[time-nuts] Phase measurement of my GPSDO

Fri Apr 3 16:57:19 UTC 2020

Your counter can measure 1e-9 *at 1 Hz*  but you are feeding it with 10
MHz's worth of noise, so divide the reading by the factor of the down
mixing (1e7) so the result is 1e-16 -- you are multiplying the effective
noise.

Though as Bob says, you don't get close to 7 digits of improvement
without paying attention to a lot of other details.

John
----

On 4/3/20 11:59 AM, Tobias Pluess wrote:
> Hi John
> 
> Yes, I totally agree with you and I also understand the difference.
> But what I still don't understand is the following:
> Obviously, my 5335A is not accurate/precise enough to measure below 1e-9
> for short tau. Currently I am comparing the 1PPS signals, but when I change
> that and use the DMTD method, I will still compare some 1Hz signals, and
> the counter is still not able to resolve stuff that is lower than 1e-9. So
> why would the DMTD work better?
> I totally see that the error is somehow multiplied, but if my GPSDO is good
> (which I hope it is :-)) the error will still be very small - perhaps in
> the 1e-9 or 1e-10 region, so too low for my 5335A. Not?
> 
> 
> Tobias
> 
> On Fri, Apr 3, 2020 at 5:34 PM John Ackermann N8UR <jra at febo.com> wrote:
> 
>> I think the difference is between *mixing* or *dividing* down to a low
>> frequency.
>>
>> When you divide, you divide the noise along with the carrier frequency.
>>
>> When you mix, you "translate" the noise.  If the signal bounces around
>> 0.1 Hz at 10 MHz (awful, I know), when you divide to 1 PPS the noise is
>> also divided by 1e7 so the ratio remains the same.
>>
>> But if you mix via a 9.999 999 MHz local oscillator, now your output at
>> 1 Hz still has 0.1 Hz of noise on it.  i.e., it's the same absolute
>> value of noise as you started with.  So you measure that absolute value
>> but don't compare it to the mixed down 1 Hz frequency, compare it to the
>> original 10 MHz frequency.  It's basically an error multiplier.
>>
>> John
>> ----
>>
>> On 4/3/20 11:25 AM, Tobias Pluess wrote:
>>> Hi again Bob,
>>>
>>> yes you describe a simple DMTD measurement. But could you tell me what
>> the
>>> difference is between that and comparing the 1PPS pulses?
>>> I mean, I could set the 10811 high in frequency by just 1Hz, and then it
>>> would result in two 1Hz signals which are then compared.
>>> Which is essentially the same as comparing two 1PPS signals, isn't it?
>>> Ok there is a minor difference: since the 1PPS signals are divided down
>>> from 10MHz, their noise is also divided down, which is not the case for
>> the
>>> DMTD.
>>> However, in the end I am comparing signals in the 1Hz to 5Hz or 10Hz
>>> region, and apparently, the 5335A is not suitable for those, at least not
>>> with the desired stability, is it?
>>>
>>>
>>> Tobias
>>>
>>> On Fri, Apr 3, 2020 at 1:45 PM Bob kb8tq <kb8tq at n1k.org> wrote:
>>>
>>>> Hi
>>>>
>>>> The quick way to do this is with a single mixer. Take something like an
>> old
>>>> 10811 and use the coarse tune to set it high in frequency by 5 to 10 Hz.
>>>>
>>>> Then feed it into an RPD-1 mixer and pull out the 5 to 10 Hz audio tone.
>>>> That tone is the *difference* between the 10811 and your device under
>>>> test.
>>>> If the DUT moves 1 Hz, the audio tone changes by 1 Hz.
>>>>
>>>> If you measured the 10 MHz on the DUT, that 1 Hz would be a very small
>>>> shift
>>>> ( 0.1 ppm ). At 10 Hz it’s a 10% change. You have “amplified” the change
>>>> in frequency by the ratio of 10 MHz to 10 Hz ( so a million X increase
>> ).
>>>>
>>>> *IF* you could tack that on to the ADEV plot of your 5335 ( no, it’s not
>>>> that
>>>> simple) your 7x10^-10 at 1 second would become more 7x10^-16 at 1
>>>> second.
>>>>
>>>> The reason its not quite that simple is that the input circuit on the
>>>> counter
>>>> really does not handle a 10 Hz audio tone as well as it handles a 10 MHz
>>>> RF signal. Instead of getting 9 digits a second, you probably will get
>>>> three
>>>> *good* digits a second and another 6 digits of noise.
>>>>
>>>> The good news is that an op amp used as a preamp ( to get you up to
>> maybe
>>>> 32 V p-p rather than a volt or so) and another op amp or three as
>> limiters
>>>> will
>>>> get you up around 6 or 7 good digits. Toss in a cap or two as a high
>> pass
>>>> and low pass filter ( DC offsets can be a problem ….) and you have a
>>>> working
>>>> device that gets into the parts in 10^-13 with your 5335.
>>>>
>>>> It all can be done with point to point wiring. No need for a PCB layout.
>>>> Be
>>>> careful that the +/- 18V supplies to the op amp *both* go on and off at
>>>> the
>>>> same time ….
>>>>
