Bode plot up to a few 100 MHz - Page 1
Forum section: electronics / testgear

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 11:43:13 am
content: For characterizing amplifiers (for example an oscilloscope or DAQ frontend) I want to measure their frequency response. Mostly gain, but knowing the phase can't hurt. My RTB2004 does this nicely, unfortunately only up to 25 MHz.What options would there be for BW up to say 100...300 MHz (I am not a RF guy, so antenna preamps and matching is not relevant for me).I have used a VNWA for thishttps://www.sdr-kits.net/introducing-DG8SAQ-VNWA3, but the installation is somewhat quirky. Very powerful instrument though.Probably other USB VNAs would also do. With NanoVNA output amplitude does not seem to be adjustable, which makes amplifier measurement quite uncomfortable (needs fiddling with attenuator kit).LibreVNAhttps://eleshop.eu/librevna-2-ports-full-vna-100khz-6ghz.htmllooks interesting.I don't want to invest in a proper bench top VNA as I neither have the need for a multi-GHz unit nor the bench space.DIY via SCPI controlled RF gen and scope would be feasible, but a lot of work I'd like to avoid. Also a proper RF generator (sine output and wide range attenuator) would set me back $2000 on top.Any ideas? Thanks!

author: RoGeorge
author_rank: Super Contributor
timestamp: October 03, 2023, 12:40:17 pm
content: I've seen on ebay second hand portable VNA/SA (often used by service man for cellular towers/antennas). The instruments were in working and sold for less than $500, with a calibration kit and some cables included. A DIY would probably cost about the same and be less reliable.

author: 2N3055
author_rank: Super Contributor
timestamp: October 03, 2023, 01:09:50 pm
content: Quote from: Martinn on October 03, 2023, 11:43:13 amStick with the VNWA....

author: Martinn
author_rank: Frequent Contributor
timestamp: October 03, 2023, 01:36:06 pm
content: Quote from: 2N3055 on October 03, 2023, 01:09:50 pmI don't own one, so I'd have to buy it. Feels a bit outdated though, installation by fiddling with USB audio codec settings is weird. But yes, tons of functions in the software and nice applications book available.Quote from: RoGeorge on October 03, 2023, 12:40:17 pmWould you have an example? I quickly scanned ebay and found nothing even remotely at $500. Range was more like $5k-$40k.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 03, 2023, 01:46:41 pm
content: No example right now sorry, it's been a while since I didn't look on ebay. You'll have to hunt for a long time until some major provider or factory renew its inventory, or got sold at an auction. Could take months or years until such bargain deals occur.

author: nctnico
author_rank: Super Contributor
timestamp: October 03, 2023, 03:42:49 pm
content: Quote from: RoGeorge on October 03, 2023, 12:40:17 pmThese typically start at 5MHz or so. There are LF network analysers (Anritsu MS4630 for example) that work from less than 100Hz to a couple of hundred MHz and have high impedance (1M Ohm) inputs as well.

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2023, 04:25:01 pm
content: Is 120 MHz enough ?SDS2202X-E with SDG2122X.To 1.5 GHz, SVA1015X Plus.

author: mawyatt
author_rank: Super Contributor
timestamp: October 03, 2023, 07:27:47 pm
content: The combo tautech mentions above should work, the SDS2000X+ and SDG2042X we have, both "enhanced", certainly do. The overall performance is quite good, albeit slow, and the Dynamic Range is impressive for a DSO.Best,

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2023, 07:59:10 pm
content: Certainly SDS2000X Plus with its inbuilt 50 MHz Arb but with limited amplitude whereas with the right standalone will hit the 120 MHz Siglent DSO Bode plot limit.Hence the suggestion of the cheaper 2ch 200 MHz SDS2202X-E for its 120 MHz Bode plot capability.Still even the 2 ch SDS2102X Plus with its ~185 MHz -3dB BW plus SDG2042X hacked to 120 MHz total cost is right up near 1.5 GHz SVA1015X where the analyzer/VNA could be the better choice if the OP doesn't need another DSO.

author: pope
author_rank: Frequent Contributor
timestamp: October 03, 2023, 09:02:15 pm
content: Why not sds1104x-e and SDG2042X? It can also do up to 120MHz, right?

