Hello David, A little late to the party but I thought I'd throw in 2c from our perspective.
The other thing we usually try to do with ADCs (this was an issue on the ROACH1s) is to put in a bit of white noise with whatever test signal we were using (sine wave usually). When you get actual sky, then you will have to fiddle with signal levels again until you get an optimum, because the sky signal will invariably be different from your lab test signals. Regards, James On Fri, Jan 18, 2019 at 7:37 PM David Marsh <davidmma...@gmail.com> wrote: > Thank you Dan! > > Using the adc snapshot, I saw that with no input the adc was toggling > between -1 and 0. This was the cause of the > spikes in the spectrum. I also looked at the adc samples with a sine wave > as the input and everything looked as > expected as you described. > > Thanks, > David > > > > On Tue, Jan 15, 2019 at 11:07 AM Dan Werthimer <d...@ssl.berkeley.edu> > wrote: > >> >> hi david, >> >> when you have no signal going into an adc, it will typically chatter >> between two values, around -1, 0 and +1, >> and you'll see a lot of RFI and interleave spurs, so the spectrum you >> sent around doesn't surprise me, >> >> but if you put in a sine wave that is much larger than the ADC chatter, >> say Vpp ranging from -100 to +100 (out of -128 to +127 for an 8 bit ADC), >> then the power spectrum spurs should be very small compared to the sine >> wave power >> (about a factor of 10,000 in power (100 in voltage)). >> >> can you use the adcsnap shot to look at the samples coming from your adc, >> and see what the relative levels are of the sine wave to the noise? >> does the power spectrum agree with what you see in the snapshot? >> >> best wishes, >> >> dan >> >> >> >> >> On Tue, Jan 15, 2019 at 9:59 AM David Marsh <davidmma...@gmail.com> >> wrote: >> >>> Hi all, >>> >>> Sorry if this has already been explained/solved. >>> I programmed Tutorial 3 onto a SNAP board and noticed some unexpected >>> spikes in the spectrum. I have set up the hardware as explained in the >>> python script with a 10 MHz, 8dBm reference going into the SYNTH_OSC SMA >>> (3rd SMA from the left). >>> >>> The attached figure shows the spectrum with no input going into the SNAP >>> board. There are large spikes in the spectrum every 50 MHz with the largest >>> at 200 MHz. >>> >>> When I add a signal to the SNAP board, I can see it in the spectrometer >>> at the correct frequency >>> but the other spikes are still present. >>> >>> Do you know what is causing these spikes and how they can be removed? >>> Any help would be greatly appreciated. >>> >>> Thanks, >>> David >>> >>> -- >>> 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 post to this group, send email to casper@lists.berkeley.edu. >>> >> -- >> 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 post to this group, send email to casper@lists.berkeley.edu. >> > -- > 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 post to this group, send email to casper@lists.berkeley.edu. > -- 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 post to this group, send email to casper@lists.berkeley.edu.
