Variable Frequency Osc circuit? Whats a good one?

[Qaiphyx]

Sinusidal wave, within 1hz to 200khz range.

 

[Mikebits]

Check this data sheet.

**broken link removed**

 

[audioguru]

Digikey doesn't have the XR2206 and it isn't listed on EXAR's site so maybe it is discontinued.
A few electronic parts distributors still have some.

 

[Mikebits]

Available from Future Electronics

**broken link removed**

 

[Hero999]

A 555 timer?

 

[audioguru]

A 555 has a square-wave output and something like a sawtooth waveform. Not a sine-wave.
A few RC filters at its square-wave output will make a sine-wave but its frequency is not variable.

 

[BeeBop]

How about a wein bridge oscillator?

Edit: yes, I edited the schematic just to change the diodes to 5.1V zeners.

 

[audioguru]

A Wien Bridge oscillator has two resistors or capacitors that must be closely matched.
I doubt you will find closely matched parts that are variable.

 

[BeeBop]

A Wien Bridge oscillator has two resistors or capacitors that must be closely matched.
I doubt you will find closely matched parts that are variable.

Yes, however the pot which controls the frequency is not one of those pairs. It worked for me just fine, but I'm not sure if it went as low as 1kHz.

 

[Nigel Goodwin]

A Wien Bridge oscillator has two resistors or capacitors that must be closely matched.
I doubt you will find closely matched parts that are variable.

Millions have been built using stereo potentiometers, and they work perfectly fine - both commercially and as home projects.

 

[Anaximander]

Millions have been built using stereo potentiometers, and they work perfectly fine - both commercially and as home projects.

A wein bridge oscillator has two matched pairs of components, two resistors and two capacitors. And the frequency of oscillation is proportional to their product. So if you wanted an oscillator with a relativly narrow frequency range (One decade, say), you have two fixed capacitors, and the resistors are given by a 'dual ganged potentiometer', these are basically two potentiometers glued together with one knob, so that turning the knob changes the two potentiometers equally.

If you want a wider range of frequencies, you use rotary switches to select different capcitor pairs. So you could set the capacitors to 1uF, and vary the potentiometers from 1k to 10k to get one frequency range, and then use the rotary switch to make the capacitors in the circuit 10uF, which, for the same potentiometer range, will give a different, non-overlapping frequency range.

There are even ways to avoid using dual-gang potentiometers, by having a gain stage in the feedback network, but you'll have to find a specific circuit to do that. I think hunting around on google will find one, otherwise you'll have to consult a book specifically about oscillators.

 

[audioguru]

My Wien bridge oscillator uses a dual variable capacitor with trimming capacitors built-in for exact matching. The circuit used to have a FET input but now has a FET-input opamp.
The ranges are selected with matched 1% resistors.

Amplitude stability used to be done with a light bulb but now is done with a FET automatic gain control circuit.

It is 10Hz to 100kHz at 0.04% distortion.
I have another oscillator that uses an over-sampled 10-steps sine-wave followed by an 8th-order switched capacitor filter IC. Its distortion is so low that it is difficult to measure, about 0.001%.

 

[on1aag]

My Wien bridge oscillator uses a
dual variable capacitor with trimming capacitors built-in for
exact matching.

Those trimming capacitors are needed to compensate for
the input capacitance of the amplifier and not for the
matching of the two cv's.
If you increase the frequency the capacitance of the cv
is at it's minimum and that's were you're getting in trouble
due to the added capacitance of the input amplifier.
That's the point were the difference in capacitance is at
it's highest.

Beebop: What's the purpose of D1 and D2 ?

on1aag.

 

[Anaximander]

...

Beebop: What's the purpose of D1 and D2 ?

on1aag.

They're non-linear components to stabalise the gain. For stable oscillation, the gain of the apamp must be exactly three. If it's less, the oscillations will decay away, if it's more the oscillations will grow without limit (Until clipping appears in the opamp).

When the amplitude of the oscillations are low, the diodes don't turn on, and the gain of the opamp is grater than three (Asuming the potentiometer is set correctly). As the amplitude grows, the diodes start to conduct, shorting out the resistor they're in parallel with, lowering the gain to less that three, so the oscillations start to decay. At which point the diodes stop conducting, and the gain rises. Etc...

They serve the same purpose as the bulb/thermistor you see in other wien bridge circuits. They cause more distortion, but are much simpler to use.

 

[on1aag]

Hi Anaximander,

You better take another look at Beebop's circuit.

on1aag.

 

[audioguru]

My Wien Bridge oscillator uses the trimmer capacitors to make the capacitance of each half exactly the same. Including the capacitance of the amplifier's input and stray wiring capacitance. I can see the output level change a little (when I sweep the frequency faster than the AGC can control it) if I move a wire a small amount.

 

[BeeBop]

Beebop: What's the purpose of D1 and D2 ?

on1aag.

They just limit the output to + and -5V.

It has been so long since I built the thing....

 

[MrAl]

How about a wein bridge oscillator?

Hi BeeBop,

Wow, i havent seen one of those things in ages now, thanks for posting.

Just a tiny problem however...
The two output diodes are connected such that neither one of them
will ever conduct, so there has got to be a problem with the connection
of those two. Perhaps you can find the error and correct this schematic?
Thanks.

 

[Tomble]

Some important questions for the OP that I think have some bearing on what is a good choice:
-How do you intend to vary it? Manually? Analogue signal? Digital signal?
-How good a sine wave do you need?
-Does the frequency need to be pretty stable/precise?
-Is startup/settling time a concern? (how much?)
-Any other constraints such as size, cost, time to make/design etc?
-If it's not a secret, what's this for?

From what little you've said so far I'd suggest either a function generator chip (IIRC these use deformed triangle waves), a reasonably high-speed microcontroller driving a DAC, or a beat-frequency oscillator, but I'm no expert on this. Others here would probably be able to give you more suitable suggestions if you can address these sorts of things though.

 

[Anaximander]

Hi Anaximander,

You better take another look at Beebop's circuit.

on1aag.

*Looks at circuit*

*Shocked face*

Eh? I have no idea what they're doing, I assumed the diodes were in parellel with each other, because that's how the wien bridge circuits I've built have worked.