooks. The 50Hz mains hum is clearly distinguishable from the 17th harmonic at 51Hz.SA44_60Hz_Square_3Hz_-41dBm_MFI happened to catch the moment when I changed the signal level from -40dBm to -41dBm. For this amplitude step some internal relay in the generator was switching and the SA44 captured some interesting looking piece of art:SA44_60Hz_Square_3Hz_-40dBm_SW_MFFinally I wanted to create a comb pattern where all harmonics are equal in amplitude – at least up to 60Hz. I used a 1ms wide pulse with 2ns rise & fall times at a repetition rate of 3Hz for that. The pulse amplitude is -40dBm again, but its energy is now spread over a wide frequency range, hence the measured level is only -80dBm.SA44_60Hz_Pulse_1ms_3Hz_-40dBm_MF

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 01:35:08 pm
content: Pretty good from the SiglentIt is within about 10dB of my HP8568B in terms of noise level and that is a lot better than a lot of expensive analysers we have at work from R&S. Because the Siglent uses FFT it's also going to sweep a lot faster than my old HP8568B with its old school analogue RBW filters.Here's a quick video of my 8GHz Tek RSA3408A when used down at 0-5kHz. This shows the noise and harmonics from an old audio generator I made when I was a student. It uses cheapo opamps so the noise and distortion isn't that great. But the Tek RSA3408A is quite fast in terms of update rate. However, this is still classed as an old and obsolete analyser at work. I got this one for free, saved from the dumpster at work because it had a minor fault. Now workinghttps://www.youtube.com/watch?v=G7SSoe3Bils&feature=youtu.be

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 01:43:43 pm
content: Quote from: G0HZU on April 01, 2018, 01:35:08 pmPity I cannot view it now with my old PC because of the HTML5 format...

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 02:09:22 pm
content: I tried uploading again here:https://www.youtube.com/watch?v=G-OkogKFxKg&feature=youtu.beDoes this one play for you or can you not watch any youtube videos on your PC?

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 02:13:16 pm
content: I usually can - but not these modern content protected (I guess) video formats - I'm still on Win XP here in the lab...EDIT: Guess it's the H.264 fromat that is not supported.

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 02:35:00 pm
content: Try again here:https://www.youtube.com/watch?v=uqthKonp9Xc&feature=youtu.beor this onehttps://www.youtube.com/watch?v=GqttvZw76_g&feature=youtu.beThe second one is really grotty with a stretched aspect ratio but it is the most basic format I can find?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: April 01, 2018, 04:27:15 pm
content: This just showed up on ebay:https://www.ebay.com/itm/Agilent-HP-35665A-DSA-Dynamic-Signal-Analyzer-w-Opt-1D2-1D4-1C2-ANA/142741758701These go to around 244 micro Hz in the right conditions.It's something of an update to these:Quote from: wn1fju on March 27, 2018, 02:20:22 pmAnd relatively similar to this:Quote from: CalMachine on March 28, 2018, 12:27:36 amDave has a video on the machine that is slightly older:https://www.eevblog.com/2013/09/21/eevblog-523-repair-hp-35660a-dynamic-signal-analyser/An eeprom upgrade to the 35665A to enable all software features is available at:http://www.glkinst.com/test-equipment/

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 04:53:49 pm
content: @G0HZU Thank you very much - both recordings play fine on my ancient machine.And about the contents - looks impressive indeed, especially for an 8GHz SA. There is not much increase of the noise floor when the signal is applied. Makes me wonder what the 1st IF in this scenario would be...

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 08:38:38 pm
content: The RSA3408A is straight to the ADC below 40MHz so it has very good performance here.For completeness I took my little AF generator downstairs and tried it on the HP8568B. See below for a 2kHz span as before but this time with the signal applied and I've used a 10Hz RBW. You can see the noise is higher and this is from my generator rather than any reciprocal effects from the analyser phase noise. Sorry the picture is a bit fuzzy, the camera doesn't focus well on CRTs.

author: Performa01
author_rank: Super Contributor
timestamp: April 01, 2018, 09:06:41 pm
content: Oh yes, direct to the ADC is certainly the method of choice for a good modern analyzer - and it certainly pays off. 40MHz is quite high - do you happen to know how many bits the ADC has?The screenshot is plenty good enough and the performance appears actually very good. All the more so considering the signal source is just an RC audio generator using standard op-amps.

author: G0HZU
author_rank: Super Contributor
timestamp: April 01, 2018, 11:08:55 pm
content: I think the ADC is 14 bit and this one has the extended memory option when in real time mode. It's at its best when capturing wideband RF signals (up to 36MHz capture BW) for post processing and that's what we use them for at work. However, we generally need more real time BW than 36MHz these days so they are slowly being replaced with modern stuff from Keysight with three times the BW.The RSA3408A user interface is classic Tektronix, poorly organised and laggy and it has some really bizarre bugs when used near its limits. However, I'm not that impressed with the Keysight replacements (PXA and EXA) in terms of the user interface. The latest versions of these analysers use a touch screen interface and you need tiny fingers and the presentation of the touch menus is poor.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: April 01, 2018, 11:41:44 pm
content: Here is a shot of the HP 35665 showing an FFT on two channels. It's showing the output of an HP 203A dual-output signal generator.The 203A was supposed to be outputting 100 mHz, but it's actually around 115 mHz. It was okay at higher frequencies. Another repair project.It takes between 30 minutes and 60 minutes at the resolution and channel setup to get that trace. The timer was around 3600 seconds, but it seemed to be updating something at twice that rate. That could be figured out more easily at a higher frequency.

