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Admittance Measurements with DSO & AWG with Bode Function
2N3055:
--- Quote from: gf on November 01, 2022, 06:00:02 pm ---
--- Quote from: nctnico on November 01, 2022, 05:24:14 pm ---Not sure whether FFT is the perfect approach; I'd probably go for an algorithm that filters a single frequency to reduce the amount of processing needed.
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Then just calculate the DFT for a single frequency only. This will do both, bandpass filtering, and act as vector detector. The shape of the filter is determined by the chosen window function. A full FFT does the same for N frequencies simultaneously with O(N*log(N)) complexity, but there are constraints on the frequencies.
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Goertzel could be used for this, for instance..
Mechatrommer:
--- Quote from: mawyatt on November 01, 2022, 05:40:54 pm ---
--- Quote from: TopQuark on November 01, 2022, 05:16:27 pm ---Don't think averaging will do much good for isolating signals from a specific frequency. Think a better way would be to measure the gain and phase shift in the frequency domain with FFT, so that you can compare in and out at a specific frequency. Sounds like a lot of work though.
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Agree, simple averaging won't help much with strong interferers, especially ones that are not totally random during the sampling intervals. We ran into this problem long ago with a special test capability, no reasonable amount of signal averaging helped much and we decided to utilize synchronous sampling and place a low frequency integrator right on top of the sampling effect which translates into a narrow bandpass filter centered at the sampling frequency. This worked very well indeed! This is also the similar technique to what quality LCR meters utilize, Synchronous Sampling shines in this type interference environment.
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i'm not sure whats a synchronous sampling is but i did use FFT to get bode plot out of DS1052E and Hantek DDS3x25 automated for each sine sweep frequency back then.. but i never tested more complicated stuffs other than a simple capacitors and inductors. iirc do the full FFT, take the largest component's magnitude (assume its the fundamental, ignore the rest of bins) and compare the phase, take another sweep frequency and redo and plot, its been a while so i could be mistaken in the process, but attached was my plot, iirc no averaging needed.
but both my DS1052E and Hantek are now inoperational, and i dont have an urgent need anymore. i even experimented with square wave, its like feed one single square wave frequency say 1 or 10MHz, do full FFT, compare magnitudes and phases for all FFT components/bins and plot, very quick! but the result is noisy and less trustworthy (2nd attachment), i guess averaging or very good DSO/AWG front end to get perfect and stable square wave is needed to improve result. fwiw.
nctnico:
@Mechatrommer: AFAIK synchronous sampling is adding all samples from the incoming signal but flipping the sign at the place where you expect a zero crossing for the real part and flipping the sign at the peak for the imaginary part of the signal (if you are interested in that). At least, that is home I implemented it.
Brain fart: An alternative for implementing an LCR meter / bode plot function could be to use 2 IQ demodulators. One for the stimulus and one for the measured signal across the DUT. Adjust the modulation frequency & phase to have the imaginary part of the stimulus at 0, Low-pass filter the IQ signals from the DUT and you should have the real and imaginary parts. This should eliminate errors due to differences in frequency between the generator's clock and the sampler. From my experience with digital function generators I've seen that there are many that have a frequency offset due to limited DDS resolution. For example: 1Hz isn't 1Hz but 1.005Hz. Since the function generator inside a DSO is relatively simple, it would not surprise me if many suffer the same issue. If you measure over a lot of cycles, then this frequency offset could accumulate to a significant error when doing synchronous sampling.
gf:
--- Quote from: nctnico on November 01, 2022, 08:14:14 pm ---AFAIK synchronous sampling is adding all samples from the incoming signal but flipping the sign at the place where you expect a zero crossing for the real part and flipping the sign at the peak for the imaginary part of the signal (if you are interested in that). At least, that is home I implemented it.
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What you describe, I would rather call a synchronous detector.
Synchronous sampling IMO just means that the sample rate is an exact integral multiple (>2x) of the signal frequency, and that the measurement interval is an exact integral multiple of the signal period. This can be helpful for building a synchronous detector in the digital domain. DFT is eventually yet another synchronous detector too (for all bin frequencies), and so is Goertzel.
mawyatt:
--- Quote from: gf on November 02, 2022, 07:15:30 am ---
--- Quote from: nctnico on November 01, 2022, 08:14:14 pm ---AFAIK synchronous sampling is adding all samples from the incoming signal but flipping the sign at the place where you expect a zero crossing for the real part and flipping the sign at the peak for the imaginary part of the signal (if you are interested in that). At least, that is home I implemented it.
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What you describe, I would rather call a synchronous detector.
Synchronous sampling IMO just means that the sample rate is an exact integral multiple (>2x) of the signal frequency, and that the measurement interval is an exact integral multiple of the signal period. This can be helpful for building a synchronous detector in the digital domain. DFT is eventually yet another synchronous detector too (for all bin frequencies), and so is Goertzel.
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Agree, that "Synchronous Detection" is the technically proper term, however most of the folks we've known use Synchronous Sampling in the same context.
Another way of thinking about this technique is that the input signal is BiPhase Modulated with a squarewave of +-1 amplitude at the sampling frequency.
A recent discovery (~2008) we called the Polyphase Mixer, others prefer N-Path Mixer, is a multi-phase version of Synchronous Detection (think of this as Synchronous Detection with multiple N samplers equally distributed across the LO phase of +-pi). This technique has proved very useful in RF/MW/MMW use as it provided direct downconversion with I and Q baseband output and because it's a bilateral function allowed creating RF/MW/MMW complex impedance matching at the input without inductors or varactors, that tracked the LO!! Also it violates (measured below 2 dB) convention Biphase mixer minimum Noise Figure theory!! If interested, be sure to thoroughly read the IEEE articles mentioned, and follow all the related threads and such.
Absolutely fascinating circuit discovery, that has proven extremely useful. Believe Apple has employed such for some time now, and maybe this new technique will show up in some new LCR meters, or FRA, understand it's in a few high performance SDRs too ;)
https://www.eevblog.com/forum/rf-microwave/polyphase-or-n-path-mixer/msg3381802/#msg3381802
Best,
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