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| Admittance Measurements with DSO & AWG with Bode Function |
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| TopQuark:
--- Quote from: nctnico on November 01, 2022, 05:08:38 pm --- --- Quote from: TopQuark on November 01, 2022, 03:50:56 pm --- --- Quote from: nctnico on November 01, 2022, 03:40:02 pm --- --- Quote from: 2N3055 on October 30, 2022, 08:49:20 am --- --- Quote from: Mechatrommer on October 30, 2022, 08:12:05 am ---i only know programmed rigol through visa and usb port, automation is possible. because thats only what i have. other brand you need to make yourself. i recently purchased owon scope meter, i saw some api doc but i dont have time for it. --- End quote --- Difference is that you pulled raw data and did all math yourself. TopQuark is talking about fact that those Siglent scopes have magnitude/phase measurement (Bode plot) built in, and he would like to automate that and get that data.. --- End quote --- How is that any different than using magnitude and phase measurement functions? Since you'll only need these measurements for a single or limited number of frequencies, that route will be much quicker. I don't see the need for using bode plot at all. --- End quote --- I have thought of rolling my own bode plot program that controls the instruments through SCPI, and reading the amplitude and phase measurements with the scope built-in math functions. Main problem with using the scope's amplitude and phase readout is, the math functions are not frequency selective, so in noisy measurements (e.g. measure loop response of a fast switching smps), the measurement might be inaccurate. --- End quote --- For bode plotting yes (even though waveform averaging could help improve things). For measuring components with a known (and controlled) sine wave source you should be able to get rid of noise using high-res and/or averaging modes. With software at the PC side, you could even average several readings and still have a reasonably quick update rate. The higher end LCR meters typically have a selectable update speed and also allow extra averaging for further noise reduction. --- End quote --- 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. |
| nctnico:
For using a known source, I assume there is a good signal to noise ratio without too much external interference. Averaging should make things better though because interference is likely to be random (noise) compared to the source's frequency. If your oscilloscope has the ability to filter, this would also help. 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. Synchronous sampling -as mentioned by mawyatt already- would be another option but for most accurate results you'd need to have the generator (DAC) clock locked to the sampler (ADC) clock. Still, it takes processing quite a few cycles to get to accurate results. In the past I have made an LCR meter-ish device to measure micro-Ohm level AC impedances on battery cells. The circuit itself is pretty crude using a relatively crappy DAC and a 10 or 12 bit ADC in a microcontroller. |
| TopQuark:
--- Quote from: nctnico on November 01, 2022, 05:24:14 pm ---For using a known source, I assume there is a good signal to noise ratio without too much external interference. Averaging should make things better though because interference is likely to be random (noise) compared to the source's frequency. If your oscilloscope has the ability to filter, this would also help. 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. Synchronous sampling -as mentioned by mawyatt already- would be another option but for most accurate results you'd need to have the generator (DAC) clock locked to the sampler (ADC) clock. Still, it takes processing quite a few cycles to get to accurate results. In the past I have made an LCR meter-ish device to measure micro-Ohm level AC impedances on battery cells. The circuit itself is pretty crude using a relatively crappy DAC and a 10 or 12 bit ADC in a microcontroller. --- End quote --- In an ideal world I'd give all the options above a try and see what works best. But given my limited time, I think I'll put my time developing my DSA project, which in theory could give a bode plot every few milliseconds (if it works) ::) |
| mawyatt:
--- 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. --- End quote --- 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. In a way the FFT is doing something similar, placing narrow Frequency "bins" to isolate energy within those frequency bins span and reject outside signals. A really good demonstration of just how good the Siglent implementation of the Bode Function is in this threads about the Peltz Oscillator Close Loop Response and Injection Locking of such. Still amazed that these kind of plots could be pulled off with a simple DSO & AWG ;) https://www.eevblog.com/forum/projects/things-coming-together-bode-plot-diy-isolation-transformer-peltz-oscillator/msg4288363/#msg4288363 https://www.eevblog.com/forum/projects/injection-locked-peltz-oscillator-with-bode-analysis/msg4424434/#msg4424434 Best, |
| gf:
--- 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. --- End quote --- 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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