A SINGLE SIDEBAND MUSA RECEIVING SYSTEM 325 



delay compensation in two of the three diversity branches when they 

 are all connected to the output at once so that the audio outputs will 

 add in phase. The branch receiving the waves at the highest angle, 

 which are always the waves which have traversed the greatest distance, 

 does not contain a delay circuit. The other two branches contain 

 variable delay circuits having a maximum of 2,768 microseconds delay 

 in steps of 31 microseconds. The band covered by the delay circuits 

 is 6,000 cycles, the same as the width of the filter in the last inter- 

 mediate frequency amplifier. The delay steps were made small 

 enough so that it would be technically possible to phase properly over 

 the entire band within less than a quarter cycle provided that the 

 phase distortions of the transmission path, and the other parts of the 

 receiver made it practical to do so. 



The delay circuits each consist of 8 units having a delay of 31 

 microseconds and 9 units having a delay of 280 microseconds. Hand 

 operated switches are provided to vary the delay in the usual decade 

 switch manner. A motor driven switch is also provided which is 

 arranged with two shafts connected by an intermittent movement so 

 that when the eight small units of delay have been added a continued 

 movement of the shaft removes these units from the circuit and 

 simultaneously connects one of the large units, which is equivalent to 

 nine small units. Further movement in the same direction successively 

 adds in the smaller units again. 



Automatic Delay Adjusting Circuits 



A block schematic of the automatic delay adjusting apparatus as 

 well as the automatic angle adjusting and recording equipment is 

 shown in Fig. 8. 



The proper delay compensation to phase the output of two diversity 

 branches can be determined by connecting the outputs of the two 

 branches to the two pairs of plates of a cathode ray oscilloscope and 

 varying the delay in the lower angle path until a straight line pattern 

 is obtained on the oscilloscope screen. This adjustment is facilitated 

 by the restriction of the band in each branch at the oscilloscope to 

 about an octave. If only a single frequency were used for this adjust- 

 ment several positions of the delay adjustment might be found to 

 give a straight line on the oscilloscope, the number of positions depend- 

 ing upon the frequency. In order that there may be only one position 

 when the maximum delay is 2,768 microseconds the phase shift caused 

 by this delay must be less than 180°, and consequently the frequency 

 of operation must be less than 180 cycles. Where a band of frequencies 

 a few hundred cycles wide is used, however, this difficulty is not 



