A Wideband Subharmonic Mixer With Low Conversion Loss and High Port-to-Port Isolations

A design of a diode based wideband subharmonic mixer (SHM) with low conversion loss and high port-to-port isolations is reported. To achieve low conversion loss, for a given diode, the radio frequency (RF) and intermediate frequency (IF) signals at the respective port should remain unattenuated over the frequency range of interest. At the same time, it has to reject unwanted signals over a wide bandwidth. The excellent wideband bandstop filtering behavior of signal interference technique is exploited for this purpose. The mixer is designed to operate over the RF band 8.5-11.5 GHz. The local oscillator (LO) frequency is tuned to keep IF fixed at 200 MHz. Measured minimum conversion loss is obtained as 6.05 dB while the theoretical limit of conversion loss for the chosen diode is 4.68 dB. Measured LO-to-RF and 2LO-to-RF isolations are better than −30 dB (maximum −50 dB) and −59 dB (maximum −73 dB), respectively, over the entire operational band. Large signal measurements show that the 1-dB compression point is −1 dBm and the third order intercept point is 10 dBm. Simplicity in circuit topology is another significant feature.