>>>> Bob
>>>>
>>>>> On Apr 3, 2020, at 5:13 AM, Tobias Pluess <tpluess at ieee.org> wrote:
>>>>>
>>>>> hi John
>>>>>
>>>>> yes I know the DMTD method, and indeed I am planing to build my own
>> DMTD
>>>>> system, something similar to the "Small DMTD system" published by
>> Riley (
>>>>> https://www.wriley.com/A Small DMTD System.pdf).
>>>>> However I am unsure whether that will help much in this case, because
>> all
>>>>> what the DMTD does is to mix the 10MHz signals down to some 1Hz Signal
>> or
>>>>> so which can be measured more easily, and I already have 1Hz signals
>> (the
>>>>> 1PPS) which I am comparing.
>>>>> Or do you suggest to use the DMTD and use a higher frequency at its
>>>>> outputs, say 10Hz or so, and then average for 10 samples  to increase
>> the
>>>>> resolution?
>>>>>
>>>>> Thanks
>>>>> Tobias
>>>>> HB9FSX
>>>>>
>>>>>
>>>>> On Fri, Apr 3, 2020 at 12:53 AM John Miles <john at miles.io> wrote:
>>>>>
>>>>>>> b) if I want to measure 1e-11 or even 1e-12 at 1sec - what resolution
>>>>>> does
>>>>>>> my counter need? If the above was true, I would expect that a 1ps
>>>>>>> resolution (and an even better stability!) was required to measure
>> ADEV
>>>>>> of
>>>>>>> 1e-12, The fact that the (as far as I know) world's most recent,
>>>>>>> rocket-science grade counter (some Keysight stuff) has "only" 20ps of
>>>>>>> resolution, but people are still able to measure even 1e-14 shows
>> that
>>>> my
>>>>>>> assumption is wrong. So how are the measurement resolution and the
>> ADEV
>>>>>>> related to each other? I plan to build my own TIC based on a TDC7200,
>>>>>> which
>>>>>>> would offer some 55ps of resolution, but how low could I go with
>> that?
>>>>>>
>>>>>> That sounds like a simple question but it's not.  There are a few
>>>>>> different approaches to look into:
>>>>>>
>>>>>> 1) Use averaging with your existing counter.  Some counters can yield
>>>>>> readings in the 1E-12 region at t=1s even though their single-shot
>>>> jitter
>>>>>> is much worse than that.  They do this by averaging  hundreds or
>>>> thousands
>>>>>> of samples for each reading they report.  Whether (and when) this is
>>>>>> acceptable is a complex topic in itself, too much so to explain
>> quickly.
>>>>>> Search for information on the effects of averaging and dead time on
>>>> Allan
>>>>>> deviation to find the entrance to this fork of the rabbit hole.
>>>>>>
>>>>>> 2) Search for the term 'DMTD' and read about that.
>>>>>>
>>>>>> 3) Search for 'direct digital phase measurement' and read about that.
>>>>>>
>>>>>> 4) Search for 'tight PLL' and read about that.
>>>>>>
>>>>>> Basically, while some counters can perform averaging on a
>> post-detection
>>>>>> basis, that's like using the tone control on a radio to reduce static
>>>> and
>>>>>> QRM.  It works, sort of, but it's too late in the signal chain at that
>>>>>> point to do the job right.  You really want to limit the bandwidth
>>>> before
>>>>>> the signal is captured, but since that's almost never practical at RF,
>>>> the
>>>>>> next best thing to do is limit the bandwidth before the signal is
>>>>>> "demodulated" (i.e., counted.)
>>>>>>
>>>>>> Hence items 2, 3, and 4 above.  They either limit the measurement
>>>>>> bandwidth prior to detection, lower the frequency itself to keep the
>>>>>> counter's inherent jitter from dominating the measurement, or both.
>>>> You'll
>>>>>> have to use one of these methods, or another technique along the same
>>>>>> lines, if you want to measure the short-term stability of a good
>>>> oscillator
>>>>>> or GPSDO.
>>>>>>
>>>>>> -- john, KE5FX
>>>>>>
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> time-nuts mailing list -- time-nuts at lists.febo.com
>>>>>> To unsubscribe, go to
>>>>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>>>>>> and follow the instructions there.
>>>>>>
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at lists.febo.com
>>>>> To unsubscribe, go to
>>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>>>>> and follow the instructions there.
>>>>
>>>>
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at lists.febo.com
>>>> To unsubscribe, go to
>>>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>>>> and follow the instructions there.
>>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at lists.febo.com
>>> To unsubscribe, go to
>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>>> and follow the instructions there.
>>>
>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com
>> To unsubscribe, go to
>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>> and follow the instructions there.
>>
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
> 