dear v. mugandhan, as you probably know, measuring the spectrum of an adc by terminating it's input or leaving it open is not very useful: adc's have a DC offset that changes with temperature. if the DC offset happens to be near an edge between two ADC staircase levels, the adc will chatter like crazy between these two levels, even a few microvolts of noise or RFI will cause the ADC to chatter between two or three levels, and the spectrum will be dominated by interleave spurs and RFI. when the offset drifts so it's right between two levels, the spectral spurs and RFI will be smaller... if you add a few LSB of noise or signal, then the pattern is not dependent on the DC offset.
best wishes, dan Dan Werthimer Marilyn and Watson Alberts Chair Astronomy Dept and Space Sciences Lab University of California, Berkeley On Fri, Sep 6, 2019 at 9:51 PM Mugundhan vijayaraghavan < v.vaishnav151...@gmail.com> wrote: > Hello, > > Since there is an interest here in red pitaya based designs, I present > here one of my recent trials with the board. I was able to implement a > single channel autocorrelation spectrometer with a 4096 point (2048 > positive frequency channels) in the red pitaya (125-14) using a combination > of the casper signal processing blocks and some HDL. The design is as > follows: > > The ADC data is converted to 2s complement, and this is input to a > simulink ip block, instantiated in the vivado block design. In simulink, > the ADC data is polyphase filtered and FFT'd to 4096 points and the FFT > spectra is accumulated for 4096 (fixed as of now) times, giving an > effective integration time of ~ 130 ms. The accumulated spectrum and the > respective data valid is the output of the simulink block. This is then > generated as an IP repository for the Zynq-7010 FPGA, and is imported as an > IP to the block design. The data from the spectrometer block is recast as a > 64 bit word (two spectral points concatenated) using the AXI DWIDTH > converter block and the data is written into a BRAM using Demin's BRAM > writer block. There is a status signal which informs the BRAM that the > samples have been written into the former, and then the PS starts reading > this BRAM as a memory mapped IP, using Pavel Demin's BRAM reader core. The > c program needed for the control of the spectrometer is executed in the > linux running on the RP's PS. > > I carried out a preliminary testing using noise signals (attached sample > spectra). I find that in the spectrum there are spurs at fs/4+/-fs/8, > fs/4+/-fs/16 and so on, which show up when using a 50 ohm termination at > the ADC input or keeping the input open. When providing a noise signal of ~ > -30dBm strength, these spurs disappear, suggesting they are additive in > nature. I'm investigating the spurs presently ! > > Attached with this mail is the github repo that contains the vivado > project tcls, block diagram files, the slx files for the simulink design > and the C code for the control and acquisition. I'd be glad if people in > the community use this and see if they are able to get similar or better > results ! I'm also working on porting the design to a 2 channel PoCo like > design, which I'll share here once the preliminary tests are done. > > github link: https://github.com/mugundhan1/rp_fft_spectrometer.git > > Note: Please include pavel demin's bram axis writer and bram axi reader > ips to the IP repo path in vivado (from > https://github.com/pavel-demin/red-pitaya-notes/tree/master/cores). I > have not uploaded all the files generated by vivado itself as the repo size > becomes close to a GB then. Any feedback is definitely appreciated ! > > > -- > V. Mugundhan > > -- > 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/CAD560x%3DaCXKWW5M3ypsHmARDfHrGqW-JMWFDQTSv2Fqc22FFHA%40mail.gmail.com > <https://groups.google.com/a/lists.berkeley.edu/d/msgid/casper/CAD560x%3DaCXKWW5M3ypsHmARDfHrGqW-JMWFDQTSv2Fqc22FFHA%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/CAGHS_vFC7g_2Z_hH6jCsgAe1SSLUjzuOwCWmQ1vGTc%3DtYieF7w%40mail.gmail.com.
