GPS Disciplined Frequency Standard
(A project in progress)
I read the article by Brooks Shera, W5OJM in the July 1998 issue of QST
(Reprint available for
download at his website)
that describes using a 1 Hertz output from a GPS receiver to discipline a
voltage controllled crystal oscillator (VCXO).
There is always a need for an accurate frequency reference. The need for an
accurate frequency becomes even more critical when dealing with weak signal
work. Especially when frequencies are multiplied up into the UHF and Ghz ranges.
Since I'm mostly interested in weak signal work, this became my first project
In the past the common practice was to periodically compare frequncies against
the 10Mhz signal from WWV. However, some path distortion shifting occurs so the
WWV signal isn't quite as accurate when recieved. GPS satellites have atomic
clocks on board and transmit a very accurate 1 hertz signal. Accurate to a few
parts in 10e11.
Original design concept
In the orginal design a comercial 5Mhz VCXO is phase locked to the 1Hz GPS
signal through the use of a PIC microcontroller and some external counter
circuits. Phase is measured by counting pulses from a local 24Mhz crystal
oscilltor that occur between the raising edges of the GPS pulse and the
5Mhz signal. Corrections are made through a proportiinal-integral (PI)
controller implemented in software.
Possible Redesign
Upon looking over the circuit, I considered that it might be possible to reduce
the hardware required. The PIC microcontroller has internal counters and
capture/compare features that could be utilized to replace the external
counters. After much sketching of schematics and interupt handlers, I came
up with a workable idea.
Design goals
- Most accuracy and stability achievable for cost.
- Use commonly available parts. Unfortunately, many designs get based on
surplus comercial equipment that becomes scarce.
- Possibly contruct my own temperacture controlled crystal oscillator to
hold cost down.
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