[time-nuts] Phase measurement of my GPSDO

Sat Apr 4 01:02:10 UTC 2020

Hi

A single mixer compares two devices. Provided you can offset the frequency
of one of your devices, it does exactly what you need to do.

Bob

> On Apr 3, 2020, at 7:56 PM, Tobias Pluess <tpluess at ieee.org> wrote:
> 
> Hey Bob
> 
> hmm how would a *single mixer* design look like? in the end I need to
> compare *two* clock signals, so a single mixer won't be of much use, would
> it?
> 
> Tobias
> 
> On Sat., 4 Apr. 2020, 01:51 Bob kb8tq, <kb8tq at n1k.org> wrote:
> 
>> 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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