How to estimate jitter and phase noise? - Page 1
Forum section: electronics / beginners

author: radiolistener
author_rank: Super Contributor
timestamp: November 14, 2020, 11:09:35 pm
content: Here is measurement of TCXO clock with oscilloscope (SDS1102X).The clock frequency is 96 MHz. Measurement taken with 10 ms delay after trigger point.We can see about 5 ns jitter after 10 ms from trigger point.Just for clarification - there is no visible jitter at trigger point.By using this data, is it possible to estimate actual jitter of clock (or total jitter of clock+oscilloscope) and phase noise of clock for example at 100 Hz from the carrier?

author: joeqsmith
author_rank: Super Contributor
timestamp: November 14, 2020, 11:15:34 pm
content: What's the trigger's jitter spec'e at? I would expect the TCXO is spec'ed much tighter. Normally, I will use a mixer and a reference with a much tighter phase noise, or I will over sample the signal and collect several periods in one sweep. Deep memory. Then calculate a perfect clock.

author: radiolistener
author_rank: Super Contributor
timestamp: November 15, 2020, 01:06:23 am
content: Quote from: joeqsmith on November 14, 2020, 11:15:34 pmUnfortunatelyoscilloscope datasheetdoesn't have specification for it's clock jitter. May betautechcan help with that.It usesADF4360-7as a clock synthesizer and VCO. It has −92 dBc/Hz phase noise at 1 kHz from the carrier according to the datasheet. So I think oscilloscope total clock phase noise is not better than that.But I wonder if such measurement can help to estimate at least something about TCXO jitter or total sum of oscilloscope and TCXO jitter?

author: MathWizard
author_rank: Super Contributor
timestamp: November 15, 2020, 02:17:42 am
content: How do you reduce jitter in say a 2-BJT multi-vibrator? Or in a Colpitt's osc, which I guess must have a lot less from the phase shift of the feedback loop?I also have the hacked SDS1104X-E, which is great overall, and sometimes I wonder about the jitter I see, but so far never compared it to my old 200MHz Tektronix DSO

author: joeqsmith
author_rank: Super Contributor
timestamp: November 15, 2020, 02:32:42 am
content: Looking at it relative to the trigger means the trigger needs zero jitter or you have to account for it. You may be surprised to find out just how bad it is. To remove it, you would just let the scope fill the memory asynchronous to the signal (who cares about the trigger) and then work the math. Yes, the scopes sampling noise needs to be accounted for. Lots of fun.Not sure how useful this would be but here's an old video showing even older software making measurements like this with a few of my old scopes.I made a different video showing my home made phase discriminator you can watch as well if you are interested.

author: pwlps
author_rank: Frequent Contributor
timestamp: November 15, 2020, 10:01:12 am
content: You can calculate the rms jitter from the phase noise spectrum but the inverse is more complicated because the formula for the jitter involves the integral of the spectrum and thus depends on its shape :https://www.maximintegrated.com/en/design/technical-documents/app-notes/3/3359.htmlequation (13).As explained in the video posted by joeqsmith you would need to measure thedistributionof jitter and then take its Fourier transform. Mathematically speaking the jitter distribution gives the clock noise autocorrelation function and its Fourier transform defines the noise power spectrum.Quote from: radiolistener on November 15, 2020, 01:06:23 amIn some sense since you are measuring therelativejitter, basically running one clock vs the other. The RMS of a time difference will indeed be the sum of RMS deviations of absolute times (assuming clocks are not correlated). Therefore the result is rather useless unless you know that one clock is much more stable than the other, but you can't tell which one it is if any from these measurements.

author: gf
author_rank: Super Contributor
timestamp: November 16, 2020, 06:14:33 pm
content: Quote from: radiolistener on November 14, 2020, 11:09:35 pm14M points of captured data @1Gs/s are IMO not enough in order that a window function could attenuate the spectral leakage from frequencies 100Hz apart sufficiently for this use case. This requires a window function with a very high side lobe suppression, which on the other hand leads to a wide main lobe. For instance, a 4-term Blackman-Harris window has a higest side lobe of -92dB but a half main lobe width of 4 FFT bins (in the frequency domain), and 7-term Blackman-Harris windows has a higest side lobe of -163dB and a half main lobe width of even 7 FFT bins (see figure 5 inhttps://www.electronicdesign.com/technologies/analog/article/21798689/choose-the-right-fft-window-function-when-evaluating-precision-adcs). But 7 FFT bins @14M points captured @1Gs/s are already 500Hz (i.e. far from the desired 100).