[time-nuts] GPS disciplined oscillators - how not to do it.
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Apr 4 01:48:53 UTC 2007
An Australian Electronics magazine recently published a circuit for a
GPS disciplined crystal oscillator.
This particular implementation is the worst I've ever seen. A partial
schematic of a frequency divider chain they used is attached.
Spot the design errors.
They even went as far as to design their own crystal oven with a
bang-bang temperature controller.
The VCXO used a logic gate as the amplifier and a varicap was added to
the circuit so that the oscillator frequency could be adjusted.
A counter was used to measure the phase error between the GPS (Garmin
GPS15L) receiver PPS output and a 50kHz frequency divided down from the
reference.
The phase error count was then used as the input to an 8 bit DAC built
using a handful of resistors and a CMOS octal flipflop. The DAC output
was then connected to the varicap after minimal analog filtering. If one
used a sufficiently long averaging time the fractional frequency error
was claimed to be better than 1E-7.
Meanwhile a PIC was used to communicate with the GPS receiver and
control a display.
Since one can obtain similar or better performance by using the GPS
receiver PPS output directly to calibrate a counter timebase, why bother
with such an inept design?
Bruce
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