Using the console command "sample" it is possible to measure the pure reference, refl and thru signal strength.
First picture shows the change over frequency of the reference signals (only look at the changes in level, the absolute levels differ due to some TAPR VNA internals)
According to the internal gain setting table in the nanoVNA there is additional gain added as the frequency moves up. These are the vertical jumps, each about 5dB at 300, 600,900, 1200, 1500 and 1800MHz. In total about 45dB gain is added in the audio path to compensate for signal loss and with the 10dB delta between lowest and highest frequencies the delta is 55dB, similar to the 55dB measured with the direct connection to the SA612 so the nanoVNA adc and dsp algorithm do maintain the dynamic range.
Next picture shows what happens when you use a clean 5V supply. Much less noise in the reference signal.
The weird thing with the reference signal is the increase in strength above 1.5GHz. This could be caused by capacitive coupling in the attenuator before the reference mixer
Third picture is the change of the amplitude signal (red line) of reflection mixer without anything connected to the SMA CH0 connector, a fairly good open. The change of amplitude over frequency does not show any weird things. The green line shows the leakage from the output signal into the forward mixer. The pattern suggest the attached cable acted like an antenna
The fourth picture is the change of the amplitude of the forward mixer. (green line) with a through from CH0 to CH1.
The bridge (red line) stops working above 1.5GHz as the characteristic pattern of the cable disappears, possibly because of insufficient measurement SNR
Till now I have not been able to get valid phase signal from the bridge above 1.5GHz, only amplitude with at least 20% noise.
Comparing the absolute levels of the signals suggests there is about 10dB headroom for increasing the thru signal and about 20dB for increasing the reflection signal. In particular the latter might have a big impact in the performance of the bridge at higher frequencies.
I hope this information is not too detailed but writing it down helps me to understand it much better.
