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.