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Undersampling with AD-FMCJESDADC1-EBZ

Question asked by Muhammet on Feb 28, 2014
Latest reply on Mar 5, 2014 by Muhammet



I am using AD-FMCJESDADC1-EBZ on ZC706 board.


I observe the input signal via ADI IIO oscilloscope. As the input signal frequency increases, the output signal level, observed from IIO-scope, decreases significantly. I take some measurements with 0 dB input signal level.


Input FrequencyOutput Signal Level (dB)

10 MHz

40 MHz10.66
60 MHz9.97
80 MHz8.55
120 MHz6.62


The decrease in the output signal level conficts with the AD9250 datasheet(ADC chip on the AD-FMCJESDADC1-EBZ).


I also take measurements with undersampling operation: (Sampling frequency is 245.76 MHz)



Input Frequency Output Signal Level (dB)
10 MHz11.54
255.76 MHz  (245.76 +10)0.80
501.52 MHz (245.76x2 + 10)-5.14
747.28 MHz (245.76x3 + 10)-9.98


As you see from the table, when the input signal is shifted from the 1st Nyquist zone to 3rd Nyquist zone the output signal level is decreased approximately 10dB. The AD9250 Datasheet has below graph SNR vs Fin but I do not have the similar values with the graph.


I also checked the rms jitter specs of the clock synthesizer on the board. The rms jitter given in the specs can not cause that much decrease in the signal.


I also used another ADC board to see the undersampling effect on the output signal, but the result was completely different. When the input signal is changed from 10MHz to 410MHz (sampling frequency is 400 MHz) the output signal decreases approximately 1dB.


What is the problem with the AD-FMCJESDADC1-EBZ? Why does the output signal level decrease 10dB in case of frequency shift from 1st nyquist zone to 3rd nyquist zone?


Thanks in advance,