**4. Packaging Methodology**

As an important part of the OTA test system, the channel emulator module is directly connected to the RF transceiver of the communication system to simulate path loss, multipath fading, and shadow fading in the wireless environment. Therefore, the input and output interface of the channel emulator should have better robustness so that it can be directly connected with the transceiver or instrument. In mm-wave low-frequency bands (below 30 GHz), interfaces such as instrument or RF transceiver outputs typically use coaxial connectors [20]. When the frequency is higher than 50 GHz, the instrument interface is usually designed as a waveguide interface for more favorable stability and cost.

This channel emulator is aimed at 66–67 GHz wireless communication system test applications, and the RF frequency range is 66–67 GHz. The IF port is compatible with the existing commercial 5G mm-wave band, the IF frequency is 27 GHz, and the IF power coverage range is −40~10 dBm. The LO input frequency range is 13~16.33 GHz. Therefore, the LO and the IF port use a coaxial connection scheme, and the RF port uses a WR12 waveguide interface. The frequency of the IF and LO ports is lower, and the influence of the gold bonding wire is small. In the module design, the RF and IF signal interfaces on the chip are directly bonded to the PCB through gold wires, and are connected to the coaxial connector through the 50 ohm characteristic impedance microstrip line on the PCB.

The working frequency of the RF port is relatively high. In order to transfer the RF port to the WR12 waveguide interface, firstly, the RF GSG PAD-to-50ohm microstrip line (as shown in Figure 8) was designed on the TLY-5 sheet with a dielectric constant of 2.2 and a thickness of 0.254 mm using bonding wires. The distance from the microstrip line to the edge of the RF GSG PAD, the height of the gold bonding wire, and the structure of the microstrip line were all optimized by electromagnetic simulation [21]. Then, a low-loss microstrip to WR12 ridged waveguide ladder transition (RWLT) structure (as shown in Figure 9) was designed using aluminum metal [22,23]. The transition structure includes a 4-stage stepped impedance transformation. By optimizing the length and height of each stepped transformation unit, a lower transition loss from the microstrip to WR12 can be obtained, and the connection loss in the 66–67 GHz frequency band is lower than 0.36 dB.

**Figure 8.** 3D model of the RF GSG PAD to microstrip transition.

Figure 10 shows the S-parameters of the transition structure between the simulated RF GSG PAD and the microstrip line and the microstrip line to the WR12 waveguide port in the entire E-band. As illustrated in Figure 10, in the channel emulator operating frequency band 66–67 GHz, the return losses (S11, S22) of the above connection structures are all better than −10 dB, and the total insertion loss between the RF GSG PAD and the WR12 waveguide port is equal to less than 2 dB.

**Figure 9.** Ridge waveguide ladder transition from WR12 waveguide interface to 50-ohm microstrip line.

**Figure 10.** The simulated connection loss of the RF GSG PAD to the microstrip line and the microstrip line to the WR12 waveguide port.

The photograph of the opened split-block and assembled module including the RF transceiver chip, RF microstrip-to-waveguide transition, and the DC biasing network is shown in Figure 11. The PCB was sintered on the aluminum structure to obtain a tighter and better grounding effect. The RF transceiver chip was installed in a groove dug in the middle of the PCB. The depth of the groove was slightly higher than the height of the chip to ensure that the top surface of the chip (after the conductive adhesive is pasted) was the same height as the PCB surface. In the design of the DC bias network, we first placed some decoupling chip capacitors next to the chip to obtain better DC bias characteristics. The DC voltage of the amplifier's drain, gate, etc. is provided through the LDO DC biasing network, and the entire module has only a 5V DC input voltage. On the side of the module, two 2.92-mm coaxial connectors were used as LO and IF ports, and the RF port is a WR12 waveguide interface.

**Figure 11.** Photo of the 66–67 GHz channel emulator module integrating the RF transceiver chip, the microstrip-to-waveguide transition structure, and the DC biasing network.
