[sdiy] VSM201 Vocorder Question

[rsdio at audiobanshee.com]

On Jan 2, 2016, at 1:12 PM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> With carrier frequency of 50kHz, 0.01% duty ratio (for 80dB attenuation) represents a pulse width of just 2ns! That's getting near the sort of time mismatch you can get in turn-on and turn-off times for the switches. So the switch might not turn on at all, or might stay on twice as long!

That's an excellent point. At 46 kHz PWM and 0.1% duty cycle, as limited by a PIC solution, we're looking at 20 ns. That seems more manageable, but with only 60 dB attenuation.

However, some 4066 chips have maximum output disable times of 120 ns to 160 ns (better with higher supply voltages). That probably means less than 60 dB attenuation, even in the best of cases. Perhaps the SPDT configuration helps this problem, since turn-on times are about half.

You could control PWM without being limited by the PIC implementation, but then the analog switch limitations come into play.

Brian