[sdiy] Synth applications of CMOS inverter as SPDT switch?
tom at electricdruid.net
Wed Oct 31 21:40:57 CET 2012
On 31 Oct 2012, at 00:03, Richie Burnett wrote:
> The main problem with this type of VCA is that its dynamic range is limited. The carrier frequency must necessarily be high so that it can easily be filtered out, but then this requires incredibly small pulse widths to be generated reliably when you want large amounts of attenuation like 60dB. Once the pulse width gets too small the switch will stop responding and just remain in the off state.
One thing that this type of VCA *is* good for is modulation signals. It's often the case that you might need a lot of cheap VCAs for modulation, and this provides a way to do it. For audio you need more bandwidth (up to 20KHz, say) and you need more dynamic range (80dB at least?), but for a mod VCA, having 256 different levels would be enough, and 512 or 1024 would be good - so 8 to 10-bit PWM resolution. Given that you might be able to filter a modulation signal at a few KHz, this becomes quite achievable. A PWM clock above a couple of MHz is enough. Another plus is that resulting VCA is linear, which suits modulation signals but is a damn pain for audio, where you'd be better with dB/V response.
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