[time-nuts] HP 5370B low frequency modulation

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jul 22 21:35:01 EDT 2007

Magnus Danielson wrote:
> Hej Bruce,
>> Oscillation/cycling of oven temperature?
>> Oven controller oscillation?
> Might be, but then my "loadingcondition" has caused the oven control out of
> stability. This would certainly supprice me. The oven control in the 10811 is
> linear and for most practical purposes behave like a first-degree linear
> diffrential system. OK, there might be a second degree aspect, but the poles
> should be fairly stable and it would be more a pure "amplitude" problem.
> Now as I was down putting the hood on (and removing some of the residues from
> the not so very smart packaging, using isopropanol) the down-spikes had started
> to become fewer and rare, and it had 490 uHz of Allan Deviation, i.e. 4,9E-11
> for tau = 5 s. I beleive it will considerably lower as the spiking stops.
> The full 10811 seems powered during standby mode. Some solutions will only have
> the oven powered in standby. What does change on powerup of the rest of the
> counter is that you turn on alot more heat, current, the fan (which has a fat
> metall wall between the flow and the 10811, but that piece of metal wrapps the
> 10811 so the cool-of-effect may be there never the less.
> After putting the lid on and letting it sit there the spikes is still evident
> and last reading gives 5,8E-11 @ tau = 5s.
> A comperative measure with another 10811 (which I just *happend* to have a
> cable for lying around on the table) but which was unheated also gave time for
> a little experiment, looking at the heat-up performance. Now, it started some
> 200 Hz hot, but went for a steady linear dive downwards which at about 400 s
> curved upwards and had a gentle overshot (i.e. lower frequency) until it
> stablized. However, subsequent measure showed a slow but steady rise and ADEV
> measures was surely limited by the driftrate. It was however much lower, such
> as 1.2E-11, and that includes the drift-rate tainting while the display
> indicated a much smoother curve.
> As I don't have a wiring aisle for my lab-bench (beleive me, I start to
> consider it as a must have even if it would be a squeeze-in variant of 3-4
> dm or so, I could not get the heated 10811 in my 5372A. I really need to hook
> the backside panels up to a BNC patch any decade now. Anyway, otherwise I would
> have a well heated alternative to measure on.
> I wish I get a phonecall with good news from a fellow time-nut tomorrow. If so
> I drive down to him. :-)
> Cheers,
> Magnus
Hej Magnus

What I had in mind was perhaps a bad joint or component failure in the 
oven regulator due to thermal cycling and or age of the oscillator.
Perhaps the oven regulator compensation capacitor??


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