DRO Phase noise measurement

[Willem]

phase noise measurement

Hi there

I have designed a 4 GHz DRO, but now i must measure the phase noise. I have a Agilent E4404B spectrum analyzer with the phase noise personality on. but when i measure the phase noise of the oscillator i just get a straight line reading, see picture. What does this mean? Anyone who knows please help.

 

[Willem]

how to measure phase noise vco dut

hi guys

What i meant in the previous post about the straight line reading is that the phase noise does not decrease as the offset frequency increases. Might it be that i am measuring the spectrum analyzers phase noise, seeing that it makes use of a phase locked loop - and thats why i measure tha same value at all the frequencies. or what????

Any help! Please.

 

[biff44]

dro phase noise measurement

A guess would be that the DRO center frequency has drifted too much during the measurement, and you got garbage output. Try phase locking the dro in a very narrow band loop and try to measure it again.

 

[Willem]

noise drift manual measure

Thanks for the repply

I am putting the DRO in a nice RF housing, that should minimize the drift quite a bit, then i am going to do the measurement again.

 

[biff44]

4ghz dro

You probably should put a big mamma capacitor across the power supply terminals on the ocsillator, like 300 Uf. Probably need 50 uF on the tune port too. Then power the thing up and go away for something like 1/2 an hour, it may have stabilized enough by then to measure. Put a piece of cardboard over the whole setup so air conditioning currents do not change the case temperature during the measurement.

Other methods, like delay line discriminator, allow for the VCO to drift a good amount without screwing up the test.

A phase noise measurement test stand solves the drift by phase locking a reference oscillator to the device under test, so if the DUT drifts slowly in frequency, it is ok.

 

[Willem]

dro measuremnets

Hi there

I still have a problem in measuring the DRO phase noise that i built. I took the circuit to a company that has a decent phase noise test set. We could not measure the phase noise due to the PLL not maintaning lock on my carrier. The carrier drifts a maximum of 30 kHz. I then read in the tests set manual that the PLL can only maintain lock to 2.5 kHz! How then does other people measure the phase noise of an unstable source like a VCO for example? I wont believe that my oscillator is the only oscillator on earth that drifts more than 2.5 kHz.

How do you do a delay line measurement?
Is that the solution?
Any help please.

 

[biff44]

hp phase noise measurement

Agilent/HP Phase noise test stand has a mixer right at the front end that converts your RF signal into a video voltage that has phase information on it. In order to do that, two signals have to be put into that mixer: your device under test, and an LO that is the exact same frequency as your DUT, but 90 degrees shifted in phase.

You have to set up the phase noise test stand to track such a signal. Since your DUT is drifting a lot, you should use an external LO signal generator, like an HP 8662 synthesizer in FM mode, multiplied up to 4 GHz, and the DC coupled FM tune port controled by the phase noise test set. You should be able to get the LO to track a 30 KHz drift.

Another entirely different method, but without quite as good a system phase noise floor, is to use a delay line discriminator. You do not need an external LO for this measurement. You start at your DUT, split the signal, put one arm into the Phase Noise Test Set's LO port, and with the other arm go thru around 50 feet of very low loss cable and then thru a variable mechanical phase shifter and then connect to the test set's other input. Tell the test set you are doing a delay line measurement, and it will tell you what to do next. You can track huge frequency drifts with this method, but like I said your system phase noise floor is not as good.

Added after 5 minutes:

BTW, you do not need the phase noise test set to do a delay line measurement. You only need the RF delay line, splitter, phase shifter, mixer, and a low frequency (0 to 10 MHz) spectrum analyzer.

Added after 2 hours 21 minutes:

Another idea. Since you have the dieletric resonator pucks already, you can substitute a transmission resonator made from a single puck for the coaxial delay line in the above. The rapid phase shift of the transmission resonator acts like a long delay line.

 

[Willem]

phase noise agilent dicriminator

Interesting methods, i will try and make public the results.

Thanks a lot.

P.S. What is the typical frequency drift for a DRO? How many hertz in other words?
I thought these types of oscillators are very stable, but now i seem to be wrong or mine is just poor. Still if you take the amount of drift and compare to the operating frequency, it is tiny - 30 kHz on 4GHz!!?? Or am i wrong?

Willem

 

[RFDave]

delay line discriminator

Oscillator drift is measured in PPM. In your case

30 kHz
-------- = 7.5e-6 = 7.5 PPM
4 GHz

This isn't too bad. I think there is a signal track option in the Spectrum analyzer to make sure it track's the carrier frequency, try turning that on.

Dave