[time-nuts] "Piezo Little Wonder" OCXO
James.P.Lux at jpl.nasa.gov
Fri May 30 12:47:56 EDT 2008
At 08:33 AM 5/30/2008, you wrote:
>Early GPS receivers used a 10.23 MHz time base.
>Probably related to 2^10-1.
yep.. the chip rate for the C/A code is 1.023 Megachips/second, the P
code is 10.23 Megachips/second, and the L1 frequency (1575.42) is
exactly 154 times the 10.23 MHz, the L2 is 120 times.
So you can see that having a 10.23 MHz oscillator is a handy thing in
a GPS receiver, especially if you can discipline it with the received signal.
These days, one might choose a reference oscillator somewhat higher,
so that when you do your 1bit A/D of the signal, you get many
samples/chip, and so that the signal directly aliases to somewhere
convenient. A lot of receivers use a sampling clock such that you get
1 bit I and Q samples at a convenient sample rate. 4*10.23 would
work nicely, eh? 40.92 MHz
>Some GPS manufacturers approached HP about making
>a 10811 on 10.23 MHz. There is a circuit modification
>for 10.23 MHz and some crystals were made (I
>have some somewhere). However, I don't believe
>any 10.23 MHz 10811's were sold. This unit was
>probably intended to meet the need not filled by HP.
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