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FMCOMMS1 IQ issues

Question asked by Balderscape on Jul 14, 2014
Latest reply on Jul 18, 2014 by Balderscape



I have been working on a project using the FMCOMMS1 board with a Xilinx KC705 dev board. I have been trying to confirm the signal path through the device sampling I and Q samples from the ADC and then routing them directly to the DAC so that the output of the DAC should reflect the ADC input exactly.


I have run into a problem where it appears I am experiencing spectrum inversion. If I inject a carrier at say 1310MHz with the CF of the Modulator and Demodulator set to 1300MHz, then at the output of the FMCOMMS1 board I see my carrier at 1290MHz. I have been looking through the schematics and I cannot for the life of me work out why this is hapenning.


If I negate the I or the Q signal as they pass through then the output spectrum looks correct, but if I swap the I and Q channels I get a double sideband style output (i.e. my carrier at 1310MHz AND 1290MHz). Does anybody know why this is hapenning and the correct way to correct for it?


The larger issue is that I have a satellite modem generating the input signal and I am feeding the output back into the satellite modem and I cannot get the modem to lock to the carrier. But I figure if I can work out this spectrum inversion problem first then I will be one step closer.