Hi Ben, I found your issue interesting and took a look at your data. I notice your sample rate period is changing every few samples. Could this be causing some aliasing in the data.
If the sample clock is being derived from a fractional n PLL this is normal behavior for the clock. If you change your sample rate to be an integer multiple of the reference clock in the system the sample periods will all be the same and the data should smooth out. This may not be the issue you have but I thought I would mention it here. I normally just read along but to pass this along. I will be interested to see what you find. Bob Stricklin On Dec 9, 2021, at 9:49 PM, 'Benjamin Godfrey' via casper@lists.berkeley.edu<mailto:casper@lists.berkeley.edu> <casper@lists.berkeley.edu<mailto:casper@lists.berkeley.edu>> wrote: Hi Tony, I do not think there is a header. At least I am collecting as much data as I expect to collect, and there isn't anything obviously amiss at the start/end of the acquisition both from plotting the ADC-count spectrum and also looking through the start/end of the text file the ADC data is stored in. - Ben On Thu, Dec 9, 2021 at 7:35 PM Tony Goodson <aggoods...@gmail.com<mailto:aggoods...@gmail.com>> wrote: Hi Ben, Does the data have a header that hasn't been removed? An unremoved header will produce this effect. --Tony Goodson On Thu, Dec 9, 2021 at 6:52 PM 'Benjamin Godfrey' via casper@lists.berkeley.edu<mailto:casper@lists.berkeley.edu> <casper@lists.berkeley.edu<mailto:casper@lists.berkeley.edu>> wrote: Hi everyone, I was hoping you could help me with a bit of a mystery. I'm working on an experiment looking for a Q~10^6 signal embedded in broad-spectrum noise. To this end, we are using a ROACH-2 board with a KatADC, to sample the incoming signal, and then offloading the data to a PC where a GPU performs a (large) FFT. I have been doing some basic tests recently, injecting a sine wave from a signal generator into the KatADC. Using a commercial swept analyzer, I have verified that the injected signal is spectrally pure. Looking at the data [2^(24) samples] sampled by the ROACH in the time domain, I see the following: <image.png> This looks a little modulated, but it could be a sampling effect. The FFT of the previous signal looks very odd though. The top image below shows the full-span view of the FFT of the same injected signal (-30 dBm at 120 MHz). The bottom two images below show zoom-ins around the central peak. <image.png><image.png><image.png> As you zoom in the FFT looks oddly modulated, and it becomes very difficult to make out much of a peak at all. I'm not windowing so I expect some scalloping loss, but this is way beyond what I would expect. I've confirmed that I'm not dropping packets by appending a timestamp to the end of each data frame so I'm not sure what is causing this. I'd greatly appreciate any input you have as to what may be behind this strange behavior. Thanks, Ben -- You received this message because you are subscribed to the Google Groups "casper@lists.berkeley.edu<mailto:casper@lists.berkeley.edu>" group. To unsubscribe from this group and stop receiving emails from it, send an email to casper+unsubscr...@lists.berkeley.edu<mailto:casper+unsubscr...@lists.berkeley.edu>. To view this discussion on the web visit https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAERfaAjmh%3D%3DOi_j-NXMko%2B%3Diy1i_BbwDiOR6sfdz4JrNZ4J25A%40mail.gmail.com<https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAERfaAjmh%3D%3DOi_j-NXMko%2B%3Diy1i_BbwDiOR6sfdz4JrNZ4J25A%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "casper@lists.berkeley.edu<mailto:casper@lists.berkeley.edu>" group. 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To view this discussion on the web visit https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAERfaAh6PP5Bm3tNbL0Y8nzabk150j5VahQ7AgjV%2BrB0o5TvAQ%40mail.gmail.com<https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAERfaAh6PP5Bm3tNbL0Y8nzabk150j5VahQ7AgjV%2BrB0o5TvAQ%40mail.gmail.com?utm_medium=email&utm_source=footer>. -- You received this message because you are subscribed to the Google Groups "casper@lists.berkeley.edu" group. To unsubscribe from this group and stop receiving emails from it, send an email to casper+unsubscr...@lists.berkeley.edu. To view this discussion on the web visit https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/A220F65D-8C69-45BE-B9DB-4E270B837858%40n5brg.com.
