[Test-Equipment] "No spectrum analyzer" phase noise test setup of a 15.6 MHz TCXO?

[Francesco Ledda]

Very accurate phase noise measurements can be made with a good PC audio card and a software spectrum analyzer. 

What you need:
-An oscillator of known low phase noise (same frequency as the unit under test)
-A mixer (Minicircuits)
-A low pass filter (about 1000 Hz to 10kHz) and an op amp amplifier with small gain

Mix the two oscillators, low pass filter the signal to remove the sum. Amplify the signal and measure spectrum. 

Note:
It is important the two oscillators are on the same frequency. Use a scope after the filter to adjust for zero beat.

Modern phase noise measurement systems use the same methodologies to measure phase noise close to the carrier.  Moving the measurement to time domain and using the Modified Allen Variance makes the measurement even easier, since MAVar is non sensitive to the frequency difference between the oscillators.

BR,
Frank
KF5RXV 





Sent from my iPad

> On Dec 28, 2015, at 19:45, David <davidwhess at gmail.com> wrote:
> 
>> On Mon, 28 Dec 2015 18:07:05 -0500, you wrote:
>> 
>> The Kenwood TS-590 S (G) list server on yahoo has an interesting 
>> discussion going on about the fairly cheap Chinese TXCO's that are being 
>> sold as an optional replacement for the factory supplied 15.6 MHz 
>> oscillator in the TS-590S(G) series transceivers.  It seems there has 
>> been reports of phase noise issues on some Chinese replacements.  This 
>> can result in unnecessary and unwanted noise being transmitted or 
>> received +/- 10 to 15 KHz from the operating frequency.
>> 
>> I am aware that today's modern procedures would use a fairly expensive 
>> spectrum analyzer to measure the phase noise of such an oscillator.  
>> However, most hams today do not have access to a SA, Therefore in true 
>> ham fashion (meaning cheap), surely someone knows of an inexpensive way 
>> to bench check the phase noise of an oscillator.
>> 
>> Has anyone any experience in doing so?  I surmise building a good 15.6 
>> MHz notch filter would be required.  In addition, a way would be needed 
>> to measure 80 to 90+ db down at 10 KHz to 5 KHz away from the 
>> fundamental.  Some of us have good HP power meters that have dynamic 
>> range of that or a little more.  That would require the notch filter to 
>> be very good (not sure one could be made that good even in a good ham lab).
>> 
>> Anyone care to share their insight on what it would take to make such 
>> measurements without a SA?
>> Your attention is appreciated.
>> Ray, W4BYG
> 
> I would try a procedure like this using a sound card and PC for the
> spectrum analyzer:
> 
> http://wenzel.com/documents/measuringphasenoise.htm
> 
> It should be sufficient for comparison to use a pair of the Kenwood
> TCXO's and a pair of Chinese TXCO's, manually adjust them for equal
> frequency while looking at the output of the mixer, and then record
> the low frequency spectrum of the two Kenwoods mixed together and then
> the two Chinese (or suspected noisy TXCO's) mixed together.
> 
> We do not care about very low frequencies so the spectrum analyzer can