In electronics and telecommunications, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way, the intersymbol interference caused by the channel can be kept in control. In RF communication, pulse shaping is essential for making the signal fit in its frequency band.
The diagram shows how the four gates contained in a 4077 may be used to build a circuit that doubles the frequency of a signal applied to it. In other words, it generates a pulse for every edge of the input signal; the pulse width is determined by the internal delay of gates IC-1a – IC-1c. To that end, the original signal at pin 12 is compared with the delayed signal at pin 12. Because of the XNOR function of IC-1d, any level difference between the two is translated into a level change at the output of IC-1d.
The quiescent output level at pin 11 is set by connecting the ± input of 1C-1a, to the ground or to the +ve supply rail. If it is taken to the ground, 1C-1d delivers a zero level followed by positive pulses for each of the edges of the input signal. If it is linked to the +ve supply rail, IC-1d outputs a high level followed by negative pulses for each of the edges of the input signal.
Instead of a 4077, a 4030 or a 4070 may be used, in spite of the fact that these have XOR instead of XNOR gates. Only the pulse width of the output signal will be slightly different owing to the changed transfer times in the gates. The current drawn by the circuit depends on the signal frequency: at very low frequencies, it is virtually nil.
