[time-nuts] HP 5370B low frequency modulation

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jul 22 19:14:25 EDT 2007

Magnus Danielson wrote:
> Hi fellow time-nuts,
> As I was going to make a casual measurement of the HP 5370B 10 MHz output, i.e.
> that of its HP 10811-60111 OCXO, I came to see an unexpected modulation. It has
> a slow modulation frequency modulation of about 5,2 mHz or there abouts. Quite
> digital/squarewave-like I might add. Not actually squarewave, but rather some
> form of PWM signal. Another interesting aspect was that it had a fairly steady
> high, abrupt fall down a slight recovery, a jump upwards and then fall up to
> the steady high level. The high frequency was the longer part of the time.
> The cycle-time of these are low, for the 200 5 sec samples I took, I had about
> 10 cycles or so. So it is about two minutes cycles. The rate varied greatly.
> I have not seen any specific details in the HP 5370B service manual about where
> the HP 10811-60111 EFC is hooked up to anything, but I have noticed that more
> or less everything is grounded on the connector. Now, if there is low-frequency
> modulations of any large current there, it may kreep in. But it seems a bit
> high for that.
> I used both a Pendelum CNT-90 as well as a HP 5372A to measure this. I used
> both the Cs and Rb as reference and it made no difference. The CNT-90 between
> the two showed only noise, so this quick and dirty three-cornered hat certainly
> pointed its crookied finger at the HP 5370B. The HP 5372A was using its own
> well heated 10811-60111 as reference but that told the same story.
> I might add that the 5370B was running in "cabed" condition, i.e. top and
> bottom lids where off as well as the right side. The top lid needs to be off if
> you want to have a chance of trimming it. The 5370B has been in bypass 9 days,
> so I consider the 10811 fairly heated.
> Has anyone else seen this? Tom?
> Cheers,
> Magnus

Hej Magnus

Oscillation/cycling of oven temperature?
Oven controller oscillation?


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