[time-nuts] Bad batch of HP10811's

[Rick Karlquist]

Didier Juges wrote:
> It may be due to the fact that the crystal in the HP 10811 is optimized
> for flat temperature sensitivity around the operating point of the oven,
> when the cheap crystals I have checked are optimized around ambient (if
> they are optimized at all).

For many years, 10811 crystals merely had to achieve a max tempco
spec around 80 degrees.  Many had no turnover, only a very flat
slope.  In later years, the process was tweaked slightly to try
to get a real turnover around 80 degrees.  While all of this has
little impact on normal operation, it can have a large affect on
the frequency at room temperature, a parameter no one worried about,
and which we only have anecdotal data on.

> Didier KO4BB

>> The possible reason for this offset is probably not the crystal, but the
>> tempco of the rest of the circuit. The oven heats up till about 82
>> degrees centigrade. A PN-juntcion at room temperature (22 degrees) will
>> increase 60 degrees in temperrature, it's forward voltage will change by
>> approx. 120 mV!!!!! This change will lead to a significant change in
>> circuit properties (e.g. of the oscillator and the attached AGC
>> circuit).
>>
>> Probably this was the reason HP specfied it's oscillator first after 10
>> mins of warm-up.
>> Jeroen PE1RGE

This is certainly incorrect, which I know both from experience with
the 10811 and also the fact that I have talked to its designers
extensively.  The oscillator circuit works just fine at room temperature
and in any event could not possibly pull the crystal even 1 Hz.  Even
the EFC circuit can only pull it a few Hz.

The 10 minutes of warmup is related to the fact that at low ambient
temperatures, it can take nearly that long for the oven current to
cut back, plus you need a few minutes, even with an SC cut, for the
frequency to settle down even after reaching oven temperature.

Rick Karlquist N6RK