I took time-series data at TP1 on NFSS. This is just after the elliptical filter which is after the demodulation on board.

As also seen in the spectrum measurement at this testpoint in CTN:2474, there is a lot of power at around 435 kHz.  But this is not noise!

As seen on the oscilloscope, this is a near-perfect sinusoid. So this must be either of the following:

• A mixed down frequency due to non-linearity of MAX4107 or the Mixer JMS1-H .
• This could be an oscillation in the closed-loop of FSS at the unity gain frequency. But last I checked (CTN:2443), the UGF for the same COM Gain (10dB) and FAST Gain (10 dB) was 369 kHz.

This measurement was taken with a 500 MHz 10x Probe with a 300MHz TDS 3034C oscilloscope at 0.5 GSa/s sampling rate.

Interestingly, there is no such oscillation or peak on the South side. However, the south sides COM Gain is 24 dB and Fast Gain is 14 dB. So it could be because it is just suppressing this non-linear effect properly or just has a very high UGF.

Data