hi xavier, it sounds like you are switching between two signal sources, one is a reference load with a very small signal level, and the other is a signal from the sky with strong RFI?
if that's the case, can you turn up the signal level on your reference level? it would be best if the RMS is around 10 on a scale of -128 to +127 for both signal sources. (so that neither source saturates the adc, and neither source has too small a signal level where you degrade the SNR from finite quantization). if you are not switching between two sources, and you have a very weak astronomical signal buried in RFI and noise, then that's fine - the weak astronomical signal can be much lower than a single bit on the ADC and you can recover it from integration in your spectrometer. just don't let your ADC saturate too often as that splatters power around the spectrum. (or if the ADC is saturating from impulsive RFI, then use one of the casper libraries that detects this saturation and replaces it with noise). best wishes, dan On Tue, Dec 12, 2017 at 4:36 PM, Xavier Bosch <[email protected]> wrote: > Hi Bob, > > Thank you for your answer. > Yes, calibrating the 8-bit 5-GSPS ASIAA ADC is a must! I worked on that > some time ago. > I always make sure that the 4-cores OGP and IODELAYS calibration residual > error is within a acceptable range before starting any data acquisition. > > We also have checked the whole LNA chain linearity and it is fine, there > is no harmonic distortion. So, we narrowed down the problem to the fact > that we do not have enough dynamic range in the ADC to accommodate the RFI, > avoiding clipping, and simultaneously having enough bits (>2) when looking > to a clean reference load. > > Thank you again, > XB > > > On Mon, Dec 11, 2017 at 2:16 PM Wilson, Robert <[email protected]> > wrote: > >> Hi Xavier, >> >> One problem with the the 8-bit 5-GSPS ASIAA ADC board is that the ADC has >> four cores and if they are not exactly aligned, a strong RFI signal will be >> modulated by and mix with the periodic errors producing a number of >> artifacts. >> >> Bob Wilson >> >> On Thu, Dec 7, 2017 at 5:10 PM, Xavier Bosch <[email protected] >> > wrote: >> >>> Hi CASPERites, >>> >>> >>> >>> We are developing a wideband spectrometer (~1.6 GHz ) in a band that has >>> a lot of RFI. Currently we are using the 8-bit 5-GSPS ASIAA ADC board and >>> the ROACH2, and the ADC resolution appears to be inadequate for our extreme >>> RFI environment. >>> >>> >>> >>> We would like to move over to a higher resolution ADC (12 to 14 bits), >>> probably with a JESD204B interface, and a Xilinx development kit. We are >>> considering something like this evaluation board KCU105 >>> <https://www.xilinx.com/products/boards-and-kits/kcu105.html#hardware>and >>> something like FMC217 >>> <https://www.vadatech.com/product.php?product=528&catid_prev=0&catid_now=0> >>> as a digitizer. >>> >>> >>> Has anyone been in a similar situation dealing with RFI? >>> >>> Has anyone ported CASPER library to Xilinx development kits and JESD204B >>> ADCs? >>> >>> >>> Thank you, >>> >>> XB >>> >>> -- >>> >> You received this message because you are subscribed to the Google Groups >>> "[email protected]" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To post to this group, send email to [email protected]. >>> >> -- >> You received this message because you are subscribed to the Google Groups >> "[email protected]" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> To post to this group, send email to [email protected]. >> > -- > You received this message because you are subscribed to the Google Groups " > [email protected]" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > -- You received this message because you are subscribed to the Google Groups "[email protected]" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected].
