[time-nuts] Sub Pico Second Phase logger

[Joseph M Gwinn]

Bruce


time-nuts-bounces at febo.com wrote on 12/08/2008 07:12:22 PM:

> Joseph M Gwinn wrote:
> > Bruce,
> >
> >
> > time-nuts-bounces at febo.com wrote on 12/08/2008 05:53:08 PM:
> >
> > 
> >> Joseph M Gwinn wrote:
> >> 
> >>> People used passive mixers driving electromechanical stripchart 
recorders 
> >>> to compare high-stability oscillators in the good old days.
> >>>
> >>> One assumes that there is a purely analog approach to measurement of 

> >>> picosecond changes in delay at 10 MHz using a single oscillator, but 
I 
> >>> have not seen any methods described, probably because the relevant 
> >>> articles appeared many decades ago.
> >>>
> >>> Can anyone suggest some articles to read?
> >>>
> >>> Thanks,
> >>>
> >>> Joe Gwinn
> >>>
> >>> 
> >> Joe
> >>
> >> Although one could in principle do this with a single diode double
> >> balanced mixer used as a phase detector all one may end up measuring 
is
> >> the effect of ambient temperature changes on the mixer phase shift.
> >> Lower mixer phase shift tempcos are possible if the RF port is 
> >> unsaturated.
> >> 
> >
> > Single diode?  Why wouldn't one use a standard (MiniCircuits or the 
like) 
> > four-diode two-transformer double-balanced mixer as the phasedetector? 

> > Many mixers have IF response down to DC.
> > 
> Oops, I meant "single diode type double balanced mixer style phase
> detector".

Ah.  Four single diodes in a ratrace ring.  Max drive +13 dBm or so. 
Called Class I or Type I.

MiniCircuits ZRPD-1 being one example.

By the way, despite the circuit diagram in the datasheet, the 
corresponding phase-detector module MPD-1 can be wired to have the IF 
output ground isolated from the common RF, LO and case ground.  A little 
work with an ohmmeter will tell the tale.  This can help to contain the 
low frequency beatnote.

 
> >> A classical dual mixer system is probably better in that with matched
> >> tempco mixers maintained at the same temperature the differential 
phase
> >> shift tempco should (with careful matching) be lower.
> >
> > Dual mixer as in DMTD (dual mixer time difference) would certainly 
work, 
> > but is pretty complex and temperature sensitive.
> >
> > I did use a loaner Symmetricom 5120A (a full digital DMTD 
implementation) 
> > to make some measurements six months ago, and after a few days of 
> > continuous operation it had settled to the point that one could see 
0.01 
> > pS changes.  (And touching one of the BNC connectors caused a 1-3 pS 
> > jump.)  This instrument costs about $30K, and is intended more for 
> > measuring phase noise and allan variance than delay changes.
> >
> > Anyway, I have to wonder what people did before DMTD was invented.
> >
> >
> > 
> >> Other than the numerous classical papers on dual mixer systems and 
the
> >> occasionl NIST paper that have some mixer phase shift tempco data
> >> (albeit sparse), I am not aware of any specific papers.
> >
> > I've read many or most of the classical DMTD papers, and have seen 
various 
> > passing estimates that diode-ring mixers have a temperature 
sensitivity of 
> > 8 to 10 pS per degree C.  (I recall your figure was 10 pS/K.)  I 
assume 
> > that the DC offset also varies with temperature and drive signal 
> > amplitude.
> > 
> The only reference I have on the offset tempco is a miniciruits
> application note from which one can deduce that the equivalent phase
> shift tempco associated with the offset tempco is a few hundred
> femtosec/C (@ 10MHz +7dBm) at some temperatures for the particular mixer
> used. The graph also indicated (if you are lucky) that the offset tempco
> may be zero at around 20C.

Do you recall the part number?


> A NIST paper indicated that mixer phase shift tempco was around 10x
> lower if the Rf port was unsaturated. It also indicated that the mixer
> phase shift tempco is much lower if the input frequency is 100MHz rather
> than 10MHz. This was one reason given for shifting to 100MHz 
> DMTD systems.

Do you recall which paper?


What I've seen that seems useful is the Watkins-Johnson application note 
from 1978 on use of mixers as phase detectors: "Mixers as Phase 
Detectors", Stephan R. Kurtz, 8 pages.  This may be the source of the NIST 
article's information.  The electrons are available on the web from WJ 
Communications (now owned by TriQuint), filename "
http://www.wj.com/archive/documents/Tech_Notes_Archived/Mixers_phase_detectors.pdf
".  Don't know how long this URL will work, as WJ is assimilated into 
TriQuint.

 
> >> A purely analog approach to phase shift measurement has to be more
> >> difficult than a hybrid one using a pair of low frequency ADCs (eg 
high
> >> end sound card).
> >> 
> >
> > Is the sound-card approach workable at the millidegree to microdegree 
> > level, if the change is spread out over an hour?  One picosecond at 10 
MHz 
> > is 3.6 millidegrees of phase.
> >
> > Joe
> >
> > 
> Preliminary (non optimum) tests by Ulrich indicate that picosecond
> stability for times up to 100sec is very easy to achieve.
> Beyond that mixer phase shift tempco mismatch may be significant.

It would not be that hard to make an oven for the mixer, as the level of 
control needed is far less stringent than for a crystal.


> ADEV noise level of around 2E-14/Tau (1s < tau <100s).
> Haven't yet seen [or] have data for longer tau.

Yes.  Need at least 10^4 seconds.


> With identical beat frequency outputs, crosstalk between channels within
> the sound card shouldn't be a great problem.

I'm not sure I believe this, as there is likely ground coupling within the 
soundcard and the ear is famously insensitive to phase.  Channel isolation 
of 60 dB isn't enough to prevent phase shifts.


> In any case it's very easy to measure the crosstalk transfer function.

Yes.

 
> One concern particularly for low beat frequencies is the phase shift in
> the sound card input coupling capacitors (usually electrolytics).
> 
> It should be easy to test the sound card phase shift stability for this
> application by driving both inputs from the same signal source.

I assume that the beatnote must be ~100 Hz for the soundcard to handle 
with low phase shift.  One might get to 10 Hz, but 1 Hz is likely 
hopeless.

One thing that will be very useful is a list of sound cards by make and 
model, annotated with their advantages and disadvantages for time-nut use.

"High-end" may not be a sufficient description.


By the way, I looked at the operating and service manual for the HP 
K34-59991A Broadband Linear Phase Comparator.  Very interesting little 
gadget, but little performance data was given.  Does anyone know how well 
phase change can be measured?  It would be easy to duplicate this with 
modern ICs.  Also, about when was this unit made?  The manual has no date.


Joe