"W3JDR" wrote in message
...
Anyone know of a PIC-based frequency locked loop for VFO control? I'm
looking for something that can be programmed with a frequency word, and
then
it aquires and maintains the frequency. I've seen frequency 'stabilizers'
but most of them just hold the frequency once it's been manually set.
There
are also PIC-assisted DDS designs, but these are more complex thatn I
want,
use more power then I'd like, and are not as clean as a simple voltage
controlled oscillator.

A number of the MicroChip MPs have 16-bit internal counters that claim to
run as high as 50 MHz. The carry out of that counter could be used to cause
an interrupt to extend the number of bits in the count.

But you would need to precede the count chain with a gate of known and
accurate count-window duration. I've thought about the subject and would
try a 74AC-series flip-flop as the "gate". The gating control signal would
be applied to its DC-reset. A 74HC4060 running with a "32" kHz clock
crystal could provide the gating signal. Properly biasing the flip-flop's
clock input might result in fairly good sensitivity.

These MicroChip MPs have a pulse-width-modulated [PWM] output, but it's only
8- or 10-bit resolution. It could be dithered for more resolution, but that
would require an even greater time-constant in the AFC applied to the VCO -
probably intolerable. A simple, inaccurate, 8-bit D-to-A chip might be used
for rough AFC voltage, with interpolation using the PWM.

WARNING: All the above is just hand-waving.

73 de bob w3otc