PA protection circuit for a single sideband radio

Claim 1

A protection circuit for a power amplifier used in a single sideband transmitter to supply RF power modulated with an audio signal to an antenna and wherein said power varies with the power of the audio signal, said protection circuit comprising: a. directional power detector means coupled between the power amplifier and the antenna and having a first output for providing thereon a first audio frequency signal varying in accordance with the modulating audio signal and indicative of forward power supplied to the antenna and a second output for providing thereon a second audio frequency signal varying in accordance with the modulating audio signal and indicative of reflected power from the antenna; b. first level comparator means coupled to the first output of said directional power detector means and providing an output signal varying in accordance with the first audio frequency signal applied thereto about a predetermined level; c. second level comparator means coupled to the second output of said directional power detector means and providing an output signal varying in accordance with the second audio frequency signal applied thereto about a predetermined level; d. summing means coupled to said first and second level comparator means for receiving the output signals therefrom and providing an output signal indicative of the sum thereof; e. peak detector means coupled to said summing means for receiving the output signal therefrom and providing a DC potential at an output thereof which is proportional to the magnitude of the output signal from said summing means; and f. attenuating means having a control input coupled to said peak detector means and an output coupled to the power amplifier for reducing the amplitude of signals supplied to the power amplifier in accordance with the DC potential at the output of said peak detector to control peak envelope power at the antenna.