FM Stereo Test Transmitter: VHF FM and audio test signals

摘要

This test signal generator is particularly notable not just because of its high precision, but also because of the original method it uses to produce its waveforms. The many features are listed in the 'Output signals' text box. Rather than using special-purpose ICs, whose exact internal workings would be hidden from view, this design just uses ordinary HCMOS logic. All audio-frequency signals are produced using binary counters and combinatorial logic from a crystal reference of 9.728 MHz. The audio output can be switched between the tone and the baseband signal. It can deliver a maximum of 2 V_(pp) from its relatively low impedance short- circuit proof output. The FM oscillator is free-running, but is very stable in frequency at around 100 MHz in the VHF FM broadcast band. The frequency can be adjusted over the FM band by changing a capacitor value, and a trimmer offers fine adjustment over a range of ±300 kHz. The RF output level, at 12 mV_(eff) into 50 Ω, is more than adequate to drive a connected receiver and obtain its maximum signal-to-noise ratio.