[time-nuts] Sub Pico Second Phase logger
Joseph M Gwinn
gwinn at raytheon.com
Mon Dec 8 22:01:53 UTC 2008
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
time-nuts-bounces at febo.com wrote on 12/04/2008 11:39:55 PM:
>
> Building a Sub Pico Second phase detector.
>
> I was inspired to build this project yesterday after
> downloading and trying
> Ulrich Bangert's 'DF6JB's Plotter 2008-10-10' program with its
> unbelievable
> flexible user Interface capabilities. http://www.ulrich-
> bangert.de/html/downloads.html
> What I needed was a Phase detector to use with the 'Plotter' program.
> I decided to see what it takes to build a simple high
> resolution, sub Pico second,
> linear phase logging detector using standard off the self IC's.
>
> How If works:
> The 5 or 10 MHz signal to be measured is buffered and toggles a
> synchronous divide by two or four FF. This gives a 2.5MHz
> square wave and its complement.
> Each side of the flip-flop connects to two of four XOR gates.
>
> The 10 MHz reference signal goes thru a matching buffer and
> then to a pair of synchronous
> Flip-Flops that provide a zero and a 90 deg phase shifted 2.
> 5MHz square wave.
> Each of these goes to two inputs of the XOR gates. The four XOR
> phase detectors
> are connected to give four PWM type XOR phase detectors, each
> separate by 90 deg.
>
> Each of the four XOR outputs are buffered by a cmos buffer gate
> that has been powered by it's own 5 volt reference supply.
> The buffer outputs then goes thru a multi-stage passive RC
> filter set up to
> give two differential filtered PASSIVE + - 5 volt outputs, 90 deg apart.
>
> Logging Data:
> For the most flexible and best performance, two differential
> 16 plus bit ADC's
> should be used, each connected to one of the dual differential
> Phase detectors.
> After using the appropriate Analog RC filters, oversampling,
> digital filters, and digital
> scaling, you get a file with a single column of data to feed
> "Plotter" the phase
> difference of the two 10 MHz signals.
>
> The Data scaling and processing:
> For simple controlled short term or lower resolution data
> taking a PC Multimeter,
> if it is isolated so that you can use it differentially will
> work. If not you need to add a differential amp.
> For best performance, process the phase data from the two
> differential phase detectors
> through two identical digital filter algorithms.
> Doing this real time on a PC or after all the data is recorder
> on a XL spread sheet both work for me.
> Besides the filtering, the spread sheet or PC needs to also do
> the linearizing by
> ( K1* Phase1_Data) + (K2 * Phase2_Data).
> K1 and K2 are the sine value of their respective Phase detectors.
>
> One of the several tricks to why this can provide orders of
> magnitude better
> performance than is generally obtained from similar type phase detectors
> is because of the four matched Phase detectors that are added,
> subtracted
> and combined and linerized in such a way as to cancel the type of errors
> found in single XOR phase detectors.
>
> Preliminary Performance
> The noise floor that I have seen while feeding the same low
> noise osc, to both inputs,
> is around 10 uv peak to peak at low Bandwidths, at zero phase,
> using a 6 digit DVM
> with a slow filter which corresponds to <<1 ps. Test are still
> underway to see what the
> lower limit is, and what the sensitivity to the environment is.
>
> This is just the start of an on going learning project, It is
> just at the breadboard stage and
> needs to be verified, critiqued, cleaned up and packaged up.
> Noted that when working with sub ps resolution, extra care
> needs to be taken.
> Although it looks to be a standard digital circuit, It is not.
> It is a very sensitive Analog circuit
> capable of giving 1 part in a million type of resolution. It
> can resolve path distance changes
> in the 1/100 to 1/1000 of an inch, and needs to be built with
> care and 'respect'.
>
> Another use (beside watching just how noisy your "GOOD " osc is),
> It can be used to compare and adjust the freq differences
> between two osc
> very quickly and with more resolution than most can use.
> 1 E-12 freq difference gave several counts per second change on
> the DVM, and with the DVM updating at several times a second,
> it made fine freq adjustments much easer than slower monitoring ways.
>
>
> If you know of other simple high resolution phase detectors,
> or see any problems or improvements
> with the idea, I'd like to hear from you.
>
> Have fun
> WarrenS
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