There are many discussion on this forum around the "noise" on CH1 limiting the dynamic range and weather the noise is influenced by the termination of CH1 or by good or bad SA612
So I decided to measure.
First 3 measurements of the "noise floor" in CH1 with a load on CH0 and an open, load or 50ohm on CH1. These are 1001 point measurements with 1/8 exponential averaging
As you can see the phase at higher frequencies is much too consistent to be caused by noise and this suggest leake of the test signal from the SI5351 to be the problem.
So using the "sample ampl" command I disabled the reference signal impact and you can see the switching of the ADC sensitivity when measuring a thru signal
The same measurement without input signal on CH1 does show approx the same as the first three measurements.
The I disabled the impact of the CH0 output signal generation by using a little known command that you can use to set the offset between the CH0 output and the SA612 LO signal to something different from the 5000Hz as hard encoded in the dsp.c (co)sine table and I set "offset 7000" to move CH0 signal outside the dsp filter window and this delivers a somewhat different picture
Apart from the step up at 300MHz where the switch to overtone is made and you loose 17dB sensitivity and this is compensated in the amplification setting of the adc, the noise floor shows a very different pattern suggesting the biggest impact on the dynamic range of the NanoVNA is leakage and NOT noise on CH1.
