Wein Bridge oscillator: is this practical?

[Skywave]

For a Wein bridge oscillator, with its two capacitors and its two resistors of respective equal values, a quick calculation indicates that for R = 300 Ohms and C = 50 pF (both values approx.), the resultant frequency will be in the region of 10.6 MHz. Now I've never met an example of this oscillator producing such a high frequency. So, is my arithmetic in error, and if not, what at the pitfalls and inherent limitations, etc., that will prevent such a high frequency from being obtained in practice with this oscillator?

Al.

[mhennessy]

I have a Farnell LFM3, which is a Wien Bridge that goes to 10MHz. By coincidence, I've tweaked mine to get 10.7MHz from it with no problems.

The manual (complete with schematics) can be downloaded here: https://archive.org/details/FarnellL...cillatorManual

This, in common with some other Farnell oscillators from the time, uses a variable capacitor for tuning. In the highest range, the resistances are very close to the 300 ohms you mention. The difficulties come at lower frequencies, where very high impedances mean that screening becomes critical. Of course, a FET - or FET-input opamp, or a valve - is needed rather than anything bipolar.

[Radio Wrangler]

There are loads out there, HP made a range of wien bridge oscillators covering into the MHz range. The HP 651A was pretty common and covers 10MHz. and the service manual can be found by a google query.

These machines used switched range resistors and variable capacitors as better matched capacitors could be bought than pots. These things had difficulty more at low frequencies where very high input impedance was needed at the input to the amplifier.

Some high frequency wien oscillators have an extra capactor section used to phase-correct the amplifier.

David