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

[Bob kb8tq]

Hi

A “proper” phase noise analyzer will get you down to < -165 dbc/Hz. It also will preserve
the frequency spectrum ( no sampling / nyquist roll over ). What you get at 132 Hz offset 
is the (DSB) noise at that offset and *only* that noise. 

With a counter ( as Magnus mentions ) the sampling process looks at and sums up all the
noise in the input bandwidth. If it’s a 0.01 second sample, you get noise from 100Hz, 200Hz,
300Hz on out to whatever the counter’s input bandwidth is. 

Since the counter likely has a bandwidth of a couple hundred MHz and you are measuring
something like 10 or 25 MHz, you sum up a whole lot of “noise floor” noise. Even if the counter
input amp has a zero db noise figure and the sampler does as well, you “fold in” 200 MHz worth
of that noise. This makes the sensitivity of the counter to low levels of phase noise pretty poor. 

If you didn’t have this sort of limitation, your counter might well have 20 fs resolution rather than 
20 ps …. (and no, this isn’t the only reason you have limited resolution)

Bob

> On Jun 19, 2022, at 10:45 PM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Bob,
> Many thanks for the guidance you provide and the phase noise measurement document.
> Can you provide feedback on this reasoning: A counter is like an ADC but in the frequency domain. So if you measure with 0.01 s tau you basically average over 0.01 s so you can only observe "phase noise" (e.g. energy that is not at the exact requested frequency) up to maximum 50 Hz from the carrier. But as you measure the true frequency changes the sensitivity of this measurement is extremely high. Translating the amount of time spend at a certain frequency away from the carrier (ADEV?) into a phase noise number in dBc is something I do not yet understand.
> With a (very good) spectrum analyzer you may be able to come close to the carrier but as there is so much energy in the carrier it will be difficult to observe phase noise energy closer than say 1 or 10 kHz (at least not with the equipment I can afford) so any phase noise plot created using a spectrum analyzer can not be better than the combined phase noise of all LO's in the spectrum analyzer and will start at say 1 or 10 kHz.
> For the frequencies between 50 Hz and 20 kHz the simplest option is to use a second LO and a mixer and a slow  (loop BW below 10 Hz)PLL to keep the mixer in quadrature and feed the output of the mixer, after low pass filtering, into a PC soundcard for FFT processing.
> Erik.
> 
> On 19-6-2022 22:45, Bob kb8tq via time-nuts wrote:
>> 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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