[time-nuts] Why using HP5370 ext-ref is (maybe) a bad idea

[Magnus Danielson]

On 01/03/14 00:06, Poul-Henning Kamp wrote:
> In message <53110BC6.6010109 at rubidium.dyndns.org>, Magnus Danielson writes:
>
>> Also, as I have told before the board doing the 10 MHz logic spews out a
>> lot of 5 MHz with overtones, which is a simple mod away.
>
> I remember you mentioning this, but I never did the mod on my counter,
> got anything I can search for in the mail-archive ?

Not from the top of my head. What I did was that I soldered one of the 
transistors base to ground (if I recall correctly) so that the 
comparator got stuck in the state. Fairly straight-forward. Look at the 
A8 board and the Q8 and Q6. That ECL loop requires the 10 MHz to be 
reasonably running for the LED to go on. Don't need that when not 
looking or suspecting problems. ECL having good rise-time creates 
shit-load of overtones.

>> Would be interesting to see if you could trim these systematics down by
>> tweaking the syntesis chain.
>
> It is not obvious to me that the 200MHz multiplier is involved in
> its own capacity, it may simply be that the 200MHz is slewed across
> the input signal and that the zero-crossing jitter therefore moves
> into the window where it matters ?

It does not have to be the 200 MHz syntesis, but it can be. The 10 MHz 
banks at the 50 MHz resonator tank every 50 ns through the transistor, 
and if de-tuned will the transitions be of the mark the further you go.
The same thing for the 200 MHz resonator tank. The filters helps to 
other frequencies out.

The resonator tanks is just re-triggered oscillators which have 
saw-tooth time-error phase which you can trim down by moving them more 
onto frequency.

Then again, 200 MHz may cross-talk into the signal path and modulate the 
trigger point. My guess is both happen to a degree.

Cheers,
Magnus