Electronics > Metrology

Phase noise measurement

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cncjerry:
I first started looking at phase noise (PN) as it related to radio transmitters and receivers.  I somehow got sidetracked for a couple of years on the other side of oscillator performance, that being stability.  I think I am now well equipped to measure stability having several different systems and references including a Cs beam,  a number of rubidium units, three or four GPSDOs as well as interval counters, Dual mixer setups, counters, etc.  So now I am going back to PN measurement and I've found that it still looks like you have to spend a fortune to get a decent PN plot.

I've read about a dozen papers on the various PN measurement methods.  I tried KE5FXs PN.exe with my spectrum analyzers and even with my 8568 and the companion 85685 preselector/preamp, I can't get a noise floor low enough to characterize anything more than maybe a poorly performing DDS or other synthesizer.   I also am looking at the old HP 3048 system because that uses a readily available interface that I might be able to couple with my 3562a baseband FFT analyzer.  If I can't put that system together, then the HP 70K spectrum analyzer has a PN measurement module I remember so I'll have to dig in there again too.

But going back to the documents like that written by KE5FX as well as Keysight, it seems like I might be able to get down to around -140dB or so by using one of the quadrature measurement methods.   There are two outlined in the docs.  The first uses a reference brought in phase with the DUT and then phase shifted 90 degrees so that when mixed within a DBM, the amplitude noise and primary frequency can be eliminated so that by then low pass filtering and amplifying the remaining phase noise, it should be able to be plotted on a baseband analyzer.  You might be able to also detect the PN by using a sound card with a 384k sample rate and 24bit or more ADC.  The second quadrature method skips the reference and splits the DUT signal itself into two paths with the second path being delayed or brought into quadrature with possibly a twisted wire quadrature device like those discussed by Breed(?) and others.

So armed with both a twisted wire quadrature module and a delay line (for 10Mhz), I tried mixing a 10Mhz signal with itselft 90 degree shifted .  I got the 20Mhz output from the mixer as expected with the 10Mhz greatly reduced in amplitude but still detectable, possibly due to shielding and coupling from input to output.  I then ran this directly into the 3562a as it has a 100Khz filter on the front end just to see what I would get.  I saw some of the typical phase noise plot but nowhere near as pronounced as expected.  For instance, and I'll post a plot as this continues, I had maybe a curve from -120dB down to -135 or so, maybe -140dB, but I would have thought the frequencies closer to 1hz to have more amplitude. I thought about it later that maybe it had something to do with the input coupling on the 3562a and I'll look at that again tomorrow.

If anyone can point me in the right direction, I'd appreciate it.  If this is hopeless within a hobbyist budget, I'll accept that advice as well.

Thanks,

Jerry

edit: Links to some doc:
Keysight:

https://www.keysight.com/upload/cmc_upload/All/PhaseNoise_webcast_19Jul12.pdf

One of the more interesting:

https://www.npl.co.uk/special-pages/guides/gpg68_noise

John's paper:
http://www.ke5fx.com/phase_noise.pdf

This link sent to me via PM  is very interesting and I'm trying to get more info.  He realized the analog mixing and baseband conversion with a high performance ADC with great results down to -170dBc:

http://www.aholme.co.uk/PhaseNoise/Main.htm

This R&S paper is better than some but the issue is with spectrum analyzers is the high noise floor:
https://www.rohde-schwarz.com/us/applications/phase-noise-measurements-with-spectrum-analyzers-of-the-fse-family-application-note_56280-15577.html

The picture in this doc is what I am trying to realize in hardware.  The issue remains though, if you can boost the PN to a level that can be measured to -170dBc by a typical baseband analyzer:

https://www.analog.com/media/en/technical-documentation/application-notes/AN-0982.pdf

Site with a simple discussion of measuring phase using correlation:

https://www.edn.com/measure-phase-difference-using-correlation/

Villain:
How much are you willing to spend? What carrier frequencies are we talking about?

mawyatt:
Jerry,

We needed to measure PN of a newly developed over-moded acoustic wave resonator back ~1990, and didn't have the proper PN equipment (and no budget for such). Recall I was at the IEEE MTT conference where MaCom presented a paper utilizing two identical oscillators, which were mixed together resulting is a zero baseband since the oscillators would self inject lock to each other. The baseband residue is the PN from each like oscillator and can be amplified and displayed with a high resolution ADC or baseband analyzer.

We funded a grad student (ended up hiring him!) to build up a couple of these setups and wrapped two of the over-moded acoustic wave oscillators with towels for thermal isolation and powered by batteries, then a high gain low noise baseband amplifier followed. The PN measurements which were better than -140dBm at 1KHz offset for a ~2GHz oscillator about 5mm square, were impressive at the time. Later we confirmed these measurements with a proper HP PN instrument at another location.

Wish I could remember the MaCom paper, but my memory is fading :P 

Anyway, know this isn't exactly what you were looking for but maybe helps in your PN endeavors since it doesn't require any expensive specialized equipment.

Best,

Edit: MACOM paper courtesy of my colleague Alberto  ;)

RoGeorge:
There are many ways to measure and characterize the phase noise.  Would be nice to link or name the documents you were talking about.

Just for an approximation, to see which oscillator is worst, I once did like this:
- set a Rigol DS1054Z oscilloscope on the biggest memory depth
- look at the oscillator's waveform at a later moment, a second later or so after the trigger.

Because constant phase noise accumulates as a time jitter over time, the zero crossing observed a second later after the trigger will slightly differ with each trigger event.  This is an example made with pulses instead of sinusoidal waveforms, just to better illustrate the idea of phase noise leading to time jitter accumulation over time.  The later we look after triggering, the more the pulse jitter around the ideal time spot.  The peaks that never move are there just as a helping reference on the screen, they are not live signals.  The bigger the delay from the trigger, the more time jitter is observed.



That method is in fact limited by the stability and phase noise of the DS1054Z internal PLL, but good enough for a demo or to compare which oscillator has lower phase noise.

An instrument can be improvised on the spot with some very clean reference oscillator and a counter, then some software to read the measured time jitter and display the statistic of the results.

SilverSolder:

--- Quote from: RoGeorge on November 22, 2020, 01:48:37 pm ---
[...] Because constant phase noise accumulates as a time jitter over time [...]

--- End quote ---

I don't understand this part -  Wouldn't the noise introduce both positive and negative jitter,  so it nets out to zero in the long run?

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