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Design >> Analog Design >> check phase noise @ what relative frequency?
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Message started by sugar on May 21^st, 2007, 6:16pm

Title: check phase noise @ what relative frequency?
Post by sugar on May 21^st, 2007, 6:16pm

I heard some designers check phase noise of a vco at 1MHz relative frequency,
if at that relative frequency, phase noise is less than say -90dbc,
then they think the design is good.

I am wondering why they chose 1MHz as the checking point, are there any criteria?

Title: Re: check phase noise @ what relative frequency?
Post by aaron_do on May 21^st, 2007, 6:54pm

The criteria is based on your blocking signals. For example if you have an interfering signal at 3 MHz offset to your operating frequency then it makes sense to measure your phase noise at 3 MHz offset. The level of PN required depends on the required SNR. For the downconversion mixer, the desired signal mixes with fLO and the interferer at f+df mixes with the phase noise at LO+df. So they end up converting to the same frequency. I think this equation is correct:

L(df) = Sensitivity + 3 dB - Pinterferrer,df - 10log(BW) - SNRmin

L(df) is phase noise at offset frequency f. Sensitivity is your minimum detectable signal level, Pinterferrer,df is the interferer power at offset f, BW is bandwidth and SNRmin is your minimum required SNR at the input to the ADC.

I think a lot of publications on VCOs don't follow a particular standard, but just try and get the best phase noise. So for comparison purposes they just chose an offset of 1 MHz or whatever.

cheers,
Aaron

Title: Re: check phase noise @ what relative frequency?
Post by sugar on May 21^st, 2007, 10:28pm

wonderful!
thank you, Aaron.