[GPS_Standard] No joy here

[Chris Howard w0ep]

It is a new day today and things are looking better.

I used my frequency counter to determine that direct
input of the control voltage did change the output
frequency.

I attempted to use a shortwave radio to hear both
WWV and my box, but I had to go to the far end of the
house to hear WWV at all.  That got me wondering whether
the little GPS board sitting 2 inches from 5 unshielded
wires carrying 10 mHz was being influenced. So
I installed shielded wiring in those places.  (probably
should also on the connection between the controller and
the VCO).  I wondered if I should also put the GPS
in it's own sub-enclosure, haven't decided.

I left the counter and the GPSDO running overnight
to awaken with it sitting on the bottom rail.
Out with the screwdriver.  Glad I had enough
foresight to include a hole in the enclosure for
access. I tweaked the gross frequency adjustment.

Now I have a lock at mid-range.  I'm running
with samples set to 8. The report is mostly
'=' with an occasional '+F'.  The statistics
page says  33 samples, Avg. Freq. Offset (acc. tot.)
-1.74e-10.

Oh, there was just a "-F" so I think I'm in
the neighborhood, finally.

Thanks for the replies, very helpful and encouraging.

Let me know if you see anything else that I should
attend to with the shielding. It may have just needed
tweaking the whole time, but I'm pretty sure
the shielding helped.


Chris



On 2/13/2012 5:51 PM, Dave Platt wrote:
> 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.
>
>
>
>
>
>