# Homework Help: How can I set a frequency in a Wien bridge oscillator

1. Apr 30, 2014

### tburn

Hi all,
Im new here :-)
So Im designing a simple Wien bridge oscillator, and need it to be set to a specific frequency.

This is my circuit:

Where C1 = C and C2 = C
and R3 = R and R4 = R

Im using ω=1/(C2R2)
where C = my capacitor values and R = my Resistor values.

My problem is, however, how do I get it to equal ω?
Should I just set either C or R to a random value, and then work out the other?

Many thanks,
Tommy

2. Apr 30, 2014

### Staff: Mentor

Yes, you can do that.

This is where the Theoretical meets the Practical. Yes you could choose a random value for one of the components and then calculate a suitable value for its partner. But in the real world there are practical considerations, too. Things like, are the resulting component values available (are they even manufactured)? Do you you have the component values in stock? How do the component values compare to intrinsic or stray values in the other components? Non-ideal op-amps have finite input impedances and gain, wiring has some resistance, stray capacitance and inductance even if miniscule, and so on.

That said, if your working frequency is reasonably low (not radio frequencies etc.) then I'd suggest choosing a standard capacitor value and seeing what the corresponding resistor value would be. If it's not a practical value, adjust the capacitor value and try again. Rinse and repeat until your component values are reasonable and obtainable.

3. Apr 30, 2014

### tburn

4. Apr 30, 2014

### AlephZero

Another practical consideration is that in the circuit diagram you have R3 and C2 in parallel. It would be "nice" if their impedances were roughly equal at the operating frequency. It probably doesn't matter to within a factor of 10, but you wouldn't want the impedance of C2 to be 1000 times bigger or smaller than R3.

Also, if you want an accurate frequency, searching component catalogs will show that close tolerance resistors are available in wider range of "preferred values" than close tolerance capacitors. You can easily get resistor values in the E96 series, but caps are likely to be restricted to E12.

5. May 5, 2014

### rude man

Not much help since as you say cap tolerances are looser and w is a function of RC.

6. May 5, 2014

### rude man

Your formula for w is incorrect.
Aleph's advice would be well taken.

7. May 5, 2014

### AlephZero

Caps with 1% tolerance are easy to get, but only in a small range of values. The next highest value above 10 is likely to be 12 or 15, not 10.1.