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</head><body style=""><div>You guys are not discussing the modulator in the VSM201 anymore right? Because in that modulator, the control range is *not* determined by how small of a duty ratio the PWM signal can get.</div>
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<div>Best regards</div>
<div>Simon</div>
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<div><br>> Op 2 januari 2016 om 23:10 schreef Tom Wiltshire <tom@electricdruid.net>:<br>> <br>> <br>> Ok, maybe -80dB is *really really* impossible! My judgement was "certainly tough, but probably not impossible", but I'm very willing to go with "extremely difficult bordering impossible" or "totally impossible" if you feel that's a more accurate evaluation! Certainly a 50KHz carrier isn't that high, so I'd probably want to cut your timings in half (e.g. 100KHz carrier), which makes it even more difficult. Certainly we need significantly sub-nanosecond switching times to get good results.<br>> <br>> The point was just that -80dB isn't that good for a modern VCA, and it's pretty much out of reach for PWM VCAs. Lots of Blackmer-cell-based designs do much better than that, and even 13600's can probably pull -80dB out of the hat ( http://hem.bredband.net/bersyn/VCA/vca_shootout.htm )<br>> <br>> Tom<br>> <br>> <br>> On 2 Jan 2016, at 21:12, Richie Burnett <rburnett@richieburnett.co.uk> wrote:<br>> <br>> > 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!<br>> > <br>> > -Richie,<br>> > <br>> > Sent from my Xperia SP on O2<br>> > <br>> > ---- Tom Wiltshire wrote ----<br>> > <br>> >> +1 totally agree<br>> >> <br>> >> If you can produce a 1% pulse width, you still only get -40dB. You need to get a 0.01% pulse wave to get -80dB. Tough to do. Not impossible, but awkward enough to make it stop seeming like such a great solution.<br>> >> <br>> >> I've thought about this a bit because of using the PIC's PWM module so much. The best case output from that is either 8-bit or 10-bit, which means that -60dB is about as good as I'd get using it for a VCA, and that implies having a switching frequency which is much too low (31KHz) for many jobs.<br>> >> <br>> >> Tom<br>> >> <br>> >> <br>> >> On 2 Jan 2016, at 19:42, rburnett@richieburnett.co.uk wrote:<br>> >> <br>> >>> <snip><br>> >>> Control range of PWM'd CMOS switches acting as VCAs isn't that great though.<br>> >>> <br>> >>> -Richie,<br>> >>> <br>> >> <br>> <br>> _______________________________________________<br>> Synth-diy mailing list<br>> Synth-diy@dropmix.xs4all.nl<br>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy</div></body></html>