FREQUENCY SHIFT TELEGRAPHY 297 



Diversity Channel Selection 



The method employed to combine or select the channels of a diversity- 

 system is of great importance. For an AM system the relatively simple 

 method of using a common load circuit for the diode detectors of the diver- 

 sity channels is generally used. By deriving a common AVC voltage from 

 the combined output and by properly adjusting the receiver sensitivities a 

 fairly constant output is obtained. The parallel connection of the diode 

 detectors causes the stronger signal to effectively block the weaker signal 

 thus giving a fairly sharp diversity selection characteristic. The problem 

 of combining the diversity channels of an FS system is more complicated 

 mainly because of the amplitude limiting. If amplitude limiting is used in 

 each diversity channel before demodulation, the resulting constant ampli- 

 tude signals convey no information as to their relative amplitudes as re- 

 ceived from the antennas. Any diversity selection must then be obtained 

 by some indirect method. It is necessary to furnish some selecting device 

 since the noise from a faded channel, if added directly to a good signal from 

 another channel, will cause high distortion. 



In an early frequency shift system employing a two-bandpass filter dis- 

 criminator (shown previously in Fig. 17) it was found that for a poor sig- 

 nal-to-noise condition the sum of the outputs of the mark and space rec- 

 tifiers increased above that for a good signal-to-noise condition. This 

 increase was utilized to suppress the output of the poorer channel and em- 

 phasize that of the better channel. Although neither the degree of diversity 

 selection nor the speed of response was as good as might be desired, fairly 

 satisfactory results were obtained. 



Another method which has been used involves the derivation of control 

 currents or voltages proportional to the amplitudes of the incoming signals 

 which in turn select the better diversity channel by some type of gate action. 

 The time constants of the control circuits must be low enough to permit 

 switching at signaling speed without introducing considerable distortion. 

 The gate circuits must also be of a type which does not introduce interfering 

 transients or otherwise allow the control voltages or currents to interfere 

 with the signal. This method permits very sharp diversity selection and 

 has the capability of approximating ideal results although it becomes some- 

 what involved in a practical form. 



A considerably simpler method has been used in some of the more recent 

 FS terminals. It is based on the use of a single-amplitude-limiter through 

 which pass the signals of both diversity channels. This is made possible 

 by arranging the two signals at the input to the limiter to be at different 

 frequencies. At the output of the limiter the two signals are separately 

 demodulated and then combined. When one of the signals is considerably 



