super simple 40106 VCO with expo or linear control

remork · Joined: Aug 02, 2009 Posts: 25 Location: bhellgium

i'm thinking a resistor/pot in series with the diode should give you saw to tri on the Q3 output. at the same time it would give you 0 - 50 % PWM on the Q2 output.
unfortunately, it would also change the frequency. :/

breadboarding as we speak. thanks synthmonger, simple saw VCO is just what i was looking for!

Tomoroh Hidari · Joined: May 04, 2012 Posts: 30 Location: Vienna

so, i breadboarded this circuit last night and overall it works pretty fine. the level of the saw out is, however quite noticeably lower than that of the pulse out. is this a feature or a bug?

JingleJoe · Posted: Wed Sep 19, 2012 3:29 am Post subject:

That is because the saw wave is only oscillating between the upper and lower bounds of the schmit trigger.
You can simply add an amplifier with offset removal or an input capacitor to remove the DC offset, which will get it up to the desired voltage.

By the way, remork, that doesn't work, it just increases the charge time but sets it to a constant time which does not alter with changing control voltages. I've tried a number of different variants of this VCO, with transistors in the feedback path and all sorts of things, but the initial design is the best. Except that I use an op amp buffer and a comparator for PWM.

Tomoroh Hidari · Joined: May 04, 2012 Posts: 30 Location: Vienna

ah, thanks for the info jinglejoe.
out of curiosity I breaboarded the 'single op-amp version' (http://electro-music.com/forum/topic-49901.html) of this circuit on a second breaboard using one opamp from a 324. rather than using a 100nf (or, as it says in the post 10nf) cap going to ground from the opamp input, I swapped it for a 470nF (I think - need to check again, once i'm at home), as with the 100nf I tried first I didn't get anything below somewhere between (roughly estimated) 700-1000 hz.
interestingly on the opamp version it is exactly the other way round - the saw is louder than the pulse (and i'm starting to suspect there's some logic - sorry, probably not the best word...hm, rather simple electronic principle - behind that as well).

i'm planning on recording both as soon as i manage to have an a/b comparison. i only have a software scope, but i could also try to take some screenshots of the respective waveforms with that, should anybody be interested.

JingleJoe · Posted: Fri Sep 21, 2012 6:44 am Post subject:

It's likely that the volume of the pulse is due to it's width, it will be nearly non-existant for both circuits but I suspect that it's faster with the op amp because op amps can usually output more current than CMOS, thus charging the cap faster.
Additionally the upper and lower voltage bounds of an op amp schmit trigger are usually higher and lower, respectively, than those of the CMOS variant.

Tomoroh Hidari · Joined: May 04, 2012 Posts: 30 Location: Vienna

wow, you really seem to be a walking compendium of electronics knowledge!

anyway, as promised, I did recordings of both circuits - from breadboard into the soundcard. mono, no editing or anything, except cutting the respective parts into single files... I did a sweep over the full range a 100k pot gave me (and some little extra turns towards the end on most files.)

it's, unfortunately, not recognizable in Soundcloud's waveform representation*, but the Schmitt-trigger saw shows obvious dc offset. also the saw waves on both circuits have a rather steep falling flank making them look close to pulses.... didn't get round to taking snapshots with a scope yet, but will try to the next days. anyways, without further ado, here's the audio:

40106 version:
http://soundcloud.com/ivorybunker/vco-40106-saw-21092012
http://soundcloud.com/ivorybunker/vco-40106-pulse-21092012

LM324 version: (this uses a 470nF cap instead of the 100nF in the schem)
http://soundcloud.com/ivorybunker/vco-lm324-saw-21092012
http://soundcloud.com/ivorybunker/vco-lm324-pulse-21092012

I think I definitely want to solder the lm324 soon, but for lunetta purposes it will of course be the 40106. and I was thinking there - would it be possible to use the pulse from that one as clock for a 4017? that should allow to create some sorts of custom wave form or modulation sequence at 1/n of the saw freq, innit?

*i enabled dl so if anyone wants they can have a look at it in a wave editor

JingleJoe · Posted: Sat Sep 22, 2012 2:55 am Post subject:

JingleJoe · Posted: Sat Sep 22, 2012 3:08 am Post subject:

I Had a look at your audio files with an oscilloscope, that weird shaped saw wave could be due to filtering effects, is there any capacitance in your signal path? The filtering in question may even be in your sound card although I doubt that as I've used mine with very very low frequency stuff with no problem.

P.S. you can also play tunes by moving the seekbar in your media player to different points in the sound file Laughing

Tomoroh Hidari · Joined: May 04, 2012 Posts: 30 Location: Vienna

I think I found out where the weird waveshape came from - I usually have this behringer 'tube ultragain" preamp between my breadboard out and soundcard/mixer in... it's the part of my equipment I wouldn't miss too much if anything bad were to happen. usually I have it in "neutral" mode without any preamp gain - anyway - looking at the signal going directly into the soundcard things are looking differently. (as in 'good')
had a try last night triggering a 4040 from the pulse out - which gave nice sub-squares and also triggering a 40106 rc osc... didn't make any recordings though and went to sleep after things stopped functioning one modification attempt later. taking up things now and trying to get that beast on perfboard...