Nelly wrote:

I need to vary the frequency of operation of a transceiver IC from
7MHz to 9MHz. The oscillator needs to feed into the crystal
oscillator input and crystal oscillator output pins of the IC (the
current design does not have a variable oscillator and is just using a
crystal). Ideally I would like to control the frequency of the
oscillator using the PWM of a micro controller (unfortunately the
current micro does not have a DAC). Any ideas on the best way to do
this?


I am currently doing a project which uses 8-bit PWM to set the freq. of
a L.O. across a 250Hz range; I use a 3-bit DAC to select one of eight
250Hz segments first. The 1st L.O. is a PLL with a 2Khz step size
and 16-bit resolution. The DAC and PWM outputs and a DC offset for
centering are summed and then drive a dual varactor to pull the
xtal of the 2nd L.O. (at 56,305,000Hz center). This gives me 1Hz
resolution.

Your 2MHz range would require a 21-bit PWM for 1Hz VFO resolution
(after scaling); a 16-bit PWM would give you about 30Hz resolution.

To me, a workable solution would involve a combination of PLL and PWM
as I have done (from what I have read I would avoid DDS for reasons
of noise).

Regards,

Michael