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

[Magnus Danielson]

Hi Poul-Henning,

On 02/03/14 23:29, Poul-Henning Kamp wrote:
>
> I have spent another evening playing around with the 5370 and the
> conclusion is pretty ironclad now:
>
> Running a 5370 with ext-ref locked to input frequencies is simply
> a bad idea and should not be done.
>
> Running it on the internal OCXO works fine.
>
> Running it on another frequency *not* locked to the input frequenc
> also works fine.
>
> In both cases the errors are statistically well-behaved, and can
> be treated with normal statistical methods, including the built-in
> STD-DEV function.
>
> But feeding ext-ref a frequency which is locked to the input frequencies
> causes the errors to become systematic, and they can no longer be
> treated as statistically well-behaved.

This comes as no surprise to me. I've expected this to be true for 
essentially all counters for ages. The relative timing of reference and 
trigger inputs interact with each others.

Running one of the input synchronous to the reference may not only 
create a maximum but also a minimum in noise. When inputs is 
asynchronous it is the average of this systematic pattern which is 
experienced.

> For instance:  The length of the coax to ext-ref suddenly affect
> your TI measurements, because it shifts the phase between the 200MHz
> and the input signal.
>
> I tried tuning up the A21 200MHz synthesizer to the best of my
> ability, and it clearly made a difference, the phase pattern
> of errors shifted around, but the errors did not get any smaller,
> they just moved.

Which then gives support to your theory that it is the 200 MHz itself 
rather than systematics of the synthesizer as I was theorizing about. 
Thus, as you tune the synthesizer you only phase-shift around the 
transitions. OK. Fair enough, that is expected to happen too.
The synthesizer probably needs to be very badly trimmed to cause 
systematics as I theorized.

> I also tried disconnecting the "10 MHz present" circuit, that
> didn't change the magnitude of the errors either, but did shift
> the phase of the peak noise a couple of degrees.

I used it to clean of the 5 MHz overtones and systematics.

> Looking at some old notes from years past which just didn't make
> sense, does now.

Good that things becomes clearer.

Cheers,
Magnus