[time-nuts] Phase measurement of my GPSDO

Bob kb8tq kb8tq at n1k.org
Fri Apr 3 23:50:35 UTC 2020 

Hi

A *single mixer* setup is something that can be done quickly and easily.
The *dual mixer* setup brings in a bunch of issues that are far more 
easily handled on a good PCB layout. 

Either way, it is going to work far better with the right sort of low noise
( = single digit nanovolt per root hz …) op amps than with whatever 
you happen across first ….

Bob

> On Apr 3, 2020, at 7:38 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> 
> Hi Bruce
> 
> I have some TUF-1 mixers in my junk box as well as some JFET OpAmps AD8626.
> So, if I connect the OpAmps appropriately with some diode limiters as you
> suggest, would you say this would give an acceptable DMTD system?
> If so it sounds like something that can easily be built on a breadbord or
> in manhattan style, as Bob already mentioned. That would be really cool.
> I think a while ago I asked a question which goes in a similar direction -
> which mixers are better as phase detectors (to build a PLL for phase noise
> measurement) and which ones should be used as actual mixers (like in this
> case).
> 
> 
> Tobias
> HB9FSX
> 
> On Fri., 3 Apr. 2020, 23:09 Bruce Griffiths, <bruce.griffiths at xtra.co.nz>
> wrote:
> 
>> One can merely add diodes to the opamp feedback network form a feedback
>> limiter and maintain the opamp outputs within the range for which the opamp
>> is well behaved whilst maintaining the increase in slew rate for the output.
>> 
>> Bruce
>>> On 04 April 2020 at 04:26 Tobias Pluess <tpluess at ieee.org> wrote:
>>> 
>>> 
>>> Jup, some of them even have phase reversal when they are overloaded, so
>> it
>>> is perhaps not a good idea in general, but I think there are opamps which
>>> are specified for this.
>>> 
>>> Tobias
>>> 
>>> 
>>> On Fri, Apr 3, 2020 at 3:30 PM Dana Whitlow <k8yumdoober at gmail.com>
>> wrote:
>>> 
>>>> Caution: opamps make terrible limiters- their overload behavior is
>>>> generally ugly
>>>> and unpredictable.  It's much better to use a genuine level
>> comparator, and
>>>> wire it
>>>> up so that it has a modest amount of hysteresis.
>>>> 
>>>> Dana
>>>> 
>>>> 
>>>> On Fri, Apr 3, 2020 at 6:45 AM 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
>>>>>>> 
>>>>>>> 
>>>>>>> 
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