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 Post subject: Phase noise observed
Posted: Fri May 18, 2007 3:19 am 

Joined: Fri May 18, 2007 3:04 am
Posts: 1
Location: Barranquilla
Hello. I am new to RF design. I want to know how can be determined the phase noise by just looking at the spectrum of a vco output.

 Post subject: Phase Noise
Posted: Wed May 23, 2007 8:17 pm 
User avatar

Joined: Sat Apr 22, 2006 11:14 pm
Posts: 20
Well, Phase noise is the stuff that makes the spectrum look like a christmas tree instead of a delta function. To actually measure the phase noise takes a little effort. If you are lucky, the spectum analyzer you are using has a noise marker function, and you can read off the phase noise power in dBm/Hz directly.


 Post subject:
Posted: Tue Jul 10, 2007 9:00 pm 
User avatar

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 70
Location: Singapore
Just few things to add, it's true that we may be able to observe phase noise from spectrum analyzer in a condition that the phase noise is higher than the noise floor of the spectrum analyzer. Some good spectrum analyzer, such as Anritsu has a good phase noise up to -100dBc/Hz.
It is good to check the capability of the spectrum analyzer noise floor before measuring it, otherwise if the phase noise is lower than the noise floor capability, you may not see the phase noise, but only the noise of the spectrum analyzer itself.

To see it clearly, you could adjust the RBW, VBW as well as frequency span.

I hope it helps.

Best Regards,

 Post subject:
Posted: Thu Mar 13, 2008 11:51 am 

Joined: Thu Oct 04, 2007 10:51 am
Posts: 2
who can give me some suggestioning setting of RBW,VBW and bandwidth when measure phase noise? thanks a lot

 Post subject: Re: Phase noise observed
Posted: Wed Apr 08, 2009 6:22 pm 

Joined: Wed Apr 08, 2009 11:07 am
Posts: 45
Well, the simple answer is that you set the RBW to be what makes sense for the region you are looking at. For instance, if you are interested in the phase noise at 1 KHz, you would set the RBW to maybe 100 Hz. That way the spectrum analyzer will "average" the noise from 950 to 1050 Hz, and display that answer at the offset of 1000 Hz. If you made the RBW much wider, the spectrum analyzer would include noise from much closer to the carrier, and since the noise close to the carrier rises so quickly, you would get an erroneous reading.

Similarly if you were out at 100 KHz offset, you could probably use a RBW of 2 KHz of so and get a pretty accurate reading.

VBW...you set that as small as you can until it looks like you are "smoothing" out important features, such as discrete spurious signals.

Practically, it is difficult to measure the phase noise of a VCO close to the carrier, since it is probably hopping around in center frequency, so there is a big change from sweep to sweep. Tried and true tricks: power the VCO and the VCO tune line off of a battery supply (no ground loops), shield the vco to keep out stray field pickup, put a piece of cardboard over the VCO to keep away air drafts, use an inside/outside DC block to connect the VCO to the Spectrum Analyzer, use a 10 dB attenuator to isolate the vco from the poor spectrum analyzer input impedance. Sometimes I hang some mondo bypass caps, like 200-4700uf, across the VCO power supply and tune lines to tame it during such a test.

If it is still hopping around too much free-running, you might have to phase lock it in a very narrow badwidth to something more stable. You might want to try a 10 Hz loop bandwidth, for instance.

Since VCO's typcially have poor phase noise, you can also make a very accurate measurement of their phase noise using a frequency discriminator technique. In this method, the VCO can hop all over the place free running, but you still get a good indication of close-to-the-carrier phase noise.

If you want to be very precise, you have to apply a small correction factor, due to the fact that 1 KHz RBW does not in fact use a 1 KHz wide bandwidth brick wall filter. The user manual will give you the correction factor.

You, of course, have to calibrate the data with each change in RBW setting. For instance if you measure -100 dBc at 10 KHz offset in a 1KHz RBW, the actual phase noise would be -100 dBc -(10 Log 1000) = -100 dBc -30 dB = -130 dbc/Hz.

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