Two Tones, One Transistor

I was wondering if there was a way to tune a single circuit for two frequencies. I get that there are two tank circuits on one transistor, but what keeps the tank circuits from interacting with each other?

In the touch-tone phone circuit, there is a loop: two tanks and a negative resistance. Each tank creates an exponentially-increasing-envelope sinewave oscillation. Given that, the amplitudes will build up until the negative resistor (the transistor) goes nonlinear. To fix that, and define the amplitudes, Bell put a varistor across each tank.

I appreciate the response, but that wasn't exactly what I was asking.

I was wondering why the two tank circuits don't interfere with the tuned frequencies. Are the two circuits totally isolated? If so, how exactly? I don't get that from the schematic, although I don't have that in front of me at the moment.

They can't be harmonically related. Look at the published DTMF frequencies.

If the tank circuits and the amplifier are linear, the superposition principle keeps the tanks from knowing about each other. Because stable oscillation requires a nonlinearity, to keep the amplitude finite, the trick (as John pointed out) is to have a *separate* nonlinearity for each tank.

We`re planning to use bolometers as the nonlinearities. We also expect to open a can of worms there, for instance the harmonics created by the nonlinear element *will* be seen by other tanks. So, one has to arrange the frequencies suitably.

Regards, Mikko