How to solve a VCO measurement problem with phase noise

[danda821]

VCO measurement problem

I am testing a VCO using spectrum analyzer. I found the frequency is changing with time esp. when the frequency span is small (5MHz). So it is very hard to get a good phase noise measurement since the carrier power is not stable. Is there any way to overcome this problem? Thanks.

 

[Mazz]

VCO measurement problem

VCOs are very sensitive to control voltage. So you need to filter it as much as you can. Supply voltage has to be filterd also, and a shield for the board can be necessary.
A good way to measure it is lock with a PLL with a very small loop bandwidth. For freqs larger then the loop bandwidth you'll see the VCO phase noise.
A more expensive solution is use a dedicated instrument.
I hope t can help.
Mazz

 

[rf_ray]

VCO measurement problem

I will suggested you testing the VCO in a room with clean ground. We run into this problem before. Later we test it in another building. The spectrum is stable.

 

[rfguy1]

VCO measurement problem

VCO are sensitive to test probes. Usually when you use an oscilloscope probe or even coaxial , the VCO unlocks . If you are using an spectrum analyzer, the best way is use a FET probe too. This comercial probe is attached to the spectrum . Valid measurements can be made from here .
If FEt probe is not available , im sure a FET amp with gain =1 could be used .