Wide Bandwidth VCO, or how to create a function generator?

Thread Starter


Joined Mar 2, 2013
I'd like to create my own function generator someday, and so I've been reading up on VCO's and finally got around to building one the other day.

The VCO I built is described as a "comparator based hysteretic oscillator", a type of relaxation oscillator. Essentially, this circuit, on the wikipedia page for "relaxation oscillator".

I somehow managed to pick values that allowed me to cover the audible range almost perfectly and stuck in a potentiometer for the top right resistor, so it can do about 50Hz - 20kHz +, not sure what the top range is because I don't have a scope but it went beyond being able to hear.

Anyways, how good quality of an oscillator is this kind? Compared to other kinds?

How could I increase the bandwidth? At higher frequencies I guess I suppose slew-rate becomes a major problem?

And can any desired frequency be obtained through frequency division? If so, wouldn't it always make sense to opt for the highest possible frequency VCO and just divide down for a function generator?

These are just a few of the multitude of questions I have.

I know this isn't really a VCO, it's an RCO, but I care more just about any kind of controllable oscilator
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Joined Jun 13, 2013
Most important - are you satisfied with the waveform?
There is another relaxation frequency generator (I personally use the term "oscillator" for sinusoidal signals only) consisting of two opamps providing squarewave and sawtooth signals at the same time.
Regarding slew rate - each opamp based circuit (oscillator, filter, amplifier) suffers from the limited slew rate. This cannot be avoided. However, opamps have different slew rates and you can select an opamp with a very high slew rate.
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Joined May 11, 2009
What kind of output do you need. Most good analog wave generators have been taken of the market a long time ago. But this one is still active http://www.exar.com/communications/timing/voltage-controlled-oscillator-vco/xr2209
It can give out Triangle, Sawtooth, Pulse, Squarewave and create FM and Sweep Generation
The analog waveform generator IC segment have been replaced by DDS ICs (Direct Digital Synthesizers). Just Google DDS, Analog devices is a good source for those kind of ICs. You can also get DDS based units from EBAY quite cheap


Joined Jul 21, 2012
You seem to have terminology problems:

A "Wide Bandwidth" Oscillator would produce something like a square wave which possesses a large number of harmonics of the fundamental,hence,it occupies a "Wide Bandwidth".

What you are looking for is a "Wide Range" Oscillator,which is capable of operating at a wide range of discrete frequencies.

A VCO is a "Voltage Controlled Oscillator",where as the name implies,the operating frequency may be varied by changing the value of a control voltage.

A more appropriate term is a "Variable Frequency Oscillator" (VFO).

Thread Starter


Joined Mar 2, 2013
Thanks for the responses. I have seen the the two op-amp circuit that produces square and triangle wave. Ultimately though, I just wanted to know what's best for a function generator and how their oscillators work, so DDS is the standard nowadays for all waveforms?

What kind of oscillators were the standard before before DDS?