The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Measurements >> Phase Noise and Jitter Measurements >> Measuring phase noise of ring-oscillator using spectrum analyzer.
https://designers-guide.org/forum/YaBB.pl?num=1401738103

Message started by Colombo on Jun 2^nd, 2014, 12:41pm

Title: Measuring phase noise of ring-oscillator using spectrum analyzer.
Post by Colombo on Jun 2^nd, 2014, 12:41pm

Hello guys,
I'm measuring the phase noise of a ring-oscillator that consists of cmos inverters. A spectrum analyzer is been used to measure the frequency spectrum, as can be seen in the figure bellow. The spectrum was measured using different values of RBW (from 100 Hz to 510 kHz) and a fixed span of 10 MHz. The oscillation frequency of ring is around 330 MHz.
As expected, reducing RBW the noise floor is reduced.
The phase noise is calculated as follow:
Phase noise at offset frequency f1 = Power (f1) - Power carrier - 10*log(RBW).
However, the obtained phase noise at 100 kHz and 500 kHz of offset is a strong function of RBW.

Which data is the correct one? is for RBW = 100 Hz since the noise floor is the lowest? How can I understand these results?

It seems that the power different between the carrier and the offset depends very weakly on the RBW, and the last term of the equation is affecting the calculation.
Thank you very much.

Title: Re: Measuring phase noise of ring-oscillator using spectrum analyzer.
Post by Colombo on Jun 2^nd, 2014, 12:44pm

The values of the calculated phase noise are: