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voltage to frequency converter circuit (VCO)

i'm working on a design project right now to create a voltage controlled oscillator using only opamps and discrete components. I'm able to create a square and triangle waveform easily and i've found many methods to create a sine waveform such as piecewise breakpoint and jfet amplifer from the triangle wave.

However, I'm having trouble finding information about a voltage to frequency interface so that i can vary the frequency of the waveforms using a DC voltage. There needs to be two ranges of voltages from

0.1-0.5V which control the frequency with 200hz/V and 1khz/V (user selectable from one of the two).

Any help is appreciated. thanks.

Reply to
macrossfreek
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I'm a bit confused whether you need the VCO to have native sinewave output or not. Assuming that a fixed-amplitude sawtooth (triangle) output is ok, the common way to do it is to use an op-amp integrator which integrates between two setpoint voltages, say +1V and -1V. A resistor feeds current into the integrator, just a capacitor from op amp out to the (-) input. The voltage which feeds that resistor determines the ramp rate of the integrator, and thus the frequency. The control voltage input can be fed to an inverter to generate a negative version of itself, and then the integrator input resistor is switched between the control voltage and its negative to ramp down or up, respectively. Which way it goes is controlled by some bi-stable circuit that switches state when the integrator output limit in either direction is reached. You can easily change either the capacitance or the resistance in the integrator to change between 200Hz/V and 1kHz/V.

If you want native sinewave output, you _could_ build a pair of higher frequency oscillators, controlled by varactor diodes, and mix (multiply) the two outputs to generate the sum and difference frequencies, using a simple low-pass filter to get rid of the sum frequency. Making such an oscillator with a very linear f vs V response is not easy.

I'm curious; is this a homework assignment?

Cheers, Tom

Reply to
Tom Bruhns

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that mite give you something to work with.

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Reply to
Jamie

I've been f****** programming all week and felt an extreme urge coming on to design something for real. (better now:) With smaller Cap, circuit is good to about 40kHz.

(max 3.5V) +5V 0 to 500mV i/p .-----o-. 10n|| Triangle o-o--------------------o1 14 | .-||---.6V pkpk Square | ___ 10k | | | || | ___ | .-|___|-. | 2o--. | | .-|___|-. | | | | | | ___ |6|\\| | | 15k | | | +5V | | IC1 | o-|___|-o-|-\\ | ___ | |\\ | | ___ |2|\\| | | | | Rset | >-o|___|-o-|+\\ | '-|___|-o-|-\\ 1 | | 9o--' .-|+/ 7 10k 10| >--o-o 10k | >--o----o8 | |5|/| .--|-/ 8 | .---|+/ | | | | 9|/ | | 3|/| | | -o- -o- | -o- -5V .-o3 5o-. 0V 0V | 0V | | | | | .---o-o6 13o-o------------------------------' | | | .-. | | Rset=7350 ohms for 1KHz/V 10k| | | 4 7 | Rset=36750 ohms for 200Hz/V | | '-o---o-' IC1 74HC4066 '-' -5V -5V Opamp TL084 (+5V p4,-5V p11) | +5 -o- "Voltage controlled Oscillator".

(created by AACircuit v1.28 beta 10/06/04

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Reply to
john jardine

On a sunny day (Fri, 20 Oct 2006 20:29:45 +0100) it happened "john jardine" wrote in :

Yes, nice, but if you want only a square wave, and use chips anyways, why not use the 74HCT4046 PLL. It has has a VCO.

But I think I can do this simpler with discrete components:

----------------------------------------- +12V | | | [ ] [ ] R1 [ ] | | | | |

Reply to
Jan Panteltje

2N2646, still got a copy of the GE transistor manual somewhere with a whole UJT chapter, (and one on tunnel diodes)

martin

Reply to
martin griffith

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