Strange things in harmonics mode settings

[erikkaashoek]

Very nice design!!!

I'm puzzled about your setting of the harmonics modes
First you define:
#define VNA_NOMINAL_3X_IF_FREQ (VNA_NOMINAL_1X_IF_FREQ/2)
So it seems you use the second harmonic of the LO when going to harmonic mode but the frequency of the LO is divided by 3 suggesting the 3rd harmonic.

frequency = frequency / 3; si5351.set_freq(frequency * SI5351_FREQ_MULT, SI5351_CLK0); si5351.set_freq((frequency + VNA_NOMINAL_3X_IF_FREQ) * SI5351_FREQ_MULT, SI5351_CLK1);

Second you seem to be using in harmonic mode the 3rd harmonic for both test signal and LO but how do you avoid detecting the 3 harmonic of the mixing of the 1st harmonics?

In my experiments I found its better to use the 3rd harmonic for the test signal and the 5th harmonic for the LO because this allows you to detect the 1st harmonic of the mixer output.