Frequency-Shift, Carrier-Current Equipment for Telemetering and Other Control Type Functions [includes discussion]
R. W. Beckwith · Transactions of the American Institute of Electrical Engineers Part III Power Apparatus and Systems · 1952
The frequency-shift crystal oscillator and the crystal discriminator, which are vital to the channel equipment utilized, are described here. The former is used to generate the carrier signal and the latter is used in the receiver to reform the transmitted intelligence.