[time-nuts] PIC Divider Performance (was Re: time-nuts Digest, Vol 56, Issue 71)

Tue Mar 31 00:39:19 UTC 2009

When Bruce raised the question a bit earlier, I was curious so set up a
quick experiment.

I happen to have two 10 MHz to 1 PPS dividers based on Tom's code.  They
lack the higher order outputs, so the modulation effects Bruce spoke of
aren't present, but they should be a good test for general jitter.  The
circuit was simple -- the PIC output fed three paralleled 74AC04
inverter gates, tied together through 47 ohm resistors on the output.
So, the PIC itself was driving a high impedance load.

I used a common 10 MHz signal to drive both dividers, and hooked their
outputs to a 5370B TIC.  Each divider ran from its own power supply, so
there shouldn't have been any coupling that way.

Over 1000 measurements at 1 PPS, I got a standard deviation of 46.9 ps.
 The delta between minimum and maximum readings was 330 ps.  Since we
were testing two independent dividers, I suppose you could divide the
standard deviation by the square root of 2, which gives about 33 ps.

I then did a test with a common PPS signal driving both inputs to the
5370B, with the stop signal run through a 4 meter cable for delay.  This
should show the noise floor of the TIC.  The results for 1000 samples
there were 23.0 ps standard deviation, and delta between min and max
readings was 140 ps.

Therefore, the PIC divider is a bit above the 5370B noise floor, but not
much.

(There is one point of caution in this comparison -- the noise floor
test with coax delay line had an absolute time interval of about 18
nanoseconds, while the interval between the two dividers was about 85
milliseconds.  In this quick test, I wasn't able to easily get the
dividers in closer sync than that.  With a longer time interval, the
effects of TIC timebase might become more significant, and I suppose
nonlinearities in the 5370B could also come into play.)

And, for what it may further be worth, I did a very rough tempco
measurement of another PIC divider and got about 50 ps/degree C over the
range of +20 to +75 degrees C.

John
----
Bruce Griffiths said the following on 03/30/2009 08:09 PM:
> Tom
> 
> Tom Van Baak wrote:
>>> Kit
>>>
>>> Probably the higher jitter and periodic phase modulation due to
>>> simultaneous switching of multiple outputs at different frequencies.
>>> The magnitude of the latter will depend on the loads driven by each output.
>>>
>>> The cure is to use an external flipflop to resynchronise the outputs to
>>> the 10Mhz clock.
>>>
>>> Bruce
>>>     
>> Kit, Bruce,
>>
>> There was no phase modulation effect that I could measure.
>> Note that in that design all pins (a single 8-but IO port) are
>> re-written each time through the loop; not just ones that change.
>> See the source code for details.
>>
>>   
> 
> 
> But for example the 100KHz output pins actually only switch state every
> 5th cycle of the 1MHz output.
> This will modulate the phase of the 1MHz output at 100KHz due to ground
> bounce.
> The magnitude of the modulation will depend on the load at the pins.
> The higher the load capacitance (or lower the load resistance) the
> greater the effect.
> The effect will always be present, although whether you can detect it
> depends on the resolution of the test setup and the pin load (C and R).
> An SR620 is unlikely to have sufficient sensitivity for detecting the
> effect with light pin loading.
> 
> In an FPGA with CMOS I/O such ground bounce and other coupling effects
> can be a few tens of picosec even though the intrinsic jitter of the
> internal logic elements is much smaller than this.
> The PIC only has a single ground pin with a bonding wire inductance of a
> few nH. If the outputs drive significant capacitance then the resultant
> ground bounce can be significant.
> 
> An external flipflop can be connected so that it doesn't share the same
> internal chip Vcc and GND wiring with outputs switching at different rates.
> External ground plane noise can be much lower than internal chip GND net
> noise.
> 
> 
>> My understanding of the PIC architecture is that all outputs
>> are essentially "resynchronized" to the clock by design. So
>> that's why the PIC divider works so well. I can't see how an
>> external off-chip flip-flop would be better than the existing
>> internal on-chip flip-flop. Might even make things worse?
>>
>>   
> 
> 
> Only if one uses a slower external flipflop and/or a poor clock
> buffering scheme.
> 
> 
>> But I don't know for sure and should not guess. In cases like
>> this I'd take an actual test over a random guess.
>>
>> As for jitter, I tested the PIC divider when I wrote it ten years
>> ago and if I recall correctly the jitter was just over what I could
>> measure with a SR620; about 25 ps. With better equipment
>> these days, one could measure how much of that is input jitter,
>> or output jitter, or measurement system jitter. But I don't have
>> anything better than a 5370 or 620 for 1PPS measurements.
>>
>> I know the PIC divider was an order of magnitude better than
>> other discrete 1PPS dividers that I had at the time, and it was
>> 100x better than the reference 1PPS out of any GPS boards
>> that I had, so I was very pleased with the performance (and
>> the simplicity, and the cost) of the one-chip divider concept.
>>
>>   
> 
> 
> Almost anything reasonable is better than a cascade of 74XX90's with a
> ripple cascade scheme between divide by 2 and divide by 5 sections.
> 
> 
>> But it would be very interesting to me if someone with a working
>> Wavecrest could make measurements of various PIC dividers
>> and refine this old data; to find out just how low the noise floor is.
>>
>> /tvb
>>
>>
>>
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>>   
> 
> Bruce
> 
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