Cheap bode plot device... - Page 1
Forum section: electronics / testgear

author: bsas
author_rank: Regular Contributor
timestamp: June 24, 2022, 09:18:50 pm
content: Can you guys recommend any cheap device that can do bode plotting for audio frequency (20Hz-20kHz)?I currently have my all time favorite Rigol DS1054Z fully "unlocked" and I have a cheap 15Mhz signal generator (Seesii something, but it clearly looks like a rebranded FY6600) but it is very annoying and hard for me to make bode plot with those...I also have a Analog Discovery 2. No idea if it can do bode plot (it should, I think) but it is annoying to have to have a laptop every time I want to use it.I saw those all in one DMM/OSC/DSG like the OWON HD272S and I was wondering if it can do it.Thanks!

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2022, 09:25:13 pm
content: Go hunt out the now quite old EEVblog video.....

author: bsas
author_rank: Regular Contributor
timestamp: June 24, 2022, 09:33:46 pm
content: If you are talking about the "EEVblog #396 - Bode Plotting on Your Osciloscope" video, yea, that is the way I am doing. But it is not practical sometimes...

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2022, 09:36:22 pm
content: Quote from: bsas on June 24, 2022, 09:33:46 pmYep, that one.Do you need a higher dynamic range ?

author: bsas
author_rank: Regular Contributor
timestamp: June 24, 2022, 09:41:17 pm
content: Well, I want something easier to setup and nicer to look at. And if possible something I could check log scales and stuff like that... Basically, an official bode plot

author: tautech
author_rank: Super Contributor
timestamp: June 24, 2022, 09:45:36 pm
content: Quote from: bsas on June 24, 2022, 09:41:17 pmWell, all is not what it seems....you still have to learn and know how to drive them properly.If you can get the results you need using the scopes measurements and cursors then it's doing the job.

author: Martin72
author_rank: Super Contributor
timestamp: June 24, 2022, 10:30:49 pm
content: QuotePC/Notebook/Tablet, Soundcard/-Interface and ARTA software.ARTA is free but you can´t save results until you buy a license.Mostly the first two things everyone got at home, so you would only spend 79€ for the license.Pic below shows a loop measure in/out, soundinterface focusrite, 24bit/192khz.You can display magnitude, phase, phase+magnitude.ARTA allows also to make distortion measures.With a couple of adaptors, it´s much more useful/powerful than simple bodeplot on a scope, IMHO.

author: thm_w
author_rank: Super Contributor
timestamp: June 24, 2022, 10:32:58 pm
content: What is the problem with needing a computer to use the analog discovery 2 exactly, you are doing bode plots in the field?Quote from: Martin72 on June 24, 2022, 10:30:49 pmOP already has AD2 which serves the same purpose.But if they are specifically doing audio measurements, then yeah, this could be better.

author: Fungus
author_rank: Super Contributor
timestamp: June 24, 2022, 10:33:51 pm
content: Quote from: bsas on June 24, 2022, 09:18:50 pmIt can, and the Digilent software will even do it via your sound card at audio frequencies.

author: Martin72
author_rank: Super Contributor
timestamp: June 24, 2022, 10:54:56 pm
content: Quote from: thm_w on June 24, 2022, 10:32:58 pmOverlooked this, you´re right of course.Basically, pc with software and a good soundinterface/-card are much more flexible/suitable for audio purposes.I´ve used this combo until I got a Neutrik A1...

author: bsas
author_rank: Regular Contributor
timestamp: June 24, 2022, 11:51:37 pm
content: OK, sorry for my ignorance, how can I do it with the AD2?

author: Fungus
author_rank: Super Contributor
timestamp: June 25, 2022, 12:37:10 am
content: Quote from: bsas on June 24, 2022, 11:51:37 pm(LMGTFY)

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2022, 02:55:09 am
content: Quote from: bsas on June 24, 2022, 09:18:50 pmIsn't it enough to have the automatic measurements on the Rigol DS1054Z? When drawing the bode plot by hand, it is useful to know that the slope of the dB/octave line is proportional to phase. I make amplitude measurements at increments of 45 degrees.

author: Fungus
author_rank: Super Contributor
timestamp: June 25, 2022, 06:00:07 am
content: Quote from: David Hess on June 25, 2022, 02:55:09 amWhat people need is a better signal generator. Back in the old days they'd have a big dial and it was easy to sweep the frequency up and down and see any dips and troughs on the oscilloscope. Now it's all buttons and rotary encoders, AWG this, waveform that... and it doesn't work any more.

author: bsas
author_rank: Regular Contributor
timestamp: June 25, 2022, 06:27:39 am
content: To be fully honest I am still quite a beginner and learning. So, doing those stuff "by hand" or doing some clever stuff like the Youtube tutorial gets me more confused than I would like to admit... I know it is my fault, but, well, I need to be honest with myself

author: Fungus
author_rank: Super Contributor
timestamp: June 25, 2022, 06:30:39 am
content: Quote from: bsas on June 25, 2022, 06:27:39 amIf you put a sine wave on screen then sweep the frequency around manually you can easily see when the amplitude changes.Another trick is to feed it white noise then look at the spectrum - all the frequencies sumiltaneously!

