[time-nuts] Phase measurement of my GPSDO

Fri Apr 3 23:56:41 UTC 2020

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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