Please see the attached jpeg that plots I vs Q samples captured by the ADC. If the number of samples captured is less than 10,000 ( or 40KB of memory with 2 bytes for I and Q each), then the plot is a smooth circle. If number of samples if more than 10,000, then the timing seems to skip and the IQ plot crosses randomly through the middle. Viewed in time domain, it seems that the ADC output simply skips some samples.
I am running the reference design of early May on Zedboard. The DAC is generating the 40MHz tone.
On a real spectrum analyzer, the output spectrum is clean, so the TXDAC is working OK.
On doing a FFT with the ADC output, the spectrum becomes noisy when number of samples is more than 10K.
I use the shell command 'dd' to read samples from /dev/iio:device5:
dd if=/dev/iio:device5 bs=$LENGTH count=1 > /root/rxfiles/capture.txt