[time-nuts] Sub Pico Second Phase logger
warrensjmail-one at yahoo.com
Fri Dec 5 04:39:55 UTC 2008
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.
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.
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.
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