author: G0HZU
author_rank: Super Contributor
timestamp: April 02, 2018, 01:02:37 am
content: Impressive stuff! I'm not sure what the RSA3408A can do 'directly' with a 0.1Hz test signal but I can easily set it up to grab I and Q data of a 0.1Hz signal. I've left it on a 5kHz span so probably sampling at something like 6.4ksps. But the idea below is just to quickly show it can at least recognise that a 0.1Hz signal is being fed to it.See below for a 20 second span taken from a 50 second data grab of a 0.1Hz test signal at this sample rate. Because it's a Windows machine it should be possible to write some code to post process and display this data in the frequency domain.Ultimately, I think I can turn down the sample rate to something like 128sps and grab up to 256M of data for post processing. But I think that would take over 100 hours to complete!

author: Performa01
author_rank: Super Contributor
timestamp: April 02, 2018, 08:22:36 am
content: I’m not entirely sure if frequencies below 1Hz still qualify as just ‘low’Anyway, the SA44 is not useful below 1Hz anymore. Not because the datasheet says so, but the lowest resolution bandwidth is 0.1Hz and quite obviously spectrum analysis becomes problematic when the signal frequency approaches the resolution bandwidth.For challenges like this, I had to pull another weapon, it’s the PicoScope 4262. Once again a little David that isn’t afraid to match with the various Goliath boat anchors out thereThe lowest analysis bandwidth is 100Hz, but we can zoom in as much as we like (x100 in this example) and take a closer look at the 0-1Hz segment.Just to make sure that David cannot lose, the RBW has been set to some 725uHz, resulting from a 524284 point FFT thus 381.5uHz frequency step and 725uHz RBW with the Blackman-Harris window.Dual channel FFT? Well, why not?!• Channel A : 1Vrms 100mHz sine wave• Channel B : 1Vrms 200mHz sine waveOne sweep takes close to 44 minutes, so should be in the same ballpark as the HP35665.Pico 4262 FFT-1Hz_2Harmonic distortion of the input signals is below -70dB and all the harmonics are clearly visible.Note the dynamic range (113dB) and SNR (110dB) in this measurement.Also keep in mind that this is just 1% of the whole picture. We can adapt the frequency window by changing position and zoom factor anytime we like. The screenshot below uses the same zoom factor, but different frequency areas. The 100mHz signal for instance has still some odd harmonics even above 27Hz. For the 200mHz signal, the range 1-2Hz is shown, where we can see the last harmonic at 1.6Hz.Pico 4262 FFT-1Hz_3

author: G0HZU
author_rank: Super Contributor
timestamp: April 02, 2018, 05:44:48 pm
content: I had a go with the RSA3408A today and this was my first attempt at a 2Hz span. I've had to post process the data in the 89600 SW but this SW could run on the 3408A in the background.Note that the full span is 100Hz and the 0.1Hz signal show distortion up to about 20Hz where there is the last distortion term visible at -98dBc. After that there is nothing in the way of distortion all the way up to 100Hz. There is a single wobbly ~50Hz term that is not quite 50Hz at -77dBc but this is going to be mains related ground leakage I think.But the plot below shows the 100mHz signal and all the harmonics to 2Hz. I went for a 1mHz RBW to keep my sample rate vs sample time numbers simple but it can go much lower than this. I suspect it could do 0.1mHz RBW (maybe even 0.01mHz) if I took enough samples as I think the buffer in the 3408A is good for >100 hours of data at this sample rate. But this is already very silly. I wonder how well this would work with just an Arduino and a decent ADC at a low sample rate

author: thanasisk
author_rank: Regular Contributor
timestamp: June 16, 2018, 01:42:16 pm
content: The attenuation/level matching problem when using a soundcard may be solved using solution like this:https://linearaudio.nl/la-autorangerOr a 20dB pad/attenuator (or even a x10 scope probe) may work well forallowing a reasonable range of input amplitudes.

author: nctnico
author_rank: Super Contributor
timestamp: June 16, 2018, 03:40:41 pm
content: Quote from: thanasisk on June 16, 2018, 01:42:16 pmWhat does it do? It doesn't say that on the website.

author: ocw
author_rank: Regular Contributor
timestamp: June 16, 2018, 04:15:45 pm
content: Attached is a look at 0.1 Hz on my Stanford Research SR760. My generator was set at 100 mHz while the SR760 measured that as 100.136 mHz.There is the inconvenience of 1048.6 seconds (about 17.5 minutes) of acquisition time for that low of a frequency span. And the SR760's span is 100 kHz or that divided by a power of two. So, I had it set at 100,000 Hz / 262,144 = 381.470 mHz.

author: ocw
author_rank: Regular Contributor
timestamp: June 16, 2018, 05:39:35 pm
content: I took my SR760 down to its minimum 191 mHz span for viewing a 30 mHz signal as shown on the attachment.

author: thanasisk
author_rank: Regular Contributor
timestamp: June 16, 2018, 06:39:40 pm
content: Quote from: nctnico on June 16, 2018, 03:40:41 pmIt is an automatic attenuator/agc, i think based on rms measurements of the input signal. It makes sure that the output is at a selected level (e.g. 1 volt) and absolute maximum of 5 volts to protect the sound cardThe info is spread here and there and there is a relevant forumhttp://www.diyaudio.com/forums/equipment-and-tools/299635-autoranger-soundcards.html

author: radiolistener
author_rank: Super Contributor
timestamp: June 16, 2018, 06:51:44 pm
content: I think it's better to use good sound card for low frequency. It will provide good dynamic range, due to 24 bit ADC...