FREQUENCY SHIFT TELEGRAPHY 269 



with the telegraph signal. All the stages are operated with full R.F. excita- 

 tion continuously to produce a constant amplitude carrier. 



As a matter of expediency frequency shift keying has sometimes been 

 provided by switching between two independent sources of carrier current 

 separated in frequency by the desired shift. In such a case the frequency 

 transitions involve sudden phase discontinuities of random values. This 

 results in the instantaneous frequency swinging considerably outside the 

 steady-state mark and space frequencies. If the band is wide, such 

 as is the case in a radio transmitter, there results a very broad sideband 

 radiation capable of causing severe interference to adjacent channels. If 

 the band is narrow, as might be the case where sending filters are 

 employed, the interference is eliminated but the amplitude transients result- 

 ing from the sudden phase shifts are capable of producing considerable 

 distortion. 



Restriction of Transmitted Band 



As seen above, square-wave modulation results in a wide spread of side- 

 band components which are of sufficient ampHtude to interfere seriously 

 with adjacent channels unless greatly attenuated. The transmitted band 

 may be restricted either by the use of a band-pass filter centered about the 

 carrier frequency or by a low-pass filter to suitably shape the modulating 

 wave form. Band-pass filters are usually used if the power level is low and 

 the frequency low enough to permit suitable filter construction. For multi- 

 channel systems the use of band-pass filters also permits efficient parallehng 

 of the transmitting channels. For radio transmitters with a transmitted 

 power measured in kilowatts, and with a frequency of several megacycles 

 which is frequently changed to suit best the prevailing conditions, shaping 

 of the modulating wave is the more practical method of restricting the trans- 

 mitted band. 



Insufficient attention has been given in the past to the envelope shape of 

 the signals from on-off keyed radio transmitters. With the ever increasing 

 crowding of frequency assignments it becomes more and more important to 

 restrict the emission of unnecessary sidebands arising from keying. The 

 envelope shape in on-off keying may be controlled by properly shaping the 

 modulating grid or plate voltage wave. It is important that the stages 

 following the keyed stage or stages be nearly linear, otherwise the wave 

 shaping will be largely destroyed. In the case of frequency shift keying, 

 on the other hand, the wave shaping is preserved after passage through class 

 C amplifier or multiplier stages, and these may be operated for maximum 

 efficiency. The greater ease of producing and maintaining the desired wave 

 shaping, so necessary for close frequency spacing of channels, is one of the 

 outstanding advantages of frequency shift keying. 



