Kit to characterise passives, alternative to network/impedance analyzer? - Page 1
Forum section: electronics / testgear

author: nihtila
author_rank: Regular Contributor
timestamp: September 17, 2018, 02:42:01 pm
content: HiI would like to expand my bench measurement capability towards spectrum and/or impedance analysis. Right now I'd like to characterise passive components to see the effects of parasitics of capacitors, for example. Or see the impedance curve of several parallel capacitors or passive filters. Every now and then it would be nice to see spectrum of power supply switching and such as well.So far I've mostly done audio and slowish digital circuits. I do not do RF, at least yet, and I'm not very familiar with RF kit either. Regarding measurement kit, I only have 1GHz Keysight 3000 series scope.VNA would be nice although I'd have little use for actual VNA features at the moment but I'd be keen to learn by doing something completely new. But VNAs are expensive. I had a look at the new Siglent SVA1015X but am slightly put down by the lower end frequency limits of the VNA and TG.Spectrum analyzer with tracking generator would be nice kit to have, for example Rigol DSA815-TG. At the moment I could do most required things with that. Obviously there is no possibility to update for VNA later if I wanted, unlike the Siglent. However, SA capabilities seem better and TG goes lower. Regarding SA, my 1GHz scope can do simple FFT analysis as well although usability is quite bad. And its built-in generator can only go up to 20MHz.One option is a piece of USB kit such as TinyVNA or similar. Are there decent USB impedance analyzers? I'm not big fan of USB kit but given hobbyist's budget it may be the only option.As a starting point, not sure if it would be possible somehow to get a signal generator and combine it with my scope to get simple amplitude vs. frequency measurement. I guess this would mean a PC software to control both?Any suggestions? I know it's a bit silly question as I do not know exactly what I need, but could at least list options. Especially when it comes to USB kit I don't know what is available. Frequency range would be mostly limited by my budget.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: September 17, 2018, 03:23:19 pm
content: Quote from: nihtila on September 17, 2018, 02:42:01 pmI did mess with it some time ago, can do rough amplitude plot up to about 4x of scope official analog bw. Basically you create ref trace with sweep. Then create response trace and calculate diff in dB, get nice flat response trace. Can have go at phase also but unsigned so you need to have clear idea what you are testing. I have RFE6GEN 24MHz...6GHz gen for these purposes. Also I have RF Explorer 6G Combo. Together they form basic SNA. It is quite limited (and of course no phase) so I messed around a little with scope.If interested I could generate some example.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 17, 2018, 03:31:33 pm
content: If you just want amplitude information, there are a bunch of options that should work fairly inexpensively, and you'd get a bode plot or similar output. If you want full impedance measurement, you need phase information as well as magnitude, and while two channels of a scope could capture it and PC based software with a connection or something similar could do the math, the accuracy of the system is highly dependent on a high resolution converter and very controlled reference phase information.... which can be tough with discrete scope channels and external cables.For a bode plot, you can basically just sweep a function generator and leave persistence on on a scope in FFT mode and just see how it fares. Not going to be the same resolution as the ADC in a VNA (which means much lower dynamic range), but it will certainly give you an idea. You can also make or buy a white noise source, its power should be flat across the spectrum needed, and just run it through your component and look at the levels coming out in FFT mode again. Some of the more modern scopes can do the sweep version with their internal AWG or an external AWG by the same company.For a more complete VNA functionality, you'd need to split the tracking gen source into an output and a reference, then digitize both the reference and the signal through your DUT to measure magnitude and phase with as much detail and stability as possible, then apply the requisite transformations needed to convert that into impedance... but even at these lower frequencies there are a lot of potential parasitics that will have to be calibrated out, which is a bunch more software and math. For full transmission/reflection, you'd need a directional bridge and a third digitizer to get the reflected power, and then you're basically just building a full low frequency VNA.I would start with a setup that will make a bode plot and an LCR meter with several test frequencies.

author: nihtila
author_rank: Regular Contributor
timestamp: September 17, 2018, 03:52:36 pm
content: Thanks for the first responses. I wasn't even considering ref trace and persistence, was thinking too complicated.As I have fairly good scope, let's say I would start with amplitude information only before I know more what I could want or get. MrW0lf already mentioned RFE6GEN, but what would be other options for signal generator that gives me sinewave (or white noise) up to at least few hundred MHz, preferably to the 1GHz my scope can do?The scope indeed has an AWG and even frequency response analysis option that could give a bode plot with a button push. However, the generator only goes up to 20MHz. I may not need 1G but would be nice to get a bit higher than 20M.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: September 17, 2018, 06:52:42 pm
content: Quote from: nihtila on September 17, 2018, 03:52:36 pmQuite trivial. Here goes, 125...425Mhz sweep on Nagoya NA-773 antenna using $7 RF bridge:RFE6GC + RFE6GEN SNA @-27.7dBm:PicoScope 2408B + RFE6GEN @-0.9dBm:Math part:1) Freq trace: norm(T)*300000000+1250000002) Ref trace (cyan): 10+20*log(average(abs(A)))3) Plot trace (green) using same formula4) FR trace (red) using LowPass({Ref} - {Plot},25)Scope is spec'd 100MHz analog and was running at 2kSa/s.As can see notches little shifted in frequency etc. This should be no problem with 1GHz scope working in spec range. I'm a little over it... Also had to crank up signal level, maybe this has some effect also (on bridge).RFE6GEN is of course bit of PITA to work with compared to normal sig. gen. I'm considering Siglent SDG6000X when they iron firmware out a bit.Edit: What I forgot to mention is capacitive loading on setup by scope. You must somehow mask scope from DUT for that reason. For some cases 10x probe is enough. I did not deal with it in any way in this quick test.Myself I still do not have VNA because first I like to learn theory hard way and also for low frequency I have several options:Agilent U1733C LCR meter (...100kHz), Scopes using internal gen and having actual FRA software: Pico 2205 (bode with phase ...100kHz), Pico 2408B (...1MHz), AD2 (...10MHz). From these AD2 is 14bit and most advanced*, but with Pico 2204A could get ...100kHz bode plots for ~100€ to "patch" VNA blind spot.* For AD2 there is special helper fixture available for Z analysis:https://reference.digilentinc.com/reference/add-ons/impedance-analyzer/reference-manualhttps://store.digilentinc.com/impedance-analyzer-for-analog-discovery/

author: David Hess
author_rank: Super Contributor
timestamp: September 18, 2018, 10:56:52 am
content: Quote from: DaJMasta on September 17, 2018, 03:31:33 pmUnfortunately this does not return phase. For simple network analysis, it is easier to find the various breakpoints using phase rather than amplitude. If the function generator provides a trigger output or ramp, then the same amplitude versus frequency display can be produced without an FFT. With long record lengths and windowed measurement, even frequency and amplitude markers are available via automatic measurements.Where limited dynamic range is acceptable, just having the oscilloscope automatically measure amplitude and phase as the frequency is swept manually works and it is what I was going to suggest except nihtila's frequency requirements require something more. Averaging somewhat improves the dynamic range.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: September 18, 2018, 11:38:04 am
content: Quote from: David Hess on September 18, 2018, 10:56:52 amWorkaround for primitive 1ch generators: If generator can do sweep to sufficient freq so signal starts dropping due to scope finite bw can trigger off negative pulse width, use trigger holdoff etc because distinct step forms.