author: Warpspeed
author_rank: Contributor
timestamp: August 24, 2022, 08:59:30 am
content: I have always wished to have something to accurately measure relative amplitude and phase that did not cost a king's ransom to take Bode plots from an actual real working circuit.I bought myself an ancient HP3575A phase meter a long time ago. These are now getting rather old, but still excellent, and I picked one up from e-bay for a couple of hundred dollars which is typical. The specifications are pretty good 3Hz to 11Mhz, phase to 0.1 degrees resolution, absolute and relative amplitude measurement to 0.1db resolution.And a full service and repair manual is available free from the internet.If you are only interested in analog circuitry, amplifiers, filters and such, its probably all you need, along with a suitable function generator of course.Sadly it will not work with switching power supplies, it needs reasonably clean signals. Trying to resolve millivolt signals below volts of switching spikes and a very noisy ground is just not practical with either an oscilloscope or something like the HP3575A.So at the moment I am building a very narrow dual channel tracking filter for it. Basically two channels tunable from a very few Hz to 40Khz with only about 3Hz bandwidth. That should block the wideband switching noise and allow only the specific frequency of interest to reach the HP3575A.Still working on this right now and so far its looking quite encouraging.Just discovered this thread while surfing the internet looking to see what other people are doing to take Bode plots on real functioning equipment, and not just playing with pie in the sky circuit simulations.Simulations are wonderful things, a world where noise problems do not exist, there is infinite dynamic range, and components are perfect.The real world is rather more hostile...

author: jasonRF
author_rank: Regular Contributor
timestamp: August 24, 2022, 11:05:31 am
content: Quote from: Warpspeed on August 24, 2022, 08:59:30 amIf you have a digital scope that can save waveforms, then you can import the data into your favorite software (I use Octave) and use FFTs to conceptually do the same thing. The longer your waveform captures, the narrower the effective filter bandwidth is, and the more gain you get on the sinusoid of interest to pull it out of the noise. The signal phase shift is the same as the relative FFT phase at the frequency of interest. Don't forget to use a window so the sidelobes of the noise don't swamp your signal of interest.The simple way of doing this does require saving one file for each frequency of interest. So it isn't fast, but it can work. I have never tried it with super noisy data, but I have done it using a square-wave source and just wanted to use the fundamental to measure frequency response.jason

author: mawyatt
author_rank: Super Contributor
timestamp: August 24, 2022, 12:28:45 pm
content: If you search using Bode Plot, you'll find all sorts of good information on Bode Plots. Some of which are related to the Siglent DSO which host a nice Bode Function that employs a "Frequency Selective" feature to help out of band signals & noise. These also use Dynamic Input scaling to achieve large effective Dynamic Range performance. Here's a few we've been associated with:https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/msg4182022/#msg4182022https://www.eevblog.com/forum/projects/things-coming-together-bode-plot-diy-isolation-transformer-peltz-oscillator/msg4288363/#msg4288363https://www.eevblog.com/forum/projects/bode-routine-shows-op-amp-output-z/msg4307539/#msg4307539https://www.eevblog.com/forum/testgear/capacitive-impedance-plots-with-sds2104x-plus-bode-function/msg4335745/#msg4335745Best,

author: jasonRF
author_rank: Regular Contributor
timestamp: August 24, 2022, 02:00:12 pm
content: Those Siglent results look very nice! Channel separation on that scope is excellent. Perhaps unless someone writes their own code to control instruments, I suspect that is the most cost-effective way to get automated Bode plots up to those high frequencies.I have a couple of Picoscopes, and the free Bode plot app also does the dynamic scaling. My lowly 2204a has a siggen that is limited to 100 kHz, and the combination of low siggen output voltage (2V peak) and scope sensitivity (10 mV/div) limit dynamic range to 70 dB or so. I posted an example herehttps://www.eevblog.com/forum/testgear/picoscope-2000/msg4129702/#msg4129702When that was my only scope, I used my slow method and an external siggen (that only has front-panel controls) to get Bode plots up to 12 MHz. Of course my slow way also has dynamic scaling, since the user needs to adjust the scope before saving each waveform.Jason

author: tautech
author_rank: Super Contributor
timestamp: August 24, 2022, 08:14:48 pm
content: Quote from: jasonRF on August 24, 2022, 02:00:12 pmmawyatt's Bode plot examples are done with the SDS2000X Plus whose capability follow on from when Siglent introduced Bode plot in the release SDS1004X-E DSO's that unlike SDS2000X Plus models which have an internal AWG the 1000X-E models can pair with any Siglent AWG via LAN or USB or control Siglent's SAG1021I USB powered AWG for the Bode plot stimulus.https://siglentna.com/product/external-arbitrary-waveform-generator-2/The 1000X-E bode plot feature is examined in depth here:https://www.eevblog.com/forum/testgear/siglent-sds1x04x-e-bodeplot-ii-(sfra)-features-and-testing-(coming)/

