A Compact W-Band Fundamental Mixer with Wide IF Bandwidth Based on a Double-Sided Coupled Substrate Structure

A wide-IF mixer serves as a key component in millimeter-wave and terahertz integrated sensing and communication (ISAC) systems, bridging the high-frequency front-end and the baseband processing module. Its primary function is to convert high-frequency RF signals into intermediate or low frequencies to meet the processing requirements of subsequent communication and sensing modules. The fundamental mixer directly combines the RF signal with the LO fundamental frequency, achieving lower conversion loss and cleaner IF output, making it particularly suitable for millimeter-wave and terahertz ISAC front ends. However, conventional fundamental mixer circuits often exhibit power nulls within the IF band, resulting in a limited IF bandwidth typically below 30 GHz. To address this limitation, this work proposes a compact W-band wide-IF fundamental mixer based on a double-sided coupled substrate structure. In the LO–IF diplexer circuit, a face-coupled substrate configuration is employed. By adjusting the length and width of resonant units on both sides of the suspended microstrip line, the coupling capacitance can be tuned to shift the IF power null toward higher frequencies, effectively broadening the IF bandwidth while enabling compact integration. Measurement results demonstrate that, under a fixed LO frequency of 70 GHz and an input power of 15 dBm, the proposed fundamental mixer achieves a conversion loss of 5.6 dB–10.1 dB. Compared with other mixers operating in the same frequency band, it offers an extended IF bandwidth of up to 40 GHz while reducing the overall size by 40.5%.