Ma Bell used LC tanks - multi tapped too! There was one transistor on the DTMF pad, acted as an amplifier.
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11 years ago
Two Tones, One Transistor
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11 years ago
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?
-- Rick
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11 years ago
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.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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11 years ago
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.
-- Rick
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11 years ago
They can't be harmonically related. Look at the published DTMF frequencies.
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11 years ago
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