[hpsdr] Hermes/Anan 10 Drift?

[Brian Lloyd]

On Thu, Mar 14, 2013 at 1:20 PM, Mike Monnier <w8bac at comcast.net> wrote:

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>
>
> **
> My Apache Labs Hermes/Anan-10 has, what looks like, frequency instability
> or
> drift that seems to be caused by temperature changes.
>

Yes, you are right. I discovered the problem a month or so ago and worked
with Phil and the folks at Apache Labs (with whom I am quite impressed and
with whom I will happily continue doing business) about it.

First, yes, you will see what appears to be temperature instability on the
internal 10MHz reference. If you look at the absolute numbers you will find
that this "instability" is well within the 1ppm specification. The
worst-case error I ever saw was just short of 2Hz at 10MHz, or 0.2ppm.
Quite a bit better than spec -- but still annoying. My first inclination
was the that there was a problem with the PLL that locks the 122.88 MHz
clock to the 10MHz reference. That was not the case. The problem turns out
to be the way that the 10MHz internal reference is temperature compensated.
Apparently it is digitally compensated and therefore the corrections are
quantized, hence the sudden jumps. Yes, it is annoying, especially if you
are a frequency measurement geek like me. But that does not mean it doesn't
meet spec or that it can't be used, even for applications like WSPR.

The other issue is the use of external reference input.


> I use an external GPS disciplined 10 MHz reference oscillator. The internal
> Hermes reference oscillator is not functional due to jumper selection. The
> GPSDO is turned on 24/7 and all diagnostic software shows the unit is
> functioning flawlessly. It is an HP 58503A. The disciplined 10 MHz
> oscillator is so close to standard, the difference is measured in parts
> per billion. I am quite sure the 10 MHz signal from the GPSDO is
> on the money. I peat this with WWV and watch the phase.
>
> I first noticed that the receiver calibration was off each time I checked
> it so
> I studied the problem. I use WWV as my calibration standard. Calibration
> is off when cold by .100 Hz roughly. To begin I note the external (skin)
> temperature of the case using an infrared thermometer. The tests start at
> switch
> on, skin temperature 66 degrees F and a frequency correction factor of
> .9999977100000. I view Phase 2 and adjust the dot rotation until it
> slows to a crawl. Almost instantly the calibration starts to shift.
>
> After one hour the skin temp rises to 92 F and correction factor is now
> 1.000003125.
>
> After two hours skin temp is 101 F and correction factor is 1.00000327.
>
> At hour 3 and 4 the skin temperature stabilizes at 103 F and the correction
> factor stays steady at 1.00000327 nominal.
>
> All tests taken with one receiver and no transmit activity. Highest skin
> temp
> area found on the right side of the box facing the front.
>
> I have a small fan for peace of mind when using a 4 watt carrier. No fan
> was
> used during these tests. No air obstructions. No heat source below the
> radio.
>
> My questions are, what might be causing this problem and can it be
> corrected?
> Thanks!
>
> Mike
>
>
>
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-- 
Brian Lloyd, WB6RQN/J79BPL
3191 Western Dr.
Cameron Park, CA 95682
brian at lloyd.com
+1.767.617.1365 (Dominica)
+1.916.877.5067 (USA)
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