[time-nuts] Metastability in a 100 MHz TIC

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jul 20 21:19:09 EDT 2007

Alan Melia wrote:
> Bruce I find this an interesting thread.......one maybe naive thought......
> "it would be nice to have a"too-good" stability on the 100MHz TIC but
> detracts from the averaging" (My interpretation), this almost suggests to me
> that a small amount of noise modulation which of course would be random,
> controlled,  and not biassed in a way to affect the accuacy of the driving,
> should be added to the 100MHz TIC OCXO. Would that counter the problems on
> uncharacterised drifting and still allow long averaging.?? Maybe even a slow
> unsynchonised low frequeny sine wave FM would achieve the same effect. It
> would seem this would be better than relying on processes which are unknown
> and not controlled to provide the effect.It is counter intuitive to
> intentionally degrade a "standard" in some respects but has been shown to
> work in some cases.
> Alan G3NYK

Using a slow unsynchronised sinewave is not the way to go a noise source 
is better and is easily implemented.
Essentially the technique used by HP in one of their counters would 
suffice, phase modulate say a 10MHz signal by a few degrees and then 
multiply the output by 10
to produce a 100MHz signal with 10x the phase modulation. This 
simplifies the design and construction of the phase modulator. A diode 
double balanced mixer can be used to phase modulate a signal by the 
required amount. Feed the LO port of the mixer with the signal to be 
modulated apply the modulation signal to the IF port and add the output 
of the RF port in phase quadrature with the original signal.

The drawback is the complexity and the fact that the resolution is still 
inadequate to achieve the maximum performance from the better GPS timing 
receivers with a good antenna and site. The simpler and cheaper D 
flipflop precedence detector used together with hardware sawtooth 
correction has far higher resolution. It also has the advantage of not 
requiring any high frequency clocks.


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