Following the American FCC rules, this 90s Radio Electronics circuit shows how to use a transformer to isolate the telephone line from an amplifier circuit.
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
In the figure we have the way to use a single control voltage to act on two varicaps...
Found in a 90s documentation, this switch that is also sensitive enough to operate capacitively uses...