Mixer Circuits and PSPICE - Laboratory 4 | ENTC 4307, Lab Reports of Engineering

Download Mixer Circuits and PSPICE - Laboratory 4 | ENTC 4307 and more Lab Reports Engineering in PDF only on Docsity! LAB  ENTC 4307 Mixer Circuits & PSPICE OBJECTIVE: To simulate a nonlinear mixer circuit using PSPICE. THEORY: To relocate a signal to another frequency requires the signal to be remodulated. The incoming RF signal in a superhetrodyne receiver is translated down to an intermediate frequency (IF). One way of doing this is to use a non-linear device such as a diode or transistor biased in the non-linear region (square-law). If we consider the characteristic of a typical diode and concentrate on the non-linear region, the current and voltage are related by the quadratic relationship  32 INININ aVaVaVi If the voltage Vd is the sum of the modulating voltage and the carrier voltage Vmcosmt + Vccosct. By substituting this into the diode equation it can be seen that the squared term will produce the cosAcosB, which when expanded, produces sum and difference frequencies. Other terms are also produced which may be filtered out. Two circuits that can produce this are the diode switching type mixer and the transfer characteristics of a JFET. For the switching type mixer, VL switches the diodes on and off. Vs modifies currents of the diode. The output consists of VL plus Vs switched by 180 at the frequency of the local oscillator, VL. The switched form of Vs can be denoted as Vs*. Then        01 01 VL VL tp tpVsVswhereVsVLVo )( )(*** Notice that the exact shape of VL is not important. That is, it can be rectangular (which fequently happens) as well as sinusoidal. Now a square wave can be represented as the odd harmonics (Fourier series):        tttp LL   3 3 14 sinsin)(