[time-nuts] Making a HP 10811 and similar types better in the real world Part I

[Perry Sandeen]

GM List,

Basic commandments of precision grade Crystal Oscillators. (1) It is both an art and a science. (2)  The devil is in many details. Unbelievers suffer.

Here is the framework of methods that really work.  (Assuming a used unit)

1.  Make sure your power supplies are stable AND meet the low noise requirements stated in the data sheet.  This means using something based on the buried zener principle for both the heater, oscillator and EFC supply using three separate circuits.  Hint: filter, filter, and more filters.

2.  The EFC voltage should be derived from something like a REF-10.

3.  The voltage references of all the supplies should be in a temperature regulated housing.

4.  Wrap the oscillator in fiberglass and insert into a wide mouth thermos bottle with the opening downward.

5.  Wrap that assembly in another 6 inches or so of fiberglass insulation and still keeping the fiberglass filled mouth downward, place into a wooden box of choice.

6.  Place box of choice  on concrete floor.

7.  Fire up oscillator.  Let it cook for at least two weeks.

8  After burn-in period use the EFC voltage to set the frequency to 10MHz (An old Bourns 10 turn linear pot with a counter dial is ideal) using your GPS or Rubidium standard using your favorite comparator.  Note dial reading and record.

9.  Wait one week and repeat step 8.

10.  Wait another week and repeat step 8.

11.  After another week reset the frequency and compare the DIFFERENCES in your recorded dial readings.  After the differences become close to the same (EFC is non-linear) week after week one has a very stable oscillator that can start to be useful.

Why this works

High stability oscillators do not want to be bumped or have their factory set temperature changed.  They are not like a car engine that needs a radiator.  The ovens are proportionally controlled.  On start-up all ovens, proportional or not, will have over-shoot. Some more, some less.  An inescapable fact of life.  

Cooling the exterior case of an ovenized crystal oscillator only causes the oven(s) to come on more frequently.  When the heater current(s) go to a minimum level, that is when the oven has reached its factory set-point and is a happy camper.

The vacuum flask minimizes heat losses.  The interior and fiberglass insulation prevents air current flow and add more insolation.

In part two I’ll expand on the 10811 temperature problem and explain my precision oscillator credentials,