[GPS_Standard] Power Supply questions

[Dave Platt]

On 02/13/2012 03:36 PM, Chris Howard w0ep wrote:

> I think the control circuit of my oscillator must be
> faulty.  I have watched the controller card go rail to
> rail with the control voltage and it doesn't seem to
> make any difference with the output frequency.
>
> I went in and manually set the DAC to skip ahead;
> I still get the same mixture of sample frequencies,
> still the same general frequency offset in the same direction.
> It's like I didn't do anything.
>
> I've tested the voltage on the pin of the oscillator.
> The controller is changing the voltage, it just doesn't
> have any effect.
>
> My oscillator is an HP 10811-60111

It's possible that your oscillator's "coarse" frequency
adjustment is off, far enough that the voltage-driven
frequency trim doesn't have enough range to pull it back
to the proper frequency.

What you should probably do is:

-  Put the FLL into "manual" mode, and set the DAC
   output voltage to one-half of full scale.

-  Compare the 10811-60111 to a "known good"
   frequency standard - e.g. beat it against WWV, or
   against someone else's GPS-disciplined or rubidium
   standard.

-  Use a small insulated screwdriver to tweak the
   frequency-adjustment cap (through the hole in the
   case of the oscillator can) to minimize the frequency
   difference - I believe you'll have to match it to better
   than 1 Hz of error (one beat per second) or even better
   than that.

-  Set the FLL into tracking mode.

It would probably also be a good idea to check the current
being drawn by the oscillator's oven heater... turn the whole
thing off, wait 30 minutes or so, turn it on, and watch the
current on the 24-volt heater line.  It should be fairly high
at first, and then drop down substantially once the oscillator
has warmed to operating temperature.  No current == dead heater.
Constant low current draw, or constant high current draw probably
means a bad temperature-sensing/control circuit.

In any case, if the oven isn't maintaining the correct temperature,
the oscillator will probably run off-frequency quite a bit.  You
might be able to adjust the frequency with the tripcap, but
without the heater maintaining a constant temperature you
probably won't get the stability you desire.