SSB Exciter for Fixed or Mobile

The simplest and most economical method of generating a single sideband signal is to employ a phasing-type transmitter of the form outlined in chapter 17 of this Handbook. If the r-f phasing system operates on the carrier frequency of the transmitter the complex frequency conversion circuits may be omitted and the complete exciter becomes inexpensive to build and simple to place in operation.

A SSB exciter suitable for fixed or mobile operation is shown in figures 1 and 4. The exciter delivers a 3 watt peak power signal, sufficient to drive a high power tetrode linear amplifier to a kilowatt level. Operation is confined to the 80 meter band, although operation on the higher frequency bands is possible with a change of coils, phasing network, and crystal. The r-f phasing circuits are balanced at some frequency within the amateur phone band and will. retain a good degree of balance over a frequency range of plus or minus fifty kilocycles of the adjustment frequency.

Circuit The circuit of the single side-

Description band exciter is shown in figures 2 and 3- A 12AU7 is employed as a crystal oscillator stage and buffer-amplifier. The first section of the double triode is used in a Pierce oscillator circuit with the crystal connected between the grid and plate of the tube. The oscillator operates directly on the chosen SSB frequency in the 80 meter band. The frequency of oscillation may be varied over a range of two hundred cycles or so by the 50 H/j.fd trimming capacitor permitting the transmitter to be "zeroed in" on a particular SSB channel. The second section of the 12AU7 serves as an isolation amplifier with the plate circuit tuned to the operating frequency. Changes in the input impedance of the diode modulator stage under operating conditions would cause frequency shift of the oscillator stage if direct coupling between these circuits was used. The isolation afforded by the buffer stage effectively prevents frequency pulling of the oscillator stage during modulation.

The output of the buffer stage is link coupled to a simple 90° r-f phase shift network wherein the audio signal from the audio phasing amplifier is combined with the r-f signals. The network is of the R-C type made up of 50 ohm non-inductive resistors and 800 fi/ifd. capacitors in a bridge configuration. The reactance of the capacitors is very close to 50

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