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2023, 09:12:03 pm
content: Quote from: pope on October 03, 2023, 09:02:15 pmBoth hacked yes but does the OP need another DSO or a 4ch one ?The little dearer SDS2202X-E also provides 50 Ohm inputs for better system matching and can be coerced to become a 350 MHz model.

author: mawyatt
author_rank: Super Contributor
timestamp: October 03, 2023, 09:37:54 pm
content: The OP mentioned scope and DAQ front ends, which might imply high impedance. Here's where the DSO and AWG might be a better choice that a VNA, which usually are centered around low Z of 50 ohms.Best,

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2023, 09:57:17 pm
content: Quote from: mawyatt on October 03, 2023, 09:37:54 pmThen old school Bode plots might the only way past the Siglent 120 MHz limit but without a Phase measurement.Cursors can then provide measurements but only a single one and not in a tabular multiple measurement format.Maybe DSO Maths can help....need do some study.

author: mawyatt
author_rank: Super Contributor
timestamp: October 03, 2023, 10:23:51 pm
content: Quote from: tautech on October 03, 2023, 09:57:17 pmWonder if Siglent has considered upping the frequency range since many of the scopes are beyond 120MHz. They have the SDG6000X AWGs which go from 200 to 500MHz. If the scope can handle the higher frequency and one has the higher frequency AWG, then "seems" possible. Of course this from "someone" that doesn't have to do the work, as we see often hereBest,

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2023, 10:29:52 pm
content: I imagine beyond 120 MHz with a HiZ system good results might be difficult to obtain and much less so for a proper 50 Ohm impedance matched arrangement.Those smarter than I should also contribute on this.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 03, 2023, 11:18:39 pm
content: Quote from: Martinn on October 03, 2023, 11:43:13 amAre you inferring that the VNWA3 does have a leveled output that can be adjusted? Does it also have sinewave drive? None of the low cost ones I have looked at do. They also don't have bias Ts, are only 2-port 1-path .....Physical size, you never mention what sort of amplifiers but just thinking if you were playing with something in the kW range, I imagine the attenuators will be much larger than the VNA.

author: dophuc
author_rank: Contributor
timestamp: October 04, 2023, 02:21:35 am
content: In the Rigol DHO800 Review Video "EEVblog 1566 - Rigol DHO800 12 Bit Oscilloscope" at minute 54:58, I see Dave using the Rohde & Schwarz MXO4 Series to evaluate the Bode Plot measuring probe with integrated 100Mhz waveform generator. That machine is the dream of many people because of its price when purchased with the "100 MHz waveform generator" optionhttps://youtu.be/S8jrpCoZyx8?si=98JFR9S5V3ep6WYF&t=3298

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2023, 02:45:11 am
content: Quote from: dophuc on October 04, 2023, 02:21:35 am200 MHz model = € 8,280https://www.batronix.com/shop/oscilloscopes/Rohde-Schwarz-MXO44-242.html

author: dophuc
author_rank: Contributor
timestamp: October 04, 2023, 03:13:08 am
content: Quote from: tautech on October 04, 2023, 02:45:11 amThis price does not include the "Waveform generator" and "Bode Plot" options, adding these 2 options costs about €3400 excluding tax

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 07:01:50 am
content: Quote from: joeqsmith on October 03, 2023, 11:18:39 pmYes, VNWA has an adjustable output. See this document on page 241:https://www.sdr-kits.net/documents/VNWA_HELP.pdfHere's an example of what I want to accomplish:https://www.eevblog.com/forum/projects/measuring-switch-mode-supply-noise-hp461a-replacement-ada4895/msg4652395/#msg4652395The last graph (frequency response up to 200 MHz) was measured using a VNWA. Power levels are small signal only, so no kW attenuators.To my knowledge none of the low cost (< $1000) VNAs have a sine output (VNWA3 certainly does not).If you know of a RF signal generator with sine output and programmable attenuator for < $2000, let me know.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 07:04:49 am
content: Quote from: dophuc on October 04, 2023, 03:13:08 amI recently purchased a R&S 300 MHz RTB2k "fully loaded" (with good discount - certainly being helped by some friendly competition from the eastern part of the world), so I am not quite looking for upgrading this.

