[time-nuts] Making a HP 10811 better

[Rick Karlquist]

WarrenS wrote:
>
> The test took a while, but I finally got around to testing the effect of
> setting a HP10811's inner oven to the xtral's EXACT turn over temperature.
> Short answer is NO Help.
> I did not see any improvement in freq stability with environmental
> temperature changes.

You have reconfirmed what was established decades ago.
The 10811 oven has high thermal gain only at the location
of the crystal.  The oscillator transistor is ovenized at
a much lower level of stability.  As explained in my 1997
paper, the 10811 frequency is quite sensitive to the transistor
temperature.  The non-hermetic versions of the 10811 are
also quite sensitive to humidity for some other unknown reason.
The oscillator transistor is hermetic.  (BTW, the transistor
is a 2N5179 selected for minimum fT at 30 mA, to assure that the
oscillator starts).

It is also worth noting that about half the 10811 crystals do NOT
have a turnover, rather they have a very low, but not zero, tempco
from 80 to 84 degrees.  The ones that do have a turnover, have a
very shallow turnover, not like an AT cut.

The 10811 uses two transistors as point source heaters,
making a zero gradient oven an impossibility (see my 1997
paper on zero gradient ovens for why this is important).
The reason for the transistors was to avoid the PWM oven controller
used in its predecessor, the 10544, which had wire heaters.
The E1938A uses linear control transistors to control the
current in flex circuit heaters.  This gives the best oven
performance, but wastes a fair amount of power in the linear
regulators, which are outside the oven.  This extra thermal
load is not a big problem because it decreases at higher
temperatures, since the oven current is dropping off.

At HP, we looked at making improvements to the 10811 and were
never able to come up with any modifications that made sense.
It's not that you can't make a better oscillator than the 10811,
it is just that the existing design has pretty well been beat
to death, and you need to start over with a clean sheet of paper
to do better.

If you want to get serious about tinkering with the 10811 oven,
you need to convert the oscillator to run in mode B, which is
very temperature sensitive.  Then you can really see what the
oven is doing, at the crystal blank.  This is mentioned in the HPJ
article on the 10811.  (Required reading for anyone playing with
10811's).

Rick Karlquist N6RK