Article : Andy Collinson
A simple test circuit to fault find audio and radio equipment. Can be used to inject a square wave signal, rich in harmonics, or used with headphones as an audio tracer.
A single pole double throw sitch is used to switch between inject and trace modes. The diagram is drawn in trace mode, the earpiece being connected to the collector of the
last transistor. Both transistors are wired as emitter followers, providing high gain.
DC blocking is provided by the 1n capacitor at the probe end, and the two stages are capacitively coupled.
when the switch is thrown the opposite way (to the blue dot) both transistors are wired as an astable square wave generator. This provides enough harmonics from audio up to several hundred kilohertz and is useful for testing AM radio Receivers.
The Circuit on Veroboard
Output Injector Waveform
Henry has also kindly provided on oscillogram on the injector in action, this is shown below.
The fast switch on time of the transistors produces the switching spike which is rich in harmonics.
Last Modified 12-5-10