This paper reports the simulation, performance, and detailed characterization of a low-noise heterodyne module with very wide intermediate frequency (IF) bandwidth in the 385–500-GHz radio frequency (RF) range. The module integrates a superconductor–insulator–superconductor (SIS) mixer with a 3–21-GHz cryogenic low-noise preamplifier (CLNA). The utilization of high current density SIS junctions offers wide RF bandwidth and facilitates better matching between SIS junctions and the CLNA. Based on an equivalent circuit of the SIS mixer chip, we calculate the IF output impedance and simulate how different matching conditions affect the performance of the CLNA. Simulations predict that low-noise module performance can be obtained over a 3–18-GHz IF range, limited mainly by parasitic elements in the SIS mixer chip. The measurement results of the heterodyne module demonstrate flat gain and a typical noise temperature of 70–80 K over the 3–18-GHz IF range, at local oscillator frequencies of 400–480 GHz. Mixer chip output impedance and CLNA input impedance have been directly measured by recording S -parameters at cryogenic temperatures. The results enabled us to quantify the contribution of various parasitic elements in the mixer IF circuitry. Calculations based on our model are in good agreement with measurements.