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2023, 07:20:47 am
content: Quote from: Martinn on October 04, 2023, 07:01:50 amAs Mike pointed to earlier SDG6000X and entry into this range is 200 MHz model but can be coaxed to 500 MHz.Then with your DSO old school Bode plot to 300 MHz would be very possible like Dave shows in this old video:

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 07:24:01 am
content: Quote from: mawyatt on October 03, 2023, 09:37:54 pmYes, I am thinking more of high impedance amplifiers, like this small design:https://www.eevblog.com/forum/projects/measuring-switch-mode-supply-noise-hp461a-replacement-ada4895/msg4652395/#msg4652395But where do you see the problem with 50 ohms? High Z input can be terminated with 50 ohms (input noise changes of course, but that's not relevant here). Output needs to be capable of driving a 50 ohms load, which might be more of a problem. I had to include cable drivers in my "HP461A" LNA design.Quote from: tautech on October 03, 2023, 10:29:52 pmI agree, that's what I wrote "few 100 MHz", which I think should be achievable with proper layout.Quote from: tautech on October 03, 2023, 04:25:01 pmInteresting options. As I wrote I just bought my RTB2K and certainly will not replace that. SDG2122X: Nice, but a bit limited at 120 MHz.What about the SDG6022X: Can it be "upgraded" to 350 or 500 MHz as well? EDIT answered by tautechSVA1015X lists "discontinued" at Batronix. Tons of other options though:https://www.batronix.com/shop/siglent/all-spectrumanalyzer.htmlHowever usable are VNA functions in these "classical" SA?Also, I own already a HMS1000 spectrum analyzer (no TG) I purchased for EMC precompliance, would need to get rid of that first (market value probably near zero due to Chinese competition and lack of magic R&S label).

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 07:29:13 am
content: Quote from: tautech on October 04, 2023, 07:20:47 amI also considered signal generator+DSO, but instead of the crude sweep method Dave shows I thought of running a sweep via remote SCPI control. Set frequency, acquire reference and output channels, calculate amplitude/phase, next frequency. But that would be a lot of work to do the interfacing and the basic sweep method might be good enough!I will definitely look into the SDG6000X!

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2023, 07:33:43 am
content: Quote from: Martinn on October 04, 2023, 07:24:01 amIt's a current model:https://www.batronix.com/shop/spectrum-analyzer/Siglent-SVA1015X-VNA.htmlQuoteSame HW as SSA3015X Plus.SVA1000X models have a # of modes with SA and single active port VNA being core functions that provide for a range of S11 and S21 measurements. Optional but 120hrs of trial use are DTF and EMI modes.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 04, 2023, 08:11:52 am
content: Quote from: tautech on October 04, 2023, 07:20:47 amWould you have a pointer where to start looking for the "upgrade"? I scanned a few posts, so far unsuccessful

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2023, 09:07:39 am
content: Quote from: Martinn on October 04, 2023, 08:11:52 amAFAIK it's the same process as SDG2000X models. Look there.Oh I saved a link,follow this:https://www.eevblog.com/forum/testgear/unlocking-late-unit-sdg2042x/msg4639576/#msg4639576

author: nctnico
author_rank: Super Contributor
timestamp: October 04, 2023, 09:16:02 am
content: Quote from: Martinn on October 04, 2023, 07:29:13 amThe downside of using a DSO is that bode plotting is pretty slow where it also differs between manufacturer. With the same settings, Siglent bode plotting is 5 times slower compared to the R&S RTB2004. A VNA is about 5 times faster compared to the RTB2004. So if you need to make a lot of measurements, using a VNA is a better option.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 04, 2023, 12:42:39 pm
content: Quote from: Martinn on October 04, 2023, 07:01:50 amThanks, I will read the document. I can't be much help on new equipment purchasing as all of mine is old. $2000 isn't much of a budget. I assume if you only want a signal generator and not an entire turn key system, you are planning to write the software or you need something that works with what ever receiver you own and their software.***Skimmed the data sheet for their synthesizer. If you like the VNWA3 and can use the squarewave drive, can you get away with just getting a step attenuator with one of the low cost VNAs?https://www.analog.com/media/en/technical-documentation/data-sheets/ad9859.pdf

