7-12] SNEAK CIRCUITS 377 



there is no overshoot when the pole frequency and the carrier frequency are 

 coincident. The envelope response of such a network is given by 



eo{t) = K 



1 +f expyj - 2f exp^|-jcos (co - w,,)/ 



1 + 



\C0o CO / 



(7-36) 



where ^ is the effective circuit ^ 

 Wo is the pole frequency 

 CO is the carrier frequency of the step sinusoidal input. 



Note that oscillatory terms are involved when the carrier is detuned from 

 bandcenter. These terms are relatively insignificant, however, for the 

 amount of detuning that would normally be tolerated. When the circuits 

 are not identical but are stagger-tuned, then the response given by Equation 

 7-36 becomes important. If the oscillatory signal is sufficiently large, the 

 output of the following stage may be blocked for a period of time in excess 

 of twice the duration of the input signal. To minimize these effects, inter- 

 stage bandpass networks are usually employed which are symmetric about 

 the IF center frequency, 



7-12 SNEAK CIRCUITS 



When considering the dynamic response of the receiver, it is not sufficient 

 to consider only the performance as a bandpass filter with saturation effects 

 under large-signal input. The transmission characteristics of the amplifier 

 in the low-frequency region of the spectrum must also be considered. 



To realize practical high-gain bandpass amplifiers the power supplied to 

 the stages must not be derived from a common-source impedance, since 

 instability will result. Fig. 7-10 shows a typical arrangement of IF stages. 

 The power leads are brought into the amplifiers near the output. De- 

 coupling filter elements CiCiC^RiRiLi Li are employed. The decoupling 

 is designed so that a single stage will exhibit adequate gain and phase 

 margin over the entire frequency spectrum when the stage is examined as 

 a feedback amplifier. In particular the stability margin must be realized 

 when the tubes operate at the peak transconductance values that would be 

 produced by a saturating signal. 



Time domain effects must also be considered. Saturating signals cause 

 the d-c currents to the various tube elements to vary. The cathode circuits 

 will attempt to degenerate the effects of a saturating signal during the time 

 that the signal exists. When the signal input ceases the cathode capacitor is 

 charged to the value which has reduced the gain during the signal on time. 



