Wein Bridge Oscillator

bananasiong said:

The frequency is determined by the frequency selective network right? Then how to change the frequency?

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You've just answered your own question!.

It's usual to use a stereo potentiometer to give variable frequency, and switched capacitors for the ranges.

So you mean, the resistors in the frequency selective network is a variable resistor with 6 leads?

On my Wien Bridge oscillator, a dual 365pF variable capacitor (from an old AM radio) tunes the frequency, and resistors are switched to change the range. The two sections in the dual variable capacitor match extremely well (much better than a stereo pot) and even have trimmer capacitors for exact matching and frequency calibration.

I think the switched resistors are 40M each for the 10Hz to 100Hz range. I selected matched 20M resistors from many of them.
It has a FET-input opamp so 40M isn't loaded down.

There are a few schematics that allow you to tune the frequency with a single pot. My W.B. oscillator has two zener diodes as the AG control. I had the chance to evaluate the THD with a spectrum analyser and I can't complain about it.

There are plenty of suitable circutis about, one using a variable capacitor is rather obscure! - but try here for a decent explanation and a decent circuit.
**broken link removed**

A variable capacitor is often better because you don't get huge bursts of noise when you change the frequency.