Instead of employing a tunable bandpass filter that is shifted to the signal frequency, the concept of a heterodyne Rx is to design a tunable Local Oscillator (LO) operating at $F_{\text{LO}}$ that moves the signal to a fixed bandpass filter, as drawn in the figure below. At the Rx side, $S(F)$ is mixed with a tunable LO with a sinusoid $\cos 2\pi F_{\text{LO}} t$ at frequency $F_{\text{LO}}$. In any case, the requirements on an IR filter are much relaxer than the TRF Rx since its only purpose is to filter out the image signal (as opposed to filtering out everything around the desired band) and the image frequencies lie away from the center frequency.