[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Dec 9 23:24:22 UTC 2008


Joseph M Gwinn wrote:
> 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.
>
>   
Yes, thats usually the case for the Minicircuits PCB mount phase
detectors and mixers except for some surface mount versions (usually the
very high frequency models).
A PCB mount mixer package is also preferable as its then much easier to
use a capacitive IF port termination (for lower noise) in conjunction
with series resistors at the RF and LO ports (for lower VSWR) than if a
mixer with SMA or other coax connectors were used.
>  
>   
>>>> 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?
>
>
>   
Supposedly an SRA-1, but some caution is in order as some statements as
to the effect of the input offset of an opamp based IF preamp in the
same application note were of dubious veracity unless one were to use an
inverting opamp input stage.
>> 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?
>
>
>   
http://tf.nist.gov/timefreq/general/pdf/971.pdf
<http://tf.nist.gov/timefreq/general/pdf/971.pdf>
Has some measurement data on mixer phase shift tempco and power
sensitivity and their frequency dependence etc.

I'll search for the paper that stated that the phase shift tempco was
lower if the RF port was unsaturated.
AFAIK there was no accompanying measurement data

> 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.
>
>   
It will be present but its effect in some cases (when the phase shift
between channels is such that the crosstalk phase is at 90 degrees to
the signal of interest) will be negligible, in other cases it is easily
measured and compensated for.
>   
>> 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
>
>   
Joe

I suspect that slow phase changes much less than 1ns or so are hard to
distinguish from gain drift given the gain tempco of the ECL phase detector.

A beat note near 1kHz appears to be even better if one is using
something like an enhanced Costas receiver or even using WKS
interpolation to locate and time stamp zero crossings.

So far only the M-Audio AP192 has been used.
Tests with an embedded motherboard 16 bit sound system show
significantly increased noise.
I've found that the noise level of motherboard sound systems varies
enormously from one motherboard model (sample of 2) to another.

Any 24 bit sound card with a performance close to or better than that of
the AP192 should suffice.
Other cards using AKM 24 bit ADCs should also be suitable.
Ideally an external sound card with balanced  XLR inputs would be best.

HP produced a number of different phase comparators each with a
different type of phase detector.
The K34-5991A design can't be older than the early 1970's because the
MECLIII devices used weren't available until then.

Warren built a similar phase detector (differential XOR or XOR + XNOR)
using CMOS ICs and for a common 10MHz input with a phase difference near
zero found short term output noise of of around 10uV or so (10V phase
detector FSR) using a passive low pass filter.
In principle an ADC like the LTC2484 could be used with a 2.5V CMOS
OR/XNOR phase detector and passive low pass filters.
The ratiometric conversion capability will significantly reduce the
sensitivity to the XOR gate supply if the XOR gate supply is also used
for the ADC reference voltage.
If one used 5V logic a resistive output attenuator would be needed which
reduce the gain stability somewhat.

All such phase detectors suffer from substantial nonlinearity near the
ends of the range due to gate output slew rate limitations.
However if operated near the centre of the range subpicosecond
sensitivity/short term stability may be possible.
Since the circuit volume is small and the tempco relatively low (few
ps/C at most) regulating the circuit temperature should be relatively
easy to do.

Bruce



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