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Stereo Test Tone Generator

Stereo Test Tone Generator

It seves the purpose of aligning an FM Stereo Modulator used in low power FM stereo transmitters. Two tone outputs are available, the low tone has a secondary output that is 180 degrees out of phase with the primary output.

Operating Voltage: 12V DC (should work ok on 9V)
Operating Current: 15ma max.
Low Output Frequency: approximately 600 Hz
High Output Frequency: approximately 800 Hz
Output Levels: approximately 0.5V – 5V P-P


IC U3B is wired to produce a virtual ground at half of the power supply rails (6V). This is used elsewhere in the circuit as a reference. The power supply is filtered with the 220uF capacitor. The 3A diode offers some protection against reverse polarity on the power inputs. For full protection, place a 1A or smaller fuse in series with the +12V input.


The circuit has two nearly identical stereo sine wave generator circuits. U1 is the heart of the low tone oscillator, U2 is the heart of the high tone oscillator. The two op-amps on the left of each oscillator chain produce square and triangle waveforms at fixed frequencies. The frequency is set with the 10n and 6n8 capacitors.

The triangle waves are passed into low-pass filters, which remove most of the harmonic energy and produce relatively pure sine waves. The two 27K and two 33K resistors set the low pass frequency.

The sine waves are each fed into an output amplifier which can be adjusted for fixed level outputs. The low tone output is also fed into IC U3A which produces an inverted copy of the low tone signal.

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