1-1. GENERAL DESCRIPTION. The FS-4 is a frequency synthesizer developed for use with the Drake Company “4” line equipment. (R-4 series, T-4X series and SPR-4.) The FS-4 plugs into an unused auxiliary crystal socket in the rear of the receiver or transmitter, and will supply the various injection signals to the equipment and allows continuous coverage without utilizing the accessory crystals. The frequency readout on the FS4 displays the received frequency band, and to this is added the frequency readout of the receiver “PTO” to obtain the correct received frequency. The FS-4 supplies all the crystal oscillator injection signals for the R-4 series and T-4X series in 0.5 MHz steps from 11.1 through 40.6 MHz, and the readout is direct. An accessory kit is available to provide proper injection signals for the SPR-4 receiver. The FS-4 can be used with the 2-C type receiver over the frequency range of 7 to 30 MHz. When used with the 2-C receiver, the FS-4 frequency readout is not direct, and a correction factor of “7 MHz” must be added to the FS-4 readout to obtain the correct operating frequency. A special accessory kit is necessary to modify the FS-4 for use with the 2-C receiver.