author: mawyatt
author_rank: Super Contributor
timestamp: October 04, 2023, 01:59:55 pm
content: Quote from: Martinn on October 04, 2023, 07:24:01 amOur comment was related to measuring a high impedance, here using a high impedance measurement instrument such as a DSO is generally better than a low Z device like a VNA. WRT to driving a low Z, most AWG generators and signal sources are fully capable of such.We have "enhanced" our SDG2000X and SDG6000X AWGs to full capability as mentioned. Both are superb performers, producing clean, accurate waveforms from 16 bit DACs, the UI is a little quirky tho, but one quickly gets used to such. Either can be employed with our SDS2000X+ for Bode functionality to 120MHz.The Bode implementation is quite good and allows user to control the input signal level across the sweep, and dynamically controls the measurement (scale factor on Ch 2) to achieve a wide dynamic range.rfloophad demonstrated quite remarkable DR for this core 8 bit DSO in Bode function, can't remember where this is but search for Bode Plot should revel such.Here's an example of using the SDS2000X+ and SDG6000X, note we are using in scope 50 ohms for Channel 2 and simply interfaced via LAN.Edit: Since we had this setup, decided to measure a Bourns 3590s 10K multi-turn precision pot set for 20dBV attenuation, note the complex frequency dependent behavior reveling the inductive nature of the resistive windings and the deep notch around 70MHz indicating a resonance within.Anyway, hope this helps.Best,

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2023, 07:15:07 pm
content: Quote from: mawyatt on October 04, 2023, 01:59:55 pmDone exploring the Bode plot limits of the $499 SDS1104X-E paired with a Siglent AWG:https://www.eevblog.com/forum/testgear/siglent-sds1x04x-e-bodeplot-ii-(sfra)-features-and-testing-(coming)/

author: tszaboo
author_rank: Super Contributor
timestamp: October 04, 2023, 09:17:57 pm
content: So instead of a vector network analyzer, you need a scalar network analyzer. There are some old HP units on ebay that would do this, but only if you can get a deal.Depending on your amplifiers, a VNA could be a bad choice. They expect 50 Ohm input and output, plus you can blow them up with just a bit of power.Considering you already have a nice 300 MHz scope, I would get a used signal generator, and program the thing up to make sweeps. Probably a Z0 probe would help on the scope side.Or you get the bonus round: Get a RF white noise generator to 1GHz, and record the bode plot with the FFT of the scope.

author: 2N3055
author_rank: Super Contributor
timestamp: October 04, 2023, 09:49:02 pm
content: Quote from: tszaboo on October 04, 2023, 09:17:57 pmAllow me a bit of pedantry here: If it is only magnitude plot (no phase info) than it should not be called Bode plot but simply frequency response plot... Just saying--

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2023, 10:15:47 pm
content: Quote from: 2N3055 on October 04, 2023, 09:49:02 pmSure with a through measurement of an active device one needs be careful VNA input limits are adhered to but Phase measurements with cheap VNA's are certainly possible and often the # of measurement types exceed the # of traces available.Quick example from a random selection of antennae tested in this thread:https://www.eevblog.com/forum/rf-microwave/what-really-is-this-antenna/

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 04:09:14 am
content: My software for the LiteVNA supports Bode, stability circles .... You stated that the squarewave drive is no problem for you. I think the issue is that the Lite's output isn't leveled. From min to 500MHz, the one I am looking at changes about 3dB. If you were trying to measure say where the amplifier was starting to compress and you were using a wideband like this, I think you would run into problems. You give up a lot of basics when you only pay $120.

author: rf-loop
author_rank: Super Contributor
timestamp: October 06, 2023, 07:12:10 am
content: Quote from: 2N3055 on October 04, 2023, 09:49:02 pmJust saying...Whole Bode plot:- - Bode magnitude plot- - Bode phase plotWe get (least with Siglent Bode Plot (hopping swept frequency response analyzer)) both but in some cases Bode magnitude plot is enough.Scalar analyze (Bode magnitude plot) is still good for many things.But example with Siglent oscilloscope Bode Plot we can not do just scalar plot only. (what can also be bit faster)This is one thing what is in wish list... so that user have also some faster options depending his variable needs.

author: tszaboo
author_rank: Super Contributor
timestamp: October 06, 2023, 07:48:07 am
content: Quote from: rf-loop on October 06, 2023, 07:12:10 amIMHO he is right. But I found phase plot to be less usefult, unless I have a sine and cosine wave I/Q data for example.I see an 1 GHz white noise source for about 30 USD (don't know how good quality is) so OP really needs to consider whether paying a lot more for phase information is worth it. Even if the white noise source is not flat to 1 GHz it's possible to measure it and compensate the results with the source magnitude difference.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 12:26:19 pm
content: Without all four S parameters, how do you measure the amplifiers stability? It's another shortcoming of the low cost VNAs out of China, 2-port 1-path. My work around was to add a transfer relay. Best thing would be to buy better equipment. You could have a look at Picotech's low cost 4-receiver VNA.https://www.eevblog.com/forum/testgear/vna-suggestions-for-10k-usd/msg5072017/#msg5072017

