THE L3 SYSTEM — TELEVISION TERMINALS 927 



within the bounds of the vestigial sideband since the quadrature re- 

 sponse function, Fig. 5(c), is attenuated relative to the ''real" response 

 function in this band. In the present case, with a 500 kilocycle vestigial 

 bandwidth and a composite video waveform for a modulating function, 

 the amplitude of Q^ for control purposes is negUgible compared with P^. 



The second term of equation (8), PQ Cos 20, contains no dc component 

 since Q itself contains no dc component and all other frequency com- 

 ponents of Q are shifted 90° in phase relative to corresponding compo- 

 nents in P, The dc control voltage therefore is not modified by the 

 existence of the second term of equation (8). However, the function PQ 

 does contain sum and difference frequencies due to the cross products 

 of the spectra of P and Q. These frequencies in the control voltage tend to 

 be large compared with corresponding frequencies due to the products 

 P^ and Q^ since the trigonometric multiplier Cos 20, equation (8), is 

 large when the phase angle error is small. The effect of the term PQ 

 Cos 20 is that of phase modulation of the receiver carrier supply and 

 its suppression determines the characteristics required of the low pass 

 filter which averages the control signal. At the penalty of sluggish 

 synchronization and restricted oscillator pull-in range the phase modula- 

 tion can be reduced to arbitrarily small values. For our purposes a pull-in 

 range of d=20 cps can be achieved with phase modulation less than ±0.1 

 degree with adequate margins. 



The control voltage is applied to a tuning element in the receiver os- 

 cillator, in this case a small saturable reactor made with ferrite as a core 

 material. This reactor is part of the series resonant quartz crystal cir- 

 cuit which determines the oscillator frequency and is capable of shifting 

 the frequency in response to the control voltage by db 20 cps, a figure 

 chosen as safely less than the first sideband components of the trans- 

 mitted signal which are ± 30 cycles from carrier frequency. This pre- 

 caution avoids possible synchronization of the local oscillator to a signal 

 sideband frequency rather than to the carrier. 



Sufficient gain is provided in the carrier frequency control loop just 

 described so that the maximum frequency difference encountered be- 

 tween transmitting and receiving oscillators is corrected by the phase 

 control voltage due to a steady state phase angle error, 0, less than J^ 

 degree. The control characteristic of the saturable reactor may be ex- 

 pressed, 



A/= A(P' - Q') Sin 20, (9) 



where A/ is the frequency shift introduced by the reactor and A is the 

 factor proportional to required loop gain. 



One other factor to be considered is the stabiUty criterion of the 



