Posted: Thu Feb 13, 2014 3:03 pm Post subject:
simpliest cmos vco
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
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.
I did not see it before either .. not in some conscious state at least ... as to why ... some ideas are too simple I guess
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
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.
Posted: Sat Feb 15, 2014 8:06 am Post subject:
Re: simpliest cmos vco
ambi-J wrote:
I am wondering why I cannot find information about it on the net and why nobody is doing it that way
because it does NOT work. At least not for DC CV, which can obviously not
pass the capacitor. What CV source did you use ? If it was a CV from a potentiometer, you got the effect from the change in Rpot ( which is,
what Richard is doing with the diode ). For AC CV there is a kind of frequency/pwm modulation, but you can't call this a VCO.
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.
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!
EDIT:I just realized that lunettas use single supply(here dual for exp.converter),
but interesting nonetheless
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Posted: Mon Feb 17, 2014 5:38 pm Post subject:
ambi-J wrote:
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.
That's a pretty wide range, even if not used as a VCO (with an external control voltage) it might already be a nice way to extend the range of the
oscillator (with another pot in the feedback path). I'll give it a try sometime _________________ "My perf, it's full of holes!" SoundCloudBandCampMixCloudTwitchYouTube The Golden Shrimp Guild
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. _________________ Revenge is a dish best served with a fork... to the eye
I used a pot as voltagedivider to my controll input.
that's exactly what I guessed in my quote.
This has NOTHING to do with voltage control at all !
Proof: disconnect your V+ power supply from the potentiometer and GND lboth ends of the potentiometer.
I claim the "pseudo VCO" -behaviour will be the same without CV !
this circuit is a tricky variant of the standard ST-oscillator. I spent quite a
lot of time to figure out how it works when I discovered it ( I don't say that I invented it, just that I stumbled about it a couple of years ago, and I did not see it somewhere before) :
with Rpot = 0, it is the well known standard oscillator.
If you rise Rpot it forms a voltage divider with R : a part of the output squarewave is fed back to the input and linearely overlays with the "triangle" voltage from the standard operation. As a consequence of this,
the" triangle" reaches the ST-thresholds much faster and the frequency rises.
Obviously there is a limit: when the fed back squarewave reaches ST-threshold voltage, frequeny would be infinit and the circuit stops working. So R / Rpot must always be smaller than V+ / Vthreshold !
Thsi circuit has some nice properties :
1) the tuning range is much higher than the standard circuit ( see graph)
2) it has a "closer to exponential" tuning range (see graph), which makes it interesting for sound generation.
3) the potentiometer is grounded and free from DC which makes it easy to replace by grounded sensors ( like Richard has shown with the VC-diode circuit above)
So why is it not popular ? :
the killing drawback is its terrible sensivity when you rise Rpot close to
the limit. The remaining triangle part of the input voltage becomes smaller and smaller and you get really LOTS of jitter and noise modulation.
But for Lunetta-style sound gadgets it can be a good solution.
the graph shows 3 tuning ranges:
a) standard ST-oscillator (Rpot =0)
b) perfect theoretical exponential tuning
c) wide range ST-oscillator ( Rpot << R )
have fun to breadboard it
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wide range Schmitt-Trigger oscillator
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tuning range of ST-oscillators
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Posted: Tue Feb 18, 2014 9:49 am Post subject:
elektrouwe wrote:
Proof: disconnect your V+ power supply from the potentiometer and GND lboth ends of the potentiometer.
I claim the "pseudo VCO" -behaviour will be the same without CV !
I just tested it and you are correct, well actually I get a slightly larger range with the pot connected to GND at both sides. You can also leave
one side unconnected. I did another test connecting the capacitor to the output of an opamp configured as a voltage follower, doesn't work. _________________ "My perf, it's full of holes!" SoundCloudBandCampMixCloudTwitchYouTube The Golden Shrimp Guild
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?
Joined: Jan 14, 2010 Posts: 6058 Location: Moon Base
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Posted: Tue Feb 18, 2014 1:19 pm Post subject:
that sounds pretty good 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). _________________ "My perf, it's full of holes!" SoundCloudBandCampMixCloudTwitchYouTube The Golden Shrimp Guild
Proof: disconnect your V+ power supply from the potentiometer and GND lboth ends of the potentiometer.
I claim the "pseudo VCO" -behaviour will be the same without CV :-) !
That is not really a proof though for the original circuit idea not working ..
You built a different, and also interesting, circuit that works too for frequency control but through another effect than the one intended in the original circuit proposed ...
A real proof would be to buffer the pot, or use a very low resistance value for it, such that it behaves more like a voltage source. When the voltage control effect disappears with that it would show that the principle does not work.
Joined: Jan 14, 2010 Posts: 6058 Location: Moon Base
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Posted: Tue Feb 18, 2014 2:44 pm Post subject:
Blue Hell wrote:
A real proof would be to buffer the pot, or use a very low resistance value for it, such that it behaves more like a voltage source. When the voltage control effect disappears with that it would show that the principle does not work.
PHOBoS wrote:
I did another test connecting the capacitor to the output of an opamp configured as a voltage follower, doesn't work.
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.
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.
It´s a pity though. Any idea how to get a more exponential response of the diode to ground idea? I mean it should be exponential aready, just not very precise...
And the resistance of the diode is only changing over 0V- ca.0,8V, is that right?
Quote:
that sounds pretty good Very Happy what flter did you use ?
That one:
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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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what flter did you use ?
