Weird VCF idea! (SC-filter!)

[Ingo Debus]

Tony Allgood wrote:
> 
> I've thought about switched cap. filters as well. In fact, I had
> originally thought about it for my Walsh generator. But remember you do
> have to follow most of them with a filter to take away the clock from
> the output. Since you would be having a variable clock frequency for
> swept filtering, wouldn't you have to have a swept LPF to remove the
> clock. Doesn't that defeat the purpose. Or am I talking crap, its been
> a long week!!

For the MF10, the clock frequency may be set to 100* the cutoff
frequency (may also be set to 50*). If you use a fixed LPF to remove the
clock from the output signal, and set the cutoff of this LPF to, say,
10kHz, the VCF cutoff must not be lower than 100Hz, otherwise the clock
will become audible.

There are some SC filters from Maxim, where the clock/cutoff ratio may
be programmed in 64 steps. But that's too coarse for filter sweeps, I
think...

Regards, Ingo