Bert:
Make sure you are working on the correct problem.
I find that the normal (in accordance with published specifications) daily aging
of the crystal exceeds the daily variation of frequency with temperature of
a stock 10811 in a benign (office or home) environment.
If you are seeing frequency variations with temperature, look at the
stability/regulation of the voltages you are feeding the oscillator.
Particularly, the stability of the reference you are using for frequency
control steering of the oscillator. There are much better voltage reference
sources available today than what Shera used.
You might be better off putting the control voltage reference in the Dewar, and
leaving the 10811 outside.
:-)
--- Graham / KE9H
==
On 1/9/2011 10:52 AM, Bob Camp wrote:
Hi
The answer to the "how far can you go" is (as always) "that depends".
You can tweak a 10811 balance wise to get it's thermal gain up 10X over the
factory settings. You may be able to go higher, but 10 is what's been
published. That will give you an oscillator who's crystal is very insensitive
to temperature. Pretty much 100% of the instability you see will be from the
oscillator circuit. Since the crystal may be going one way when the circuit
goes the other, the performance may actually be worse with the higher thermal
gain.
The real answer is to do temperature runs and to adjust things based on the
transient and stabilized performance of your part. That way you can observe
data that directly relates to the thermal control system.
If you do decide to go with the high thermal gain case, doing a quick mod to
put the unit on the B (thermometer) mode is definitely the right way to do it.
You get things done in a couple hours that would take weeks otherwise.
Bob
On Jan 9, 2011, at 9:07 AM, [email protected] wrote:
Being a Shera Fan I finally broke down and bought a Tbolt. I experimented
with a foam enclosure with roughly 1 centimeter on all sides. What I found
that it only increased the board temperature by 19 C. It was still as
sensitive to ambient. Thanks to Lady Heater it even shows an increase of 40 mC
when I walk up to it with one minute delay.. Changing to an aluminum
extrusion the increase is only 7 C and the response is much slower. In its
final
assembly it will be inside a larger metal mass, I do not think that the
higher temperature of 56 C is conducive to longer life. Opinion: putting foam
around a 10811 will only give you a warm feeling unless you make it much
larger than shown in the picture of of KH6GRT.
Based on the above test results I feel mass is more important. So I took a
aluminum enclosed dewar weighing 943 gr. and did first put a resistor in it
and heated it up with 2 W. Temp. did rise to 70 C. Next I disassembled my
worse 10811, which is very easy, since it is a nice compact unit and
inserted it in the dewar. Monitoring temp with my YSI it shows 65.2 C with 24 C
ambient. Power dissipation is 1.575 W. Will track it over time before I
replace the 10811 with one of my better ones for frequency tests.
Questions to our experts:
A) will Removing the foam mess with the temp. control loop
B) How low can total power into the 10811 be before temp control can become
a problem, I think I am ok now, but once in a chassis with Shera and
power and battery the temp. surrounding the dewar may become a problem.
C) What is the preferred orientation of the unit.
I asked already once before if any one has data on the 1 1 PPS output of
the Tbolt over time, since I am considering it as an alternative to a stand
alone GPS receiver, and if there are GPS receivers that outperform a Tbolt
what is the recommendation. It will be used to control a Rb.
Bert Kehren
In a message dated 1/7/2011 8:22:51 P.M. Eastern Standard Time,
[email protected] writes:
Has anyone experimented with the amount of insulation on a
10544 or 10811 oscillator? They are meant to run hot by
design and I worry that adding any insulation, or too much
insulation, will either cause over-heating or limit the ability
of the oven control to maintain the set point.
None of the hp/Agilent bench test equipment that uses these
oscillators uses insulation. Perhaps that's a clue.
/tvb
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