simpliest cmos vco

ambi-J · Joined: Apr 12, 2007 Posts: 20 Location: belgium

trying to make a very simple vco I experimented with the easy 40106oscillator.
since the frequency is created by charging and decharging a capacitor wich is coupled to massa I asked myself: if it's not coupled to massa but to a voltage it will quicker raise to its trigger level and the frequency should be higher.
I tried it and it works, of course not with every voltage and depending the vallue of the cap and the setting of the resistor the voltage range versus frequency change.
it's certenly not 1v/oct exponential and I think it's linearity is not good eigter, but it is a vco since the frequency change applying a voltage.
I think this truck will work with almost every oscillator.
I am wondering why I cannot find information about it on the net and why nobody is doing it that way

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richardc64 · Posted: Thu Feb 13, 2014 5:25 pm Post subject:

I've done it, but in a more "complicated" way. Rolling Eyes

The diode acts as a voltage controlled resistor varying "how much" of the cap connects to ground. A quirk of this method is the higher the voltage the lower the frequency.

With the cap directly to ground and an npn transistor across it, synthmonger made a VCO with a normal CV-to-freq. response.

http://electro-music.com/forum/topic-28799.html

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Blue Hell · Posted: Thu Feb 13, 2014 5:26 pm Post subject:

Nice find!

I did not see it before either .. not in some conscious state at least ... as to why ... some ideas are too simple I guess Laughing

Apart from that .. yeah it has the drawbacks you mention ... it would depend very much on the actual threshold voltage of the port too .. but hey .. this is the Lunetta forum Cool

Blue Hell · Posted: Thu Feb 13, 2014 5:29 pm Post subject:

DUBmatze · Posted: Sat Feb 15, 2014 1:57 am Post subject:

elektrouwe · Joined: May 27, 2012 Posts: 140 Location: Germany

ambi-J · Joined: Apr 12, 2007 Posts: 20 Location: belgium

I used a pot as voltagedivider to my controll input, so that is DC.
There has no frequency (AC) to pass trough that cap.
All I do is changing the voltage level from which the cap charges and to what it decharges. So for instance when this is 0v the frequency is what you expect a normal 40106 should give. At a higher voltage the cap charges and decharges quicker, so the tune is higher.
When the controll voltage is near the trigger level, which is approx 8v with a chip supply of 15v the oscillator stops oscillating. But in between it gave me a range from about 10hz till above my hearing: 12khz

I challenge everyone to give it a try, it's easy to breadboard.

next weekend I will post some record

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L´Andratté · Posted: Mon Feb 17, 2014 3:06 pm Post subject:

In this schematic is an interesting sawtooth vco made from a 4093 NAND chip.
It is more or less v/oct. And for high frequency, so the 330pF timing cap should be lowered...(I used an LED instead of the diode).
http://www.schmitzbits.de/
(Three-Phse-LFO)

But the diode thing rules for simplicity! Shocked

EDIT:I just realized that lunettas use single supply(here dual for exp.converter),
but interesting nonetheless Wink

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PHOBoS · Posted: Mon Feb 17, 2014 5:38 pm Post subject:

L´Andratté · Posted: Mon Feb 17, 2014 5:45 pm Post subject:

wow, now I get what you mean!
it´s called lateral thinking...
challenge accepted, you´ll hear from me tomorrow surprise

richardc64 · Posted: Mon Feb 17, 2014 9:34 pm Post subject:

Eliminate the PNP from the Rene Schmitz VCO and it will be single-supply. The response won't be V/Oct and probably not Oct/V, either.

I favor replacing the diode in the feedback path with a resistor or pot. That should give a more square-ish waveform, instead of a spiky saw.

elektrouwe · Joined: May 27, 2012 Posts: 140 Location: Germany

PHOBoS · Posted: Tue Feb 18, 2014 9:49 am Post subject:

L´Andratté · Posted: Tue Feb 18, 2014 12:08 pm Post subject:

Ground pot day today, but I tried with a 4093 (the only chip I had on hand).
But this got me only some wobbly modulation... Maybe the trick works only with single inverter? I checked resistance between 10k and 1M with caps between 470n and 20u.

The diode thing is really cool though, I recorded some stuff,
it is 3 4093 cascaded with the second feeding back on the first (:
I totally forgot how good they sound with all this strange harmonics
put through a resonant lp, who really needs a MOOG?

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PHOBoS · Posted: Tue Feb 18, 2014 1:19 pm Post subject:

that sounds pretty good Very Happy

what flter did you use ?

I couldn't find my 40106's so I used a 4093 aswell, with a 150K resistor in the feedback path,
a 22nF capacitor and a 100K pot. (don't forget to ground all non used inputs).

Blue Hell · Posted: Tue Feb 18, 2014 1:49 pm Post subject:

PHOBoS · Posted: Tue Feb 18, 2014 2:44 pm Post subject:

Blue Hell · Posted: Tue Feb 18, 2014 4:02 pm Post subject:

Ah ok, convinced now Confused

ambi-J · Joined: Apr 12, 2007 Posts: 20 Location: belgium

you are right. when I feed the cap from an opamp voltage folower it loose the vco effect almost. the change in frequency is verry little then.
maximum effect is attchieved this way by using a high cap (1uf) and a very low R1 : 1.5 octaves, and the change in frequency is reversed then.

commathe · Posted: Tue Feb 18, 2014 7:13 pm Post subject:

I don't think this is voltage control. It looks to me like you are affecting how quickly the cap can discharge itself by adjusting the resistance to ground. If you disconnect the pot from the power rail and just use it as a series resistance it should probably still work nearly the same.

elektrouwe · Joined: May 27, 2012 Posts: 140 Location: Germany

L´Andratté · Posted: Wed Feb 19, 2014 4:10 am Post subject:

L´Andratté · Posted: Thu Feb 20, 2014 8:23 am Post subject:

Now I´ve tasted blood...
Twisted Evil

Think I´ve got to look into 4093 vco module!
It´s not lunetta, so I´ll take my hat here, with one more clip for you Laughing

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rossics · Posted: Sat Apr 12, 2014 4:00 am Post subject:

I was able to come up with a pretty gnarly sounding noise maker using this idea; use a switching jack for CV in so when there is no jack inserted the point goes to ground

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L´Andratté · Posted: Sat Apr 12, 2014 4:50 am Post subject:

Nice Rossics, I mean´extra noise´ All Ears

I think those CMOS vcos, especially cascaded, are much underappreciated,
they yield very interesting sonic textures.

I´m tending to the cv-way explained by richardc64, diode to ground (while vactrols are nice, I need them for my vcf´s already).

I´m thinking about and will be trying soon if it is possible to use bipolar cv in a scheme like the Steiner VC Phaser circuit...
http://electro-music.com/forum/phpbb-files/sp_phaser_1_153.jpg