[time-nuts] Phase measurement of my GPSDO

John Ackermann N8UR jra at febo.com
Sat Apr 4 01:35:25 UTC 2020 

Hi Tobias --

Several years ago, with a bunch of help from Bruce and John Miles, I did
a very high isolation, very low phase noise buffer amp design that TAPR
sold for a limited run.  It's built with surface mount parts but they
are user-friendly sized.

Details and schematic are at https://www.febo.com/pages/TNS-BUF.  The
TAPR product page and link to manual are at
https://tapr.org/product/tns-buf-isolation-amplifier/

It is possible we might still have some bare boards available; I need to
check on that.  It's also possible that if there's enough interest, we
could do another small production run (we'd need at least 25 orders to
make it economically feasible).

John
----

On 4/3/20 8:09 PM, Tobias Pluess wrote:
> Hi Bruce
> 
> the NIST design you mentioned - do you mean that publication where they
> used 2N2222's for a diode ring mixer? if so I can perhaps build this as
> well because I think I even have some 2N2222s in my home lab :-)
> Concerning the RPD vs. TUF mixers - what is the actual property which makes
> the RPD "better" than the TUF?
> 
> Thanks,
> Tobias
> 
> On Sat., 4 Apr. 2020, 02:01 Bruce Griffiths, <bruce.griffiths at xtra.co.nz>
> wrote:
> 
>> Tobias
>>
>> That would certainly work for a start and have a better performance that a
>> counter front end.
>> The performance can be estimated using the tools at the link Bob provided.
>> Lower noise opamps will improve the performance somewhat.
>> A wider bandwidth opamp with a higher slew rate may be useful for the
>> final stage of a Collins style zero crossing  detector.
>> The RPD series of phase detectors will have better performance than the
>> TUF-1.
>> For the ultimate performance at low offset frequencies one can build a
>> mixer using diode connected BJTs as NIST have done.
>>
>> Bruce
>>> On 04 April 2020 at 12:38 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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