[time-nuts] Making a HP 10811 better
Bob Camp
lists at rtty.us
Wed Mar 31 00:38:24 UTC 2010
Hi
.... and part of the gain is the insulation ... and the 10811 has thin insulation (compared to what people try to put on it ....).
--------------
I seem to remember going to a paper that talked about a lot of those nasty little component tempco's by ... ummm..... err...
Yes, I think it was your paper :)...
I also remember a comment about the numbers in the paper being "later proved optimistic". I don't think that part got into the final paper.
Bob
On Mar 30, 2010, at 8:18 PM, Rick Karlquist wrote:
> 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
>
>
>
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