[time-nuts] Re: Fixing PN degradation via ADEV measurement

[Bob kb8tq]

Hi

As HP found out back around 1973 or so, translating ADEV to phase noise 
is not possible. This is true, even if you have the ADEV numbers for a variety
of Tau values as opposed to some sort of “average” kind of number.

There are a number of things ( like spurs ) that can strongly influence a counter
based ADEV reading, and have very little impact on a phase noise ( or signal to
noise reading.  There also are ways the shape of the phase noise curve can
impact ADEV and have very little signal to noise impact for a specific signal. 

By far the best way to do this is to properly measure phase noise at various 
offsets from carrier. You can then look at the dbc/Hz numbers at each offset. 
This lets you see what your devices are doing to the signal. You can then track
down the offending bit or piece and fix the problem. 

The easiest way I know of to do phase noise is to quadrature lock two identical
sources into a double balanced mixer. You then put in a simple amplifier stage
to drive the mix down output into a sound card or spectrum analyzer. Total cost
if you already have a sound card should be < $50 ( US dollars …) for a DIY version.
That assumes you have the usual junk box parts and do a point to point wire
version. 

Some example ADEV plots:

http://leapsecond.com/museum/manyadev.gif <http://leapsecond.com/museum/manyadev.gif>

http://leapsecond.com/museum/manyadev.gif <http://leapsecond.com/museum/manyadev.gif>

Some plots of a number of measurements:

http://www.leapsecond.com/pages/fe405/ <http://www.leapsecond.com/pages/fe405/>

Quick primer on phase noise measurement 

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

( The easy approach starts on page 21 :) )

Bob


> On Jun 19, 2022, at 11:40 AM, Karen Tadevosyan via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi 
> 
> 
> 
> Thank you for the clarification and rf-tools link.
> 
> 
> 
> Agree with your calculation. That’s why I raised this question regarding a fixing PN degradation by Pendulum CNT-91.
> 
> 
> 
> Could you please explain where is the error in my reasoning of the experiment :
> 
> 
> 
> *	There is one 10 MHz OCXO with ADEV = 5 mHz
> *	There are two boards (DUT1 and DUT2) which multiply 10 MHz OCXO signal by 2.5 using the PLL method
> *	DUT1 has 25 MHz output signal with high PN  (checking by air and by measurement of S/N)
> *	DUT2 has 25 MHz  output signal with low PN  (checking by air and by measurement of S/N)
> Experiment’s steps:
> *	Step 1: DUT1 ADEV measuring gives me a value of 60 - 70 mHz instead of the expected 12.5 mHz  (5 mHz x 2.5)
> *	Step 2: DUT2 ADEV measuring gives me a value of 10 - 12 mHz which matches the expected 12.5 mHz  (5 mHz x 2.5)
> *	Step 3: based on ADEV values which in the first case (DUT1) are much greater than expected and in the second case (DUT2) coincide with the expected I conclude that PN of the output signal from DUT2 will be lower than from DUT1.
> I can see this PN degradation using Pendulum CNT-91 only as R&S FSQ8 does not fixate any PN degradation between DUT1 and DUT2
> 
> Karen, ra3apw
> 
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