Improvement of the phase rejection of the image channel in radio-frequency mixers

Operation of the classical scheme of the mixer with phase rejection of the image channel is analyzed. It is shown that addition of controlled phase and amplitude correctors that operate in the intermediate-frequency (IF) range between low-pass filters and the quadrature equal-arm adder at the IF enables fine adjustment of the mixer and implements minimum phase and amplitude imbalances, which leads to a substantial increase in the rejection level of the image channel, down to −40 dB and lower. This result was backed by measuring the parameters of a mixer that was manufactured for operation in a frequency band of 135–175 MHz with 25-MHz IF. Since phases and amplitudes are corrected at the IF, the proposed circuit becomes universal, i.e., it is independent of the range of converted frequencies.