[time-nuts] HP5370B anomaly

[Bruce Griffiths]

Magnus Danielson wrote:
> Hi!
>
> On 12/27/2010 06:35 AM, Bruce Griffiths wrote:
>>> Considering that:
>>>
>>> 1) I've measured the same signals on similar or higher performance
>>> counters and not seen it with those units.
>>>
>>> 2) The frequency difference is so low that I do not experience a
>>> phase-wrap for the 2476 s long measurement run.
>>>
>>> 3) The phase slope is much lower than the amplitude of the
>>> oscillation, so the linearity error can be ruled out completely.
>> Why?
>> What about a periodic effect due to crosstalk at the interpolator mixer
>> FFs inputs?
>
> The noise seems independent of the slope of the signal, but do depend 
> on how I set up the input signals. The three last plots are 
> interesting...
>
> http://rubidium.dyndns.org/~magnus/time/hp/hp5370/HP5370B-inputwander.png
>
> SEP plot:
>
> 1 Hz to EXT ARM from TADD-2
> 5 MHz to the START channel
> 5 MHz to the STOP channel
> http://rubidium.dyndns.org/~magnus/time/hp/hp5370/HP5370B-4.tim
>
> COM plot:
> Same as above, but changed the setup from SEP to COM.
> http://rubidium.dyndns.org/~magnus/time/hp/hp5370/HP5370B-5.tim
>
> COM no STOP plot:
> Same as above, but with the STOP channel disconnected.
> http://rubidium.dyndns.org/~magnus/time/hp/hp5370/HP5370B-6.tim
>
> So, I *do* think that there is some disturbance being injected into 
> the signal-path, but I do not think it is normal non-linearity which 
> would depend on rate of phase. This disturbance is however shifting 
> with what signals I have hooked up, even if they are not used.
>
> Looking up on details, the LED detection on A22 is clocked by a 10 Hz 
> coming from the A11 board where a RC-based oscillator generates it, so 
> it's frequency is separate from any other. And notice how my 10 Hz 
> sampling experiment does not give much difference than the 1 Hz 
> sampling rate.
>
> So, I suspect the noise I see is the beat frequency between the 10 Hz 
> oscillator and my 1 Hz or 10 Hz signal.
>
> The use of ECL FF for LED sampling isn't quite the best choice... 
> sharp edges...perfect for injecting noise.
>
>>>
>>> 4) The amplitude is stable and does not change with the slope.
>>>
>>> 5) The unit does not have the 5 MHz noise as I disabled that "feature".
>>>
>>> I do howver realize that the START and STOP channels blinks at a
>>> fairly high rate. These LEDs are driven with signals coming from the
>>> A22 board, where all the critical timing also passes by... will try to
>>> figure out a way to handle it, but right now it severely degrades the
>>> performance of my HP5370B.
>>>
>>> I think I discovered the effect at least, with a suspect mechanism for
>>> it.
>> You suspect the LED drive signal?
>
> Or some form related signal. I suspect the LED detection...
>
>>>
>>> It would be nice if others would see if they could achieve the same
>>> effect.
>>>
>>> Cheers,
>>> Magnus
>>>
>>>
>>
>> What is the amplitude of the effect?
>> How good a source pair is required to see it?
>
> Some 200 ps or so in amplitude...

I haven't seen anything that large with my 5370A.
The noise of your setup appears to be at least 50% higher than I usually 
see.
What are the signal levels at the START and STOP inputs?
What are the START and STOP attenuator settings, coupling, impedance and 
trigger threshold settings?

>
> It degrades the performance of the HP5370B to the level of my HP5372A. 
> Even when setting up the less useful case it falls down a little.
>
> See how the same sources behaves when tossed to a DTS-2070C.
>
> Cheers,
> Magnus
>
>

Bruce