[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 6 04:30:32 UTC 2008


WarrenS wrote:
> Bruce
>
> Thanks, for pointing out my FF mistake. I do know that at this level of resolution 
> you can not have two edges that are close together in the same AIR Space, 
> let alone the same IC without some interaction.I would not put them on the same PCB or PS
> when checking what the best possible resolution is.
>
> I was thinking because of the dual 90 deg paths, the IC in which the 
> two phases got close together would not be used at that time anyway so it did not matter too much. 
> Obviously I blew it big time by forgetting to consider the first set of FFs whose edges can and do cross. 
> In my case this is not a problem because these FF are in fact on the reference osc and not in my breadboard. 
> & I messed up, Thanks for pointing  what out.  
>
> The description I gave is not my present phase detector. 
> I include all the extra possibilities I could think of and some planned upgrades,
> to make it into a more General purpose & flexible tool instead of the simple 
> one function tool it now is. 
>
> I need to remember the KISS concept. The best is the simple way.
> For sub pico second measurement a single dual XOR differential phase detector is all that is needed. 
> All that extra stuff I is only needed if someone wanted to make a very general purpose 
> wide range less accurate XOR phase detector. 
>
> The trick to keeping it simple and accurate is the willingness to change the phase 
> of the low noise (2.5 MHz divided down) Osc to 90 deg offset at the start of the test.
> The way I do it is: on my test OCXO Osc I have added a semi calibrated, 10 turn dial that I can 
> fine set its freq to 1 part in 1E-12 resolution. 
> It also has an added switch that allows the output of the dual differential sub pico second Phase detector 
> to change the phase of  the test Osc in a short time period by way of the ECF.
> Once the two signal are phase locked at 90 deg, indicated by the dual phase detector at zero volts out, 
> I open the lock phase switch, re-adjust the fine freq trim pot if necessary so that the phase detector does 
> not drift off too fast, and rezero the detector again with the switch when required, etc. 
>
> To record data, I set my cheap little isolated 4+ digit PC logable DVM to its mV range, 
>  and log data at once per second which is good for as long as the phase detector's output  
> stays around zero. This all requires a manual set up, It takes a little extra time, BUT it gets the job done.
> It is also Very cheap to build and it seems to perform better than most near 'state of the art' home equipment.
> The whole thing takes 3 Ics including some extra non related stuffs. 
>
> Thanks for your feedback, If you see or think of anything else you feel would be helpful to me do please let me know.
>
> WarrenS
>
>   
Warren

Some commercial instruments share flipflops in the same package between
2 channels each with a slightly different clock frequency.
The crosstalk between channels is largely capacitive in nature and can
be canceled to some extent by injecting a small portion of the
(antiphase) clock, crosstalk from which is to be canceled, via a
capacitor (of appropriate value). Whilst some improvement is possible
this isnt a complete cure.

For the less stable oscillators, dividing down the frequencies before
comparison with the limited range XOR phase detector is desirable if
they are to be monitored over a relatively long time.

HP used to make the K34-5991A phase comparator that used an ECL XOR
phase comparator.
This instrument was intended for driving a strip chart recorder.
It allowed the user to set the output to the ideal value for 180 degree
and zero degree phase shift between the inputs for calibration purposes.

Ratiometric conversion of the low pass filtered XOR output will reduce
the dependence of the measurement on power supply voltage.

Bruce



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