[time-nuts] Making a HP 10811 better

[Rick Karlquist]

The problem you described was more of an issue in the 5061.
Len Cutler was extremely relieved that the 5071 didn't have to
have an analog integrator.

In the 10811, the biggest problem is that the capacitor is
barely large enough for loop stability purposes.  That prevents
you from increasing the gain regardless of how many megohms
you can achieve.  The last time I checked, you still could not
get a larger capacitor value that would physically fit in the space in
the 10811.  Everything in the 10811 is max'ed out, per
discussions with the designers, who recently retired.

It is also worth noting that you can get a thermal gain of over
1000 by optimizing the power sharing between the resistors.
This experiment is done with the oscillator in B mode so the
crystal is a thermometer.  Unfortunately, this thermal gain
maximization does not optimize the 10811 tempco because it
ignores the tempco of the electronics.  You have to make the
oven tempco cancel out the electronics tempco.  This cannot happen at
a turnover of course, which leads to another discussion.  A large
fraction of 10811 crystals do not have a turnover, only a
region of very low tempco.  Thus the discussion about getting
right on the turnover point is moot.

In the E1938A, we had crystals that definitely had turnovers,
and set each one at a turnover.  What a nightmare in production.
The 10811 paradigm looked really good by comparison.

You can now see how complicated it gets if you fool around with
a 10811.

Rick Karlquist N6RK



Bob Camp wrote:
> Hi
>
> Here's the gotcha with the "integrator". The poor thing starts out with a
> closed loop gain of a bit over 4. That's *very* low by oven controller
> standards. The gain gets up to 40 or so by the time the gizmo labeled 2 uf
> gets up to 40 meg ohms. It's going to have a hard time going 10X above
> that. Getting up to 400 Meg on the pc board is going to be a challenge.
> You have to guess what the cap is made out of, so coming up with an exact
> mega ohm microfarad product for it is a challenge. It's likely that it's
> going ot cut in below 1 G ohm. That's when everything is new, clean and
> dry.
>
> Then if you just happen to have dust / dirt / humidity / spider webs ...
> there goes your 400 Meg. Humidity in particular is nasty. It goes in
> easily and it's very hard to drive out.
>
> The DC gain is unlikely to make it past a few hundred under the best of
> conditions. You are getting maybe a 10X boost over a normal controller
> when it all works right. When the insulation resistance starts to go down,
> you pay for it with shifts.
>
> Bob