[time-nuts] 10811 Extra Lagging etc
Tom Clark, K3IO
K3IO at verizon.net
Wed Jun 21 00:27:20 EDT 2006
If you are using the 10811 as part of a GPSDO the PLL
should take care of any number of slow-moving changes
in frequency; whether it's temperature, humidity, voltage,
OCXO ageing, DAC drift, phase-of-the-moon, etc.
So I don't see a compelling need to "protect" the OCXO
in the ways being proposed.
If you are in a harsh, or fast-changing environment then
do the math to see if your OCXO dF/dt exceeds what the
PLL can close relative to the dF/dt of the GPS reference.
There are several elements of the GPSDO that can have sever
1. The GPS receiver itself. Of particular concern is the group delay
thru the ~2 MHz wide IF filters and also the several MHz wide
filters in the RF front-end. The IF filters are often SAW (Surface
Acoustic Wave) devices, and the RF bandpass filters are
frequently coaxial ceramic devices (functionally similar to tuned
cavities, but built inside high dielectric constant ceramic). I
have seen tempco's of as much as a few nsec/ºC for some brands of
2. The GPS antenna. Most systems employ patch antennas which are
manufactured with ceramic dielectric loading of the patch
elements. Since these are located outdoors, they can see several
tens ºC temperature swing throughout a day.
3. Most GPSDOs use a fairly long divider to bring the standard
oscillator down from 10 MHz to 1 PPS. If you use ripple counters
for these dividers, they will show a significant temperature
sensitivity. I once used a string of 3½ 74HC390's to bring 10 MHz
down to 1PPS and was appalled to find several hundred nsec of
delay change with ambient temperature in a trailer. But when you
realize that 7 BCD counters with 4 flip-flops each account for
something like 72 CMOS gates, each with a tempco ~1 ns/ºC, the
results were not surprising. The moral to this story is that the
counters in your GPSDO should be synchronous counters. Better
still, I found that TVB's single chip PIC divider had unmeasurable
I would be surprised if you find a significant humidity or pressure
pull unless you have a problem with moisture condensing on some
component; the dielectric constant of liquid water (or ice) differs
significantly from unity!
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