Marc and Yuhang

To understand if ~20kHz noise is present as sidebands of RF signals, for DDS2, we checked RF signals coming from CH0 and CH1 with spectrum analyzer.

Figure 1 shows 78MHz signal coming out from CH1 on spectrum analyzer. For this singal, we could see:

1. It has noise shoulder, which is between 0~3kHz.

2. We don't see any noise peak around 20kHz.

Then we checked also CH0, which shows the same behavior.

Since we don't see 20kHz peak around RF signal, we tried to use CH0 to demodulate CH1 and we did't expect to see ~20kHz peak.  (Note that RF amplifier was used for CH0 in this measurement) Figure 2 shows the demodulated signal spectrum. Peaks around 20kHz show up clearly in this spectrum. This means that these peaks doesn't come from RF signals but they were just there (actually everywhere).