Dougal McDougal of that Elk wrote:
> You need to watch the stability - ie phase margin. Make sure you sample at
> about ten times the unity gain cross-over freq.  Your unity gain freq could
> be around 12Hz - this should filter out the 2f at 120Hz. You may also need a
> phase-lead compensator depending on the number of integrators you want (ie
> for just the VCO you have one integrator - but you can add a phase-lag to
> increase the slope). You need a Bode Plot in your head or on paper.If you
> use a low-pass filter this will introduce phase lag and the amount depends
> on the order of teh filter. The more you filter, the less 2f carrier
> feedthrough but the more instability.

Since this is being done in software, can't one get rid of all the
phase lead/lag problems by using identical linear phase FIR filters on
both the reference input and the VCO?

Thanks.
-- 
rhn A.T nicholson d.0.t C-o-M