I've looked at the output spectrum of ADF4351 and AD5385, the 3rd harmonic is always highest among all harmonics, what is the reason for that?
Are you looking at the output of the ADF4351 or the ADL5385?
I've looked at the output from both. And in both cases the 3rd harmonic is the highest harmonic.
I'm not complaining that it's too high. I just want to know the reason behind this.
This is because the output signal is a square wave (or almost square wave); assuming you're using a divider output.
If you do a Fourier transform on a square wave, the odd order harmonics are higher power than the even order harmonics. More info: https://en.wikipedia.org/wiki/Square_wave
Yes I am using divider on ADF4351. However ADL5385 should be pure analog, right? I don't understand how it would putout square wave or stepped square wave by any means. Can you shed some light on that?
I suspect the ADF4351 3rd harmonic is mixing with the ADL5385 I and Q signals to generate an output at the 3rd harmonic frequency.
Retrieving data ...