author: alm
author_rank: Super Contributor
timestamp: October 06, 2023, 12:51:05 pm
content: The VNWA 3SE (latest version) also incorporates a transfer switch so it can measure all four S parameters without changing connections. I believe there's support for using an external bridge to deal with power amplifiers. Seethis recent topicfor some pointers.I'm skeptical about the ability to get useful measurements at hundreds of MHz using a high impedance environment, since there is no such thing as high impedance at higher frequencies. Similar to using 10x scope probes for hundreds of MHz. They work, assuming you are have a very well behaved low impedance source. The impedance fluctuations over the bandwidth would be huge, likely affecting the measurement.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 01:11:11 pm
content: Does their software also calculate stability? Hard to say about working with high impedance's and RF without knowing more of what the goals are. Waiting for lasmux to release their active probe.https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4973839/#msg4973839

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 01:18:51 pm
content: I've been playing around a bit with rectangular waveguides in the 8-12GHz range. An open with waveguide is a bit meaningless when it comes to calibration. Instead, we mechanically offset the short to shift the phase. Here I walk through the calibration for another member and compare the results with my old Agilent PNA:https://www.eevblog.com/forum/rf-microwave/experimenting-with-waveguides-using-the-litevna/msg4801919/#msg4801919

author: mawyatt
author_rank: Super Contributor
timestamp: October 06, 2023, 02:28:34 pm
content: Quote from: alm on October 06, 2023, 12:51:05 pmHere's some interesting links to making relatively high impedance measurements at higher frequencies, but not necessarily Bode Plot type measurements (especially "Closed Loop"), but more of a relative type measurement.Dr Cripps simple yet highly effective "SMUI" an unterminated semirigid cable with SMA has proven quite useful in our past, allowing effective but not too intrusive voltage measurements. We also used some of these voltage probes with micro-coax and wafer probe fixtures for finer measurements with our custom broadband chips and hybrids. Of course these are for relative measurements in response to the near E-field, but our experience was they did not perturb the DUT too much and proved quite useful.Another interesting use of such was the near field microwave microscopyAnyway, thought some might find this useful!!Best,https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiv6OrmxuGBAxXSM0QIHWfTBMgQFnoECCIQAQ&url=https%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F6668%2F4753970%2F04753992.pdf&usg=AOvVaw2RuytMvnomfmQi2IvpcRSw&opi=89978449https://www.academia.edu/28719448/Cripps_Steve_C_RF_power_amplifiers_for_wireless_communications_ourdev_672470C259C

author: Martinn
author_rank: Frequent Contributor
timestamp: October 06, 2023, 03:41:21 pm
content: Quote from: tszaboo on October 06, 2023, 07:48:07 amLike this onehttps://www.aliexpress.com/item/4001032161351.htmlWhat about dynamic range/linearity requirement vs. swept sine?Regarding "Bode" phase or just amplitude: I guess for verifying a 100 MHz scope frontend amplitude only would be fine. Not sure how much extra information there would be compared to looking at a 3 dB roll-off only. Maybe if there's some peaking inbetween might be more obvious in the phase plot.

author: Martinn
author_rank: Frequent Contributor
timestamp: October 06, 2023, 03:51:40 pm
content: Quote from: joeqsmith on October 06, 2023, 12:26:19 pmI'm not a RF guy so bear with me. Why would I need anything else than in-out (S21) for verifying if a DC-100 MHz low noise amplifier would be stable? You could either look at the transient response (excessive ringing) or the bode amplitude (peaking) to find out its stability margin.Quote from: alm on October 06, 2023, 12:51:05 pmAgain, why would I need this (besides S21)?Quote from: alm on October 06, 2023, 12:51:05 pmYes and no.Yes above some MHz you have to be very careful with layout and wiring.No - high Z scope inputs with 500 MHz BW exist, so do probes - so why not? If I have any high Z amplifier, just solder a 50 ohms terminator at its input, place a SMA connector across it and feed it with whatever source. This should be OK for 100 MHz easily (again, I am not talking about GHz measurements, I guess above 300 MHz things get complicated).As a matter of fact, one thing I'd like to try is a classical DC servo JFET analog scope frontend (which most likely even my RTB2004 has). For measuring, I'd terminate the input with 50 ohms; output would be somewhat lower impedance anyway or I'd insert a buffer (normally, next stage would be PGA/VGA driving a differential ADC input, which I don't want to mess with.

author: alm
author_rank: Super Contributor
timestamp: October 06, 2023, 04:06:56 pm
content: Quote from: Martinn on October 06, 2023, 03:51:40 pmWhat is the impedance of such scope input or probe at 500 MHz based on the capacitive reactance? Would you really call that high impedance?

