How to make a Voltage Controlled Oscillator

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Hello all,

What is the simplest VCO that can be done with off-the-shelf components?
It doesn't have to be very fast. Audio range is fine.

Cheers,
Cem

Re: How to make a Voltage Controlled Oscillator
There are simpler, but this works out well enough, and it's a familiar
circuit.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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Re: How to make a Voltage Controlled Oscillator

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Hello Tim,

Either you have forgotten to paste the link, or somehow my newsreader
does not show it.

Cheers,
Cem

Re: How to make a Voltage Controlled Oscillator
Derp derp...

It's one thing to forget an attachment, but a link, that's just
embarassing :)
http://myweb.msoe.edu/williamstm/Images/Cheap_VCO.png

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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Re: How to make a Voltage Controlled Oscillator
On a sunny day (Tue, 26 Oct 2010 01:03:23 -0500) it happened "Tim Williams"

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You can make it linear by replacing the 100k resistors by PNP current sources.

Re: How to make a Voltage Controlled Oscillator

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Wow! A modified RC flip flop. Never thought of that. Thank you very much
Tim!

Jan: Will this thing work logarithmically this way? That would be must
more suited to my application than linear then? I'm going to use it to
generate tones.

Re: How to make a Voltage Controlled Oscillator
On a sunny day (26 Oct 2010 11:16:03 GMT) it happened Cem Uzunoglu

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I knew somebody who designed a dB VU meter using RC constants,
You need a minimum starting voltage here though, depending on the Vbe.
For very high values of the resistors,
and high values the control voltage relative to the supply voltage,
it will also approach linear.

Maybe to make it log better use a linear version and add a log amplifier in
front.
or a log potentiometer perhaps?

Re: How to make a Voltage Controlled Oscillator
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Resistors will give you RC time constants, which have an exponential sort
of thing.  For voltages close to Vbe, time goes to infinity (or the gain
is too low to turn on the transistors anymore and it stops completely).
For large voltages, it looks linear (in which case, the resistors
approximate current sources).

If you require linearity, the good old hysteretic comparator with current
sources is hard to beat.
http://webpages.charter.net/dawill/tmoranwms/Circuits_2008/Triangle.gif
This particular circuit has a 4.5 million times range of frequency in a
single range.  For simplicity, the discrete comparator could be replaced
with an IC like so:
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Pulse_Width_Modulator.png
using a current source to discharge the capacitor, instead of a resistor.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: How to make a Voltage Controlled Oscillator
Tim Williams:

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It may be a byproduct of bad quoting.

Re: How to make a Voltage Controlled Oscillator
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The VCO in a 4046. See figures 14B and D on page 11

http://www.onsemi.com/pub_link/Collateral/MC74HC4046A-D.PDF

The Analog Devices AD654 is simpler

http://www.analog.com/static/imported-files/data_sheets/AD654.pdf

and Farnell still stocks it, so I guess it is also off-the-shelf.

--
Bill Sloman, Nijmegen


Re: How to make a Voltage Controlled Oscillator
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The 74HC4046 has problems when the control voltage is low.
Many people find they can't get better than a 10:1 ratio.

Use a 4000 series 4046 instead, they work over a 1000:1 range.

Clifford Heath.

Re: How to make a Voltage Controlled Oscillator
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Damn! I knew that, and thought that I'd dug out a 4000 series data
sheet! I must have failed to copy the URL and pasted an earlier
reference.

Sorry.

--
Bill Sloman, Nijmegen

Re: How to make a Voltage Controlled Oscillator
On Wed, 27 Oct 2010 02:18:59 +1000, Clifford Heath

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Depends on use.  Too large a tuning range makes for a noisy,
cantankerous-to-control, oscillator.

You _could_ roll your own using a package of LM339's
        
                                        ...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Re: How to make a Voltage Controlled Oscillator
On a sunny day (26 Oct 2010 02:16:16 GMT) it happened Cem Uzunoglu

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[74HC]4046


Re: How to make a Voltage Controlled Oscillator
On Oct 26, 4:16A0%am, Cem Uzunoglu

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I like this one : TL084 + 2N3819
both triangle and rectangle waveforms output :
http://cjoint.com/?0kAogzIgPf

Re: How to make a Voltage Controlled Oscillator

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Looks cool. There are no resistor values on the schematics though. Are
they important?

Re: How to make a Voltage Controlled Oscillator
On Oct 26, 4:02 pm, Cem Uzunoglu

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Sure it is - and nice to study too.

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Of course they are, with cap C1
they set the VCO response Hz/V.

AOP #1 is an amplifier w/ gain +1 or -1
(R1=R2) depending of the conduction of Q1.
AOP #2 is an integrator w/ time constant R3*C1.
AOP #3 is a trigger with treshold R8/R7.

Not the simplest, but quite fine for audio.

Re: How to make a Voltage Controlled Oscillator



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A 74HC05, capacitor and three resistors:

http://www.abvolt.com/misc/vco_74hc05.jpg


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

Re: How to make a Voltage Controlled Oscillator
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Pffbt.  Too many resistors, and way too many pins:

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If you'll accept a control current, instead of a control voltage, it can
be done in two components :)

Worked fine in the toob days.  Back then, they had a hundred volts to play
with, so the required headroom wasn't a problem.  It is quite drifty and
noisy, and should be kept away from external light or radiation.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: How to make a Voltage Controlled Oscillator
On a sunny day (Tue, 26 Oct 2010 10:45:49 -0500) it happened "Tim Williams"

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Same idea works with an UJT too:

 Uc              +12 V
  |              |
 [ ] R1       |--
  |---------- | UJT
  |          \|--          
 ===             |------- pulse out
  | C1          [ ] R2
  |              |
 ///            ///

I used a PNP current source for R1 to make it controllable.
  

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