This is an application where two Zener diodes were placed in series, in a back to back configuration. They were placed across the primary winding of a transformer used to apply modulation to an AM transmitter. The modulation was in the form of a single frequency, and the modulation level was not to exceed 30% by specification. The modulation level showed some variation with temperature, so that the diodes were selected to limit the voltage across the transformer primary to ensure that the 30% modulation limit would not be exceeded. The drive for the transformer/limiter combination was from a low impedance source, adjusted to provide 30% modulation peaks during final test. The pole mounted transmitter was required to operate in all weather, unsheltered conditions, at any airport in the …
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