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

[Karen Tadevosyan]

Hi

 

Bob, many thanks for your explanations/recommendations and links.

 

According your advices I will try to make PN measurement on new SA with cross-correlation to get more clear picture of the output signals.

 

The next my step will be a quadrature method measurement  and comparison of the results.

 

Karen, ra3apw 

 

From: Bob kb8tq <kb8tq at n1k.org> 
Sent: Sunday, June 19, 2022 11:46 PM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Karen Tadevosyan <ra3apw at mail.ru>
Subject: Re: [time-nuts] Fixing PN degradation via ADEV measurement

 

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

 

Some plots of a number of measurements:

 

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

 

Quick primer on phase noise measurement 

 

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 <mailto: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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