author: Warpspeed
author_rank: Contributor
timestamp: August 24, 2022, 10:54:09 pm
content: Some excellent suggestions there Jason.My problem is bulk noise swamping the low amplitude signal, mainly high amplitude switching spikes producing broad band hash.I will continue on with my tracking filter as its already about half built.The idea is that I have an LC oscillator tunable between 100Khz and 140Khz. There is also a 100Khz crystal oscillator using a high quality reference crystal. These two sinusoidal signals go into an analog multiplier which produces a nice clean 0 to 40Khz output after some low pass filtering.This 0 to 40Khz drives an injection transformer and becomes the testing frequency.The two noisy recovered signals go into two identical channels. Each consists of another analog multiplier fed with the recovered signal (0 to 40Khz) and the output of the LC oscillator (100Khz to 140 Khz). That then passes through a crystal bandpass filter at 100Khz which has a -6db bandwidth of about 3Hz and pretty steep attenuation either side.The filtered 100Khz signal then go into a third analog multiplier again mixed with the LC oscillator frequency to get back down to baseband 0 to40Khz.Its all fairly simple. So far I have both the 0 to 40Khz test signal and the 100 to 140Khz carrier frequency generated.I have breadboarded the filters and up/down frequency conversion, but need to get a proper board made before I can do any serious test and measurement and get some performance figures.The bandwidth is so narrow, its not really possible to sweep the frequency, it would take forever to cover any reasonable bandwidth. But I can turn the knob on my LC oscillator to any frequency of interest and the whole thing settles down in a couple of seconds and I should be able to make pretty noise free measurements.I may need some switched gain/attenuation ahead of the first analog multiplier, but it may also be workable without. Its been a bit of work, but not an expensive project. Results so far have been very encouraging.

author: Someone
author_rank: Super Contributor
timestamp: August 24, 2022, 11:31:45 pm
content: Quote from: Warpspeed on August 24, 2022, 08:59:30 amQuote from: Warpspeed on August 24, 2022, 10:54:09 pmIt is common/routine to measure sub milivolt phasor signals in switchmode power supplies, to characterise the control loop(s).Here is a good reference going into some of the dynamic range limitations:https://fscdn.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/fra_phase_margin_appli-e.pdfsmall signals which scopesareable to measure, as jasonRF mentions above you need more data to improve the filtering of the noise. Just as a narrow bandpass filter has a long settling time, the fundamentals apply in analog or digital. Now that cheap scopes have 10M or 100M of sample memory its often easier to use (offline) digital processing rather than analog hardware. There can be multiple frequency selective steps, modern scopes have high-resolution, averaging, and sometimes other filtering available each with their own multiplicative effects through the processing chain.

author: Warpspeed
author_rank: Contributor
timestamp: August 25, 2022, 12:05:23 am
content: No doubt its possible, but the wide available bandwidth of an oscilloscope is pretty much self defeating when dealing with very broad spectrum high amplitude noise.There is also the problem of judging exact phase and amplitude difference between two fuzzy waveforms even after a huge amount of sample averaging.I will continue on with my project just out of sheer curiosity. It may be the old fashioned way of doing things, but it should still work for me.Radio receiver technology does pretty much the exact same thing, pick out one very weak signal out of a whole spectrum of chaos.

author: mawyatt
author_rank: Super Contributor
timestamp: August 25, 2022, 01:03:07 pm
content: @Warpspeed,Understand the"curiosity"aspect, see note belowSince you are using a tracking filter to select the signal of interest, have you considered "Synchronous Sampling" or a "Commutating Filter" techniques. These are easily implemented with standard CMOS and tune with the clock.Best,

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 25, 2022, 10:11:06 pm
content: Quote from: Fungus on June 24, 2022, 10:33:51 pmDid not know Waveforms worked with other hardware, will be trying this at work today! That could be super handy for callouts to studiosEDIT- Actually... thinking about this I think I DID know this, but had issues with it and didn't go any further. Will try again either way

author: Warpspeed
author_rank: Contributor
timestamp: August 25, 2022, 11:08:39 pm
content: @MawyattYes indeed, many ways to skin a cat.From the completely nerdy software/mathematical approach, to the down and dirty nuts and bolts hardware approach.I am at home and retired a long time ago, plenty of time on my hands, and this project is mainly a time filler, something I have been thinking about doing for a very long time..Also, have a lot of hardware here, remnants of a lifetime of messing about with home brew amateur radio. So the tracking filter approach has a lot of appeal, and is something I feel very confident of achieving good results with.I also have quite a few very nice 100Khz vacuum mounted crystals of Russian origin, that are perfect for a pair of matched super sharp bandpass filters.Its all coming together quite nicely too, and there have been far fewer problems than I expected.Its becoming a rather interesting project.

author: JeremyC
author_rank: Regular Contributor
timestamp: November 04, 2022, 05:35:14 pm
content: Quote from: bsas on June 24, 2022, 09:18:50 pmIf you are working with audio, you may find interesting links below:- RightMark audio analyzer,https://audio.rightmark.org/index_new.shtml- REW,https://www.roomeqwizard.com/QuoteYes, the AD2 has Bode plot option, in the WaveForms it’s called “Network”, check thelink, it may give you some ideas.