[time-nuts] Metastability in a 100 MHz TIC
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Jul 20 20:54:23 EDT 2007
Tom Van Baak wrote:
> I like your point about the random quantization error in the
> sawtooth. Yes, that would help the noise by a few dB.
> On the other hand it would also seem the 10 ns resolution
> of the TIC is the limiting factor (by an order of magnitude)
> over the 1 ns resolution limit of the sawtooth corrections,
> so improving the quality of the sawtooth corrections has
> limited gain.
> Now, I'd still like to pursue the issue of noise in the 100 MHz
> oscillator. Do we agree one doesn't need a cesium for this?
> Or even an XO?
As long as one keeps track of the frequency drift of the timebase
oscillator this is true.
> True, you want some accuracy in the 100 MHz. But the counts
> are only integers from 0 to 160 so the accuracy requirement
> is just 3 digits, 0.1% (so cheap quartz, at 1e-6, or cesium, at
> 1e-13, is extreme overkill). I mean, almost anything wiggling at
> 100.0 MHz will serve as an adequate timebase.
> Also, as you point out, instability or jitter is your friend, not
> your enemy in this case. Would it be possible to introduce
> the +/- 5 ns jitter deliberately in the 1pps trigger level instead
> of in the timebase? I.e., slow down the rising edge enough
> so that you get jitter for free?
Yes adding stochastic jitter to the leading edge of the PPS signal is a
good way of injecting the required noise into the measurement.
The most predictable way is to slow down the PPS edge (a simple RC
filter should be more than adequate ) and feed it into one input of a
comparator whilst the other input is connected to a noise source.
> Another solution might be to deliberately choose an inaccurate
> and unstable oscillator; use the 1pps to count oscillator cycles
> per second, as well as to count the time interval. The larger
> count can be used to calibrate the smaller count on every count.
> This gives all the jitter you need and avoids any injection issues.
Yes a suitably noisy LC oscillator should work, will probably need to
reduce the tank Q somewhat to achieve sufficient noise/jitter.
Deliberately using resistors to introduce predictable noise may be a
Attenuating the sinusiodal ouput of an oscillator that is too stable
using a resistive attenuator with a high output resistance may also be a
viable technique for producing a sufficiently noisy clock.
> While you're at it, how about N of these oscillators, each making
> its own out of phase measurement of the same OCXO-GPS 1pps.
> Another couple of dB of resolution...
This is getting way to complicated, surely the method of using the
inherent noise in the hardware corrected PPS signal in conjunction with
a D flip flop acting as a simple precedence detector (as proposed
several weeks ago) is easier, cheaper and simpler, it also has more
resolution than almost any other method one can devise. It can easily be
elaborated slightly to allow detection and rejection of phase error
Clock the D flip flop with the hardware corrected PPS signal, connect
the divided down OCXO (or other standard) being disciplined to the D
input. Interrupt the microprocessor on the trailing edge of the PPS
signal, read the Q output of the D flip flop (the 200 milliseconds PPS
width of the PPS signal from an M12M or similar GPS timing receiver is
more than adequate to allow the D flipflop to settle with an extremely
low probability of being in a metastable state- even a few microseconds
is probably more than sufficient) and add 1 to the phase count whenever
D is 1 subtract 1 whenever D is 0. The EFC voltage is then adjusted to
keep the phase error at zero (corresponds to a 50% probability of the D
flipflop output being 1 when read by the micro). This simplistic
algorithm can be replaced by a more optimum algorithm as required.
This can all be built with a few DIP ICs to ease assembly, however a
well designed 2 layer PCB with a ground plane is probably advisable.
Other than a programmable delay line and a high resolution DAC no
unusual parts are required.
Instead of using one microprocessor to do everything several of simpler
processors each dedicated to a particular function may be better.
One processor could be dedicated to hardware correction of the PPS
signal, whilst another can implement the phase error measurement and the
OCXO control loop.
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