The Synchronous AM Problem in FM TV Transmitters

The FM sound signal of a broadcasting TV transmitter is usually affected by spurious AM caused by the resonators of the vision–sound combiner. A simple expression is derived which allows the quantitative evaluation of this unwanted effect. Since the frequency stability of the resonators is the only factor which falls within the control of the designer, some considerations are made regarding the mechanical design criteria to be followed in order to improve it.