The unit provides a square-wave output that is rich in harmonic content. The circuit`s output frequency can be varied from 50 Hz to 15 kHz. The heart of the circuit is a 555 astable connected in its equal mark/space mode. The frequency is controlled by potentiometer R2 and capacitor Cl.
Signal-injector - schematic

Resistor R3 controls the output level with the output ac-coupled through C3.

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