From gstopp@fibermux.comTue Dec 5 11:15:32 1995 Date: Tue, 05 Dec 95 09:35:24 PDT From: gstopp@fibermux.com To: analogue@hyperreal.com, electronica@andrew.cmu.edu Subject: Re: Pulse width circuit?????? Here's a circuit that will create a variable width pulse waveform output from a sawtooth or triangle wave input: PWM1 91K 100K 0---\/\/\/\-o----/\/\/\/--+ | | 91K | |\ | 0---\/\/\/\-o----|- \ | PWM2 | \---o---+ | / | +----|+ / | | |/ IC-1 | ----- | |\ IC-2 PULSE --- +---|- \ 3K OUTPUT - | \---o--\/\/\/\-o--0 | / | | +---|+ / | \ | |/ | / WAVEFORM IN | | 1.5K \ 0---------------------\/\/\/\-+---\/\/\/\--+ / (SAW or TRIANGLE) 10K 1M \ | ----- --- - Inputs "PWM1" and "PWM2" are the pulse width modulation inputs. Typically one will go to the wiper of a pot that is connected between V+ and V-, to serve as the initial pulse width pot. The other will then serve as a pulse width modulation control voltage input. The values of the 91K input resistors to this pulse width control voltage summing amplifier (IC-1) can be adjusted up or down to suit individual requirements. A lower value will mean a wider range at the (-) input of comparator IC-2. A higher value will mean a narrower range. The "WAVEFORM IN" input should come from either a sawtooth, triangle, or sine wave source. The positive and negative peaks of this waveform should fall just within the pulse width control voltage range as mentioned above. If the waveform is smaller in amplitude than the control range, then there will be a "dead zone" on either side of the initial pulse width pot. This is a good thing because you can dial in teeny weeny narrow pulses that are right on the edge of existence if you want. Op-amp IC-1 can be any run-of-the-mill op-amp - 741, 307, half a 1458, half a TL082, quarter of an LM324, etc. There are no slew rate or bandwidth needs here. Op-amp IC-2 should be a 748. If you can't get that use an LM301. Both of these are externally compensated, and in this circuit the intention is to leave off the external compensation cap entirely so the slew rate is the highest possible. This will provide the most vertical edges possible on the output pulse wave. If you really just want the basic pulse width modulation sound you can use the same "normal" op-amp types as listed for IC-1 - in fact if you use a dual you can cut the parts down. However you will miss out on the really high harmonics if you compromise here. Since the output of the comparator IC-2 bangs against the positive and negative power supply rails, the 3K/1.5K attenuator is used to bring it back down to a level that is less harmful to VU meters. As shown the values are chosen for plus and minus 15 volt supplies, with the pulse width control voltage range matching the amplitude of the input waveform. The output is set to plus and minus 5 volts with an output impedance of 1K. - Gene gstopp@fibermux.com ______________________________ Reply Separator _________________________________ Subject: Pulse width circuit?????? Author: PPilgrim@Teleglobe.CA at ccrelayout Date: 12/5/95 7:18 AM Hello again, Can anyone describe how to build a pulsewidth circuit? I would like to add pulse width to the osc's in my prodigy. All i have to start with is a square, tri, or ramp waveform. Unfortunately all my schematics have curtis chips with pw pins on the chip. With the prodigy, I am dealing with discrete VCO's :) Thanks From s9506190@minyos.xx.rmit.EDU.AUWed Dec 6 10:10:17 1995 Date: Wed, 6 Dec 1995 12:30:14 +1100 (EDT) From: The drink coaster formerly known as Damien Miller To: PPilgrim@Teleglobe.CA Cc: analogue@hyperreal.com, electronica@andrew.cmu.edu Subject: Re: Pulse width circuit?????? On 5 Dec 1995 PPilgrim@Teleglobe.CA wrote: > Hello again, > > Can anyone describe how to build a pulsewidth circuit? I would like to > add pulse width to the osc's in my prodigy. > > All i have to start with is a square, tri, or ramp waveform. Unfortunately > all my schematics have curtis chips with pw pins on the chip. With the > prodigy, I am dealing with discrete VCO's :) take the sawtooth out, add a CV and stick the lot thru a comparator. thus: saw o-----/\/\/\/-----| |\ o------------|+\ CV o-----/\/\/\/-----| | \ | |-----o PW out | / ---|-/ | |/ | ----- / / / gnd I would use a TL071 for the opamp, they are cheap and pleantiful. the resistors should be about 10k from memory (I don't think that they are critical), also you might need a resistor between ground and the input to the op amp. Hope this helps, Damien /-------------------------------------------------------------------------\ | Damien Miller a.k.a s9506190@minyos.xx.rmit.edu.au a.k.a Silicon Dreams | | http://minyos.xx.rmit.edu.au/~s9506190 || No nuclear testing, thanks! | | CAUTION: WEIRD LOAD | \-------------------------------------------------------------------------/