PSPICE is a general circuit analysis program for microcomputers developed from SPICE (Simulation Program with Integrated Circuit Emphasis). It was developed using the FORTRAN language and is mainly used for computer-aided design of large-scale integrated circuits.

Designers use PSpice primarily to simulate analog circuits. It can also be used to simulate digital filters. The main components in a digital filter are delay elements, adders, and multipliers. Adders and multipliers can be implemented with operational amplifiers, and delay elements can be simulated with a transmission line. The transmission line in PSpice is a long-forgotten component that can achieve a delay of several seconds.

For example, Figure 1 shows a second-order regression digital filter. The transfer function of this filter is:

Where H(z) is the transfer function of the digital filter, z is the z-transform variable, Ai is the coefficient of the polynomial in the denominator of the transfer function, and Bi is the coefficient of the polynomial in the numerator of the transfer function. These coefficients can be obtained using filter design software (Reference 1). The relationship between the sampling frequency fS and the transmission line delay is t=1/fS. For example, a bandpass digital filter has a 3dB passband from 900Hz to 1kHz and a sampling frequency of 6kHz. The Butterworth characteristic analysis can obtain the following transfer function:

At this time, the delay of the transmission line is 1/6000 = 166.67ms. If an additional 1Ω impedance Z is set for the transmission line, the parameters of the transmission line are Z0 = 1Ω and t = 166.67ms. Figure 2 is a PSpice circuit. VCVS (voltage-controlled voltage source) E1 and E2 simulate voltage followers, while VCVS E3 and E4 and the resistors connected to them simulate adders. Figure 3 is the simulation result.