[time-nuts] Allan Deviation -> continuing saga...
Didier Juges
didier at cox.net
Sat Oct 28 00:18:52 EDT 2006
In the quest to check the stability of my OCXOs and make a GPS
disciplined oscillator, here is my current progress:
Using parts I had on hand and a couple of free AD8561 samples, I made a
synchronous divider for the 10 MHz OCXO to get it closer to 1 Hz. Since
I only had two synchronous binary dividers (74F161), I used 7 stages for
a ratio of 128, which gives me 78.125 kHz. That is the lowest whole
frequency (in Hz) I can get with straight binary counters. The F series
is old and thirsty, but fast (120 MHz for these counters). I should be
able to run that against the 1 PPS from the GPS, and I will post results
when available, I hope sometime during the week end.
The clock shaper uses a commercial 10.7 MHz IF transformer, readjusted
down to 10 MHz (see schematic at
http://www.ko4bb.com/ham_radio/Manuals/2_GPS_Stuff/Clock_Shaper.jpg).
It provides light loading to the OCXO through the 390 ohm resistor, yet
provides about 2V p-p to the comparator, to get it out of the noise. I
do not know the temperature stability of the transformer, or the loaded
Q, I will check that later. I realize this is probably not an optimum
design, but it's what I could do with what I have on hand. I am not
using the fine tooth comb and tweezers yet, I am still with the rake and
the fork lift :-)
Didier KO4BB
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