*3.3. Star Mixer*

The passive star mixer has been widely used in mm-wave on-chip systems since being proposed by Basu and Maas [17]. Its highly symmetrical star topology can offer better port-to-port isolation and high spurious rejection [18,19]. Different from the traditional

star mixer, the traditional straight symmetrical Marchand Balun is modified to S type with two bends, which can reduce the chip size while ensuring the performance of the mixer, as shown in Figure 6. To reduce the coupling between transmission lines, the decoupling ground wall consisting of metal-vias array is inserted between four S type balun and the IF port. Additionally, double side coupling lines of the Marchand Balun are grounded separately in the end, rather than joint together by the bottom metal layer for better spurious rejection. Four diodes measuring 1 × 15 μm are arranged in a symmetrical layout for broad IF bandwidth. In addition, the LO input matching network is co-optimized with a pre-stage HPF output network. Furthermore, an extra BPF with the same topology as the one after the tripler is integrated after the star mixer. Finally, the micrograph of the complete channel emulator is shown in Figure 7, with a chip size of 2.7 by 0.9 mm2.

**Figure 6.** Schematic of the modified star mixer.

**Figure 7.** Chip micrograph of the channel emulator.

Simulation results of the mixer with the following BPF show that both the input and output return loss is better than −10 dB, and the conversion loss of the mixer is about 6.5 dB from 66 to 76 GHz. Furthermore, the insertion loss of the BPF after the mixer is between −1.4 and −2 dB within the RF operating bandwidth.
