Help with VCOs characterization

hello, I've sent to fabrication some ring VCOs. I am checking their simulations and place a load of 50Ω emulating a spectrum analizer, but the signal falls with this load
There is no buffer in my designs so I wonder if there are some methods in lab to be able to measure frequency against control voltage.

I'll appreciate.

i think it will be hard , coz u need a buffer for ur oscillator coz , the spectrum analyzer will load ur vco , and u can't say this is the oscillation frequency coz the loading will change the frequency of ur oscillator

khouly

Hi Stoikov,

Without the buffer any load (measurement or in-circuit) hanging off your VCO output will pull the frequency as well as drop your output level.

A simple technique to improve the loading of the spectrum analyzer measurement is to carefully add a series 1k resistor to the output of your VCO. That will drop your level by 26 dB but should give you sufficient isolation to measure your frequency vs. control voltage characteristic. Alternatively you can make a high-Z RF probe, a coax cable with a 1kohm chip resister on the probe end.

An article on the VCO isolation subject:

h**p://www.mwrf.com/Articles/ArticleID/5513/5513.html

Cheers.