A Wideband Subharmonic Image-Reject Mixer With a Fixed IF Using Schottky Diodes

This work presents a Schottky anti-parallel diode pair (APDP)-based wideband subharmonic image-rejecting mixer (SH-IRM) with high image rejection and port-to-port isolation. Although the RF can bevaried over a wide range, the intermediate frequency (IF) remains fixed. The RF and image frequencies overlap in a wideband SH-IRM. Therefore, a fixed bandstop filter (BSF) cannot be used for image suppression in the RF path. The novelty lies in a single-varactor-based tunable wideband BSF implemented in the RF path, capable of suppressing image frequencies according to requirements in the overlapping RF-Image band. A tunable BSF using a single varactor diode having a 77.45% tuning range is used in the RF path for the image rejection. A wideband band pass filter (BPF) with over 70% FBW is synthesized for sharp roll-off and at least 35 dB rejection over the LO bands. A wideband RF matching networks with 64.2% FBW is also implemented. The co-design approach is used to obtain the filtering and IF matching using a single block. The SH-IRM implemented in PCB technology operates over 4.76-9.26 GHz RF band with a fixed IF of 600 MHz when the LO varies over 2.08-4.33 GHz with an optimum power of 8.5 dBm. Measurement results show an average conversion loss of 12.2 dB, at least 25 dB image rejection, LO-to-RF isolation better than 30 dB, and 2LO-to-RF isolation better than 52 dB over the entire band.