**1. Introduction**

6G is a new generation of mobile communication system developed for the needs of future mobile communication. According to the law of mobile communication development, 6G will have ultra-high spectrum utilization and energy efficiency, and will be an order of magnitude or higher more efficient than 5G in terms of transmission rate and spectrum utilization. Its wireless coverage performance, transmission delay, system security, and user experience will also be significantly improved [1]. Traditional mobile communications mainly use the frequency band below 6 GHz, which has become increasingly saturated. With the huge demand of 6G for system capacity, in some 6G applications, spectrum resources of several GHz may be required to meet specific transmission requirements. Due to the abundance of spectrum resources in the millimeter-wave (mm-wave) band, the World Radio Conference (WRC-19) has approved multiple mm-wave spectra for future mobile communications research and development, including the 66–67 GHz frequency range [2].

Different from frequency bands below 6G, the mm-wave band has poor penetration ability and large path loss. In order to effectively increase the propagation distance of the link, an effective solution is to adopt a large-scale multiple-input multiple-output (MIMO) system architecture [3]. In a MIMO system architecture, the number of antennas is greatly increased, and the systematic conductive testing becomes more impractical due to the influence of the long calibration time [4]. Moreover, the cost of testing is expensive, and the complexity of the test system increases exponentially. Therefore, multi-probe over-the-air (OTA) testing in millimeter-wave shielded anechoic chambers is becoming the mainstream testing solution [5–7].

**Citation:** Zhou, P.; Wang, C.; Sun, J.; Chen, Z.; Chen, J.; Hong, W. A 66–76 GHz Wide Dynamic Range GaAs Transceiver for Channel Emulator Application. *Micromachines* **2022**, *13*, 809. https://doi.org/10.3390/ mi13050809

Academic Editors: Lu Zhang, Xiaodan Pang and Prakash Pitchappa

Received: 31 March 2022 Accepted: 7 May 2022 Published: 23 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

As a vital part of the mm-wave OTA test system, a wireless channel emulator can accurately simulate and measure the wireless environment's degradation of system performance, including the free space path loss, shadowing, and multi-path fading of the transmitted signals from the antenna ports [4]. Transceivers in the channel emulator send/receive up/down mm-wave signals to/from OTA probe antennas. The channel emulator is an important high-end general-purpose instrument for verifying the performance of system equipment and terminal equipment in various complex channel environments. However, due to high demands on dynamic range, compactness, cost, and power consumption, the mm-wave channel emulators are rarely seen, especially when operating at frequencies above 50 GHz. At present, the known channel emulator with the highest operating frequency is PROPSIM released by Keysight, which can support up to 43.5 GHz [8].

This paper presents a 66–67 GHz transceiver monolithic microwave integrated circuit (MMIC) in waveguide module for massive MIMO channel emulator application. The proposed transceiver is integrated by cascading a tripler chain for LO drive, a mixer, and a band-pass filter using a 0.1 μm pHEMT GaAs process.

A high dynamic output power range, up to 50 dB over 66-to-76 GHz, is achieved by carefully dealing with all unwanted harmonic signals employing highly selective band-pass and high-pass filters in the transceiver. Total power consumption of the chip is 645 mW with the supply voltage of 5 V. A low-loss ridged waveguide ladder transition was designed so that the output interface of the module is a WR12 waveguide interface, which is convenient for direct connection with an instrument with E-band waveguide interface. To the best of our knowledge, the proposed integrated module is a competitive E-band transceiver system for radio channel emulation application.
