Abstract: An offset frequency converter for the intermediate frequency (IF) stages of a multi-channel radio frequency (RF) phase/amplitude measurement receiver, particularly useful for measuring the performance characteristics of antennas. The converter allows use of noncoherent transmitter and receiver local oscillator sources which utilize digital frequency synthesizers and enables harmonic mixing to be employed when the tuning resolution of the receiver local oscillator is incremental. The converter samples an incoming first intermediate frequency signal IF.sub.1 in a reference channel, which typically is obtained by mixing an LO source in the receiver with an incoming signal from a reference antenna. The IF.sub.1 sample is then upconverted with an upconversion signal to a higher frequency, filtered, and the downconverted with a downconversion signal to an IF.sub.1 +IF.sub.2 signal offset from the IF.sub.1 sample by a lower second intermediate frequency IF.sub.2. The added frequency offset IF.sub.