This circuit uses inexpensive, common components to generate a precise dial
tone for phone applications (see the figure).
U1 (an Intel 82C54 timercounter) generates 350- and 440-Hz square waves that
are filtered by R1/C1 and R3/C2
and mixed together by resistors R2 and R4.
An operational amplifier configured as a 395-Hz, Sallen-Key, second-order
bandpass filter (halfway between 350 and 440 Hz) removes unwanted signal harmonics.
Almost any timer-counter can be used as the signal source, so long as it produces
roughly square wave outputs.
If a counter-timer device that provides an overflow pulse is used, flip-flops
configured as dividers should be connected between the counter and the filter-mixer
to generate square waves.
The node at the noninverting (+) input of op amp U2A has a high impedance
and should be well shielded to prevent noise from entering the circuit.
A 220-pF (C5) capacitor was added between the (+) and (-) inputs of the op
amp to reduce the high-frequency impedance at the noninverting input and also
to reduce its susceptibility to noise.
