The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> Spectral purity relation between oscillator and divided frequency.
https://designers-guide.org/forum/YaBB.pl?num=1503907439

Message started by Jacki on Aug 28^th, 2017, 1:03am

Title: Spectral purity relation between oscillator and divided frequency.
Post by Jacki on Aug 28^th, 2017, 1:03am

Hi all,

I am calculating the spectral purity of the output of a ring oscillator. I am a little confused of the definition.
I get the spec. of spectral purity of the output oscillating frequency of a DLL, based on the spec., I need to calculate the requirement of spectral purity the oscillator. For example, if the pre-scaler is 32, and it means the output frequency of the oscillator will be divided by 32 as the output of the DLL, how to do the calculation. By the way, the spec. is not given by phase noise, it is given by integrated energy in the given bandwidth, for example [-fm + f0, f0 + fm].
If the oscillator has the frequency as fosc, and the output of the DLL is f0, and N = 32, the integration of the spectrum of the DLL output is from -fm + f0 to f0 + fm, how to build the equation between these two spec.?
I writes my calculation below, correct me if I am wrong (by the way, if the noise contribution from the frequency divider is not much, we may skip its contribution).
The integration of the oscillator frequency spectrum is from -32fm + fosc to fosc + 32fm, since it is the energy integration, not the SNR, I would think the spectral purity would be the same as the integration from -fm +f0 to f0 + fm. Any comments? Thank you.

Title: Re: Spectral purity relation between oscillator and divided frequency.
Post by Jacki on Aug 31^st, 2017, 6:55am

Hi,

I think I solve my problem since phase noise doesn't move. Thank you.