Electronics > RF, Microwave, Ham Radio

Best option to measure phase noise

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I'd like to measure the phase noise of my RF signal generator, however I don't have a spectrum analyzer with good-enough phase noise; Like at least -110dBc/Hz at 10KHZ offset at 1GHZ.
(My best spectrum analyzer is SignalHound BB60D which uses a superhet architecture and also doesn't have good phase noise characteristics)
I am looking for something affordable (below 10K) and preferably not too old/heavy (like ancient HP devices)
These are the only options I know:
1. SignalHound SP145 (10K. released just last month)
2. Aaronia ECO 18 (not released yet. Not much data available on its phase noise, but previous models using OCXO had good phase noise)

Anything else I've missed?

I'll attempt to answer since you've got no responses.  I don't know what your experience level is and what level of phase noise you're trying to measure but good phase noise measurements at low levels (good sources) are generally expensive and difficult.  HP/Agilent and Rohde & Schwartz have great app notes and R&S even has good you tube videos.  Here are a few tidbits of advice.  Maybe others can chime in and enforce or disagree- this is how this board generally goes.  Comments follow.

1. If you have high a quality piece of gear that that you're concerned about, you can rely on the manufacturer's specs.  Phase noise is a design type spec and if the SG works generally and passes calibration/selftest, etc, there is no reason to think that the phase noise is worse than the specs.

2. The RS and HP App notes go through it better than I can, but for the brute force method using just a good SA, you will need an SA that has a 10-20 db better spec that your DUT.  This is difficult at low phase noise levels.  There is also the problem of dynamic range.  You've got this big carrier and you are trying to measure small noise components close in.  One technique is to put an ultra narrow notch filter at the carrier frequency, this eases the DR problem significantly- you're just measuring residuals similar to THD measurements.  This filter itself can be expenisve.  You also have issue in the brute force technique of the true shape and BW of the piece of offset spectrum you're measuring.  The HP and RS notes go through this, it's not difficult but isn't obvious either.  Some good SA's might account for this internally.

3. There is a clever technique that you can look up (again look at HP or RS) that is called "the cross correlation method".  This method uses two simultaneous measurements using two uncorrelated receivers feeding an autocorrelator.  Some clever math makes a lot of the measurement system's phase noise drop out and you're left with just your DUT phase noise.  This is the way most good phase noise analyzers work rather than just brute force.

4. Hams and others without a NASA budget can infer phase noise with techniques like reciprocal mixing where you build a receiver with your source as LO and A-B test it with a really low phase noise source and see what your close in noise looks like.  One of the old Ham standards was the HP8640, a 50 pound HP beast that has pretty exceptional phase noise to this day.  They can be had for a few hundred and maybe a few hundred more to send to a Cal Lab for a look.  Journals like QEX, a specialty technical Ham journal would be a good place to start.

5. If you really need to measure phase noise, you might consider renting a good Agilent or RS box.  You might be able to get a short rental time of a week or use the instrument at the supplier's site for a day.  I would think that you can get your hands on a good instrument for < $1000 USD/Day if you look around.

I don't know of any general purpose instrument that would be useful and not very specialized.

Good luck.


Thanks for your detailed answer John.

--- Quote ---HP/Agilent and Rohde & Schwartz have great app notes and R&S even has good you tube videos.
--- End quote ---
Yes, I have been watching/reading those recently (plus other resources like TheSignalPath)

--- Quote ---If you have high a quality piece of gear that that you're concerned about, you can rely on the manufacturer's specs.
--- End quote ---

--- Quote ---using just a good SA, you will need an SA that has a 10-20 db better spec that your DUT.
--- End quote ---
That was pretty much my conclusion so far: I won't get access to something affordable enough to measure e.g my OCXO's phase noise, and even for the signal generators I am building, the difference between my DUT's phase noise and the spectrum analyzer I was looking at (SignalHound SP145 and SM200C) is still less than 10dB in some offsets.

For a few discrete frequencies one can build a oscillator + mixer setup to convert the DUT frequency to a lower frequency range and this way higher relative phase noise, so that than the SA may be good enough. For something like an OCXO this may need having 2 similar OCXOs to test the sum of the phase noise of the two.

This is what I was generally referring to in my number 4 response above of how Hams approach this.  The idea of using fixed OCXO modules is very good.  I don't have much direct experience but see fixed TCXO/OCXO modules in trade magazines with phenomenal specs.


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