How to calculate periodic jitter from a spectrum of a signal
How to calculate periodic jitter from a spectrum of a signal
(OP)
I am trying to find a function that can calculate the periodic jitter given a power spectrum such as captured from a spectrum analyzer.
For example, I have a 1.25GHz clock that is being FM modulated by another signal running at 1MHz. I can capture the power spectrum of the 1.25GHz on a spectrum analyzer. From the power spectrum, how do I calculate how much the 1.25GHz is being modulated in term of mUI or ps?
Is there an equation I can use to calculate?
For example, I have a 1.25GHz clock that is being FM modulated by another signal running at 1MHz. I can capture the power spectrum of the 1.25GHz on a spectrum analyzer. From the power spectrum, how do I calculate how much the 1.25GHz is being modulated in term of mUI or ps?
Is there an equation I can use to calculate?





RE: How to calculate periodic jitter from a spectrum of a signal
Jitter is integrated phase noise, so if you are able to capture a phase noise plot of the clock in terms of dBc/Hz, then you can numerically integrate the phase noise to get jitter. I've attached a spreadsheet I grabbed from RF Globalnet as a starting point.
Dave
RE: How to calculate periodic jitter from a spectrum of a signal
I knew how to integrate the noise power spectrum to obtain the phase noise in either mUI or ps-rms.
What I am looking for is kind of a special case I think where you got a carrier being FM modulate by a another signal at a much lower frequency.
RE: How to calculate periodic jitter from a spectrum of a signal
I'm certain this concept would be fraught with practical difficulities.
Can you measure the jitter of the carrier, the modulating signal and the modulator [with no input signals] and combine them to get the nmber you want?
RE: How to calculate periodic jitter from a spectrum of a signal
However...if it is just little FM deviation(ie. small angle approximations apply), then the sidebands at +/- 1 Mhz from the carrier in dBc are approximately:
sideband dbc = 20 LOG (delta f/2Fm)
where delta f is the frequency deviation, and 2Fm in this case is 2 * 1 Mhz.
So, if you can see how many dBc down the sideband tone is, you can calculate the frequency deviation in Hz to the carrier at 1.25 Ghz. That will yield the pk time jitter.
Rich
www.MaguffinMicrowave.com
Microwave and wireless design consulting
RE: How to calculate periodic jitter from a spectrum of a signal
I like that! Where did you find that equation?
Z
RE: How to calculate periodic jitter from a spectrum of a signal
I do keep a couple of big file cabinets of important paper print outs, and found it there:
Review of the Specification and Measurement of Short-Term Stability, by Robert B. Shields
The copy I had did not say what publication it was from, but it was probably an IEEE or IRE paper.
www.MaguffinMicrowave.com
Microwave and wireless design consulting