Need help with YIG Oscillator output purity

[induzer]

Hi,

after I got the 2nd LO of my R&S FSA fixed now, me found a further issue.

If the span is >= 5MHz everything is just fine. (see upper part of 1st diagramm)

But if i set it right to 4,9999 MHz width, i get some peaks at the lower end of the spectrum between 100Hz to 4,9985 MHz.

The peaks show up at 87,9 kHz / 175,8 kHz and 263,7 kHz. (see lower part of 1st diagramm)



I was able to determine the signal from the YIG-oscillator to get "dirty" when you set a span lower 5 MHz.

The dirty signal


Sweep clean


Sweep dirty


I have been thinking about this for 2 weeks now and I am a bit to lazy to check the 5 modules involved in the control loop.



Could it be a mixing product of the mixer right to the YIG ?

Or may the multiplier have spectral purity issues ?

Maybe someone knows which module could NOT be the source of the "noise" ....


hack on,
induzer