[time-nuts] Phase measurement of my GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Apr 4 01:26:36 UTC 2020 

Tobias

The diode connected BJT (2N2222) mixer is compared with various commercial mixers and phase detectors in a NIST paper that has a graph showing the PN of various mixers as a function of offset frequency.

The RPD series phase detectors have a higher output and lower PN than most mixers.
The output depends on the input characteristics of the lowpass filter at the IF output.
A capacitive load at this port increases the output at the expense of isolation between ports etc.
These interactions are clearly shown in Spice simulations of such mixers.

Bruce 

> On 04 April 2020 at 13:09 Tobias Pluess <tpluess at ieee.org> 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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