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 04:18:22 pm
content: https://eng.libretexts.org/Bookshelves/Electrical_Engineering/Electronics/Microwave_and_RF_Design_V%3A_Amplifiers_and_Oscillators_(Steer)/02%3A_Linear_Amplifiers/2.06%3A_Amplifier_StabilityQuote from: alm on October 06, 2023, 04:06:56 pmIf I could only find scope probes that didn't load a circuit.I posted some data for the probes I have in that thread. Also showed the loading effects with one of my 10x probes compared with a resistive probe and where the cross over was. Agree with your point, it's not simple and it's costly. We will wait and see how Lasmux's low cost (<$250USD) probes work.https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4988716/#msg4988716************Video on stability:

author: mawyatt
author_rank: Super Contributor
timestamp: October 06, 2023, 05:19:53 pm
content: Quote from: Martinn on October 06, 2023, 03:51:40 pmAnother "tool" from way back 4~5 decades ago was what we called the "Pinger". This simply was a narrow pulser (CMOS ACT logic) configured into an old Tek scope probe which produced a Dirac Doublet type waveform (+ and - Impulse with 0 average to not upset the DUT DC bias balance). A Pot and High valued R allowed setting the amplitude to a low enough level not to over drive the DUT, and not load the DUT too much.Since just about any practical multi-ordered "linear" system can be distilled into a simple 2nd order system with reasonable fidelity, the "Pinger" would excite the system and one could monitor the response on a scope. The stability could easily be deduced from the response and related to a 2nd order system with an effective dampening and/or closed loop phase/gain margin.The "Pinger" proved quite useful and allowed signal injection into various parts of the circuit, especially valuable for "Closed Loop" systems, where opening the loop to inject the test signal can be difficult or dangerous.Somewhat surprised that something similar has not been offered commercially, or least we are not aware of such??Best,

author: joeqsmith
author_rank: Super Contributor
timestamp: October 06, 2023, 06:29:21 pm
content: Looking at the step response for low frequency amplifiers is still common. Ping it as you say with different loads.https://www.ti.com/lit/an/slva381b/slva381b.pdf?ts=1696608820902&ref_url=https%253A%252F%252Fwww.google.com%252F

author: Martinn
author_rank: Frequent Contributor
timestamp: October 08, 2023, 02:34:52 pm
content: Quote from: joeqsmith on October 06, 2023, 04:18:22 pmI'm also a big fan of active FET and Z0 probes.Yes I see your point. 10 pF will have an impedance of around 50 ohms at 300 MHz. So the "DC" approximation is only valid for either very low input capacitances or low bandwidth.Quote from: tautech on October 04, 2023, 07:33:43 amIs the SVA1015X as "upgrade-friendly" as the SDG6000X? I think when one would buy a VNA, it should cover obvious applications like bluetooth antenna matching, which would already require the much more expensive 3.2 GHz version.

author: tautech
author_rank: Super Contributor
timestamp: October 08, 2023, 05:35:41 pm
content: Quote from: Martinn on October 08, 2023, 02:34:52 pmNo.You need get to the 2.1 GHz (different HW) SSA3021X Plus analyzer before any upgrade tricks can be applied to get to 3.2 GHz VNA capabilities. Further study will disclose paths to follow.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 08, 2023, 06:17:36 pm
content: Quote from: Martinn on October 08, 2023, 02:34:52 pmI would have liked to have found a used one that would cover 9kHz or so for PDN and other measurements. The old Agilent PNA I bought is limited to 300kHz and so I still use the old HP network analyzer for low frequencies.Some time ago I made an attempt at measuring a BLE antenna and matching circuit. You can see my botch job of attaching coax to it and then looking at the radiation pattern of the antenna. For S11/S21, the LiteVNA is fine. It's problem is active circuits like you asked about. Still, for a $120, it's a nobrainer to learn on.https://www.eevblog.com/forum/testgear/sub-$1000-vna-for-antenna-matching/msg4552187/#msg4552187