About C/N of VCO

时间：04-12 整理：3721RD 点击：

When evaluate a VCO performance , there is a characteristic that is C/N ratio far from carrier . Could u tell me what it means to VCO performance.
thanks!

Hi,

C/N is the Carrier to Noise floor. However measuring it in VCO in open-loop is very tricky and I am not so sure that it is possible. (please currecty me anyone if I am wrong about this one).
However from previous experience we have measured it in close loop.

Enjoy,
RF_Router

What you said above means the C/N is like as phase noise far from the carrier , such as the 160 Hz from carrier?

The noise floor is dependent on the filter that you are using. And yes you can say 160Hz but. Anohter easy thing to do. Is measure once with the carrier and once withough the carrier and you will get the best measurment. Still the filter make a difference to how much noise you measure. I don't believe you have a 1Hz filter but you may have somthing like 30Hz or somthing around this so make your calculation so you will give your answer as per 1Hz.

I hope I helped.
RF_Router

RF Router , what i said for C/N is from VCO.Do u explain that for me? thanks. and why you mentioned the filter ? Is it related to the filter?

Hi,

About the C/N, as far as I know it has to be measured in closed loop. I don't think it is possible to measure it in open loop. About the filter, this dependes with what equipment do you make the measurement. If you are using a spectrum analyzer which will usually be used then look at RBW (resolution bandwindth). The RBW will actually define your noise floor. Now previously I told you to measure the noise with carrier and without it. Now, I don't think you will be able to do that. So you can measure like you said at a certain distance from the carrier. Don't forget to mention C/N @ offest 160Hz.

Enjoy,
RF_Router

C/N is used stand for "Carrier to Noise Ratio". Namely, Phase Noise from the carrier at certain offset.

Isn't this expressed as SSB Phase Noise, often in dBc/Hz?

Hi GaAs FET,

yes, it is. From some reason most manufactores measure dBc/hz @ offest of 10KHz.

Enjoy,
RF_Router

C/N is carrier power to the total noise power ratio. Total noise power includes the phase noise, AM noise and other noise (you name it). If the AM noise and other noise power is very low, then C/N is approx equal to the phase noise.

Best way to do this is to lock the VCO using a very narrow loop bandwidth for the PLL. If you use a loop bandwidth of 100Hz, then far outside the loop bandwidth you will just be looking at the VCO phase noise with no impact from the loop.

Hi all,

I have to agree with toonafishy cause that's the way to do it. Like I said measure it in close-loop.

Enjoy,
RF_Router

Hi,
I feel that Closed loop measurement is advocated mainly because of the instability (short term) of the free runnung oscillators. If you use closed loop the frequency will be more "bounded" within the range controlled by loop components but the measurement of phase noise is erroneous. The ideal would be to measure the oscillator phase noise with the frequency being "unchanging". This may be achieved by providing a stable control input to VCO.
BRM