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

[Magnus Danielson]

Erik,

A counter actually measures a number of phase measurements. Then, as you 
process that you get a frequency readout based on the difference between 
them (event-count divided by time between phase measurements). Now, as 
you want to do frequency read-out, you can do a handful of filtering 
mechanisms, and the CNT-91 can do the linear regression. This filtering 
takes a number of samples and provides a filter to estimate frequency. 
The consequence of that is that variations you see now have a different 
scale than if you did the original calculation of only two 
phase-samples. This creates a bias function and variations needs to be 
corrected for to get numbers you can relate to the normal scale. It's 
great for giving better frequency readings, but if you aim to quantify 
the variations you end up fooling yourself.

Also, your assumption on observation frequency in Nyquist is wrong. 
Turns out that aliasing of higher frequencies is very problematic. It is 
only very recent instruments that can have the ability to avoid aliasing 
(by using digital decimation), but a counter is not one of them, it is 
fully exposed to the aliasing problem.

There is translations charts to convert the noise-amplitude of each 
noise type into phase-noise and ADEV readings. If you have truely random 
noise obeying the rules, you can convert between them. Toss in a spur, 
and it works differently, and well, you need to convert those too 
according to other rules. Look for "Enricos chart".

Noise types reaching for high frequencies compared to measurement tau0 
will affect the resulting ADEV for sure. The bandwidth of that even 
affects white phase modulation directly, and flicker phase modulation to 
some degree.

So, a counter is really like an ADC for phase, with wide bandwidth input 
and a sub-sampling mechanism (trigger/time-base). Through processing 
frequency estimates can be provided. Aliasing occurrs in the 
sub-sampling. Modern counters can provided estimation filters than goes 
from a higher sub-sampling rate to a lower, which to some degree removes 
aliasing, but not fully. These frequency estimation methods form a form 
of decimation filter.

Cheers,
Magnus

On 2022-06-20 08:45, Erik Kaashoek via time-nuts 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 :
>>>
>>>
>>>
>>> *Â Â Â  There is one 10 MHz OCXO with ADEV = 5 mHz
>>> *Â Â Â  There are two boards (DUT1 and DUT2) which multiply 10 MHz OCXO 
>>> signal by 2.5 using the PLL method
>>> *Â Â Â  DUT1 has 25 MHz output signal with high PNÂ  (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:
>>> *Â Â Â  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)
>>> *Â Â Â  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)
>>> *Â Â Â  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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