Abstract

Communication is an integral part of people’s daily life, and its demand will never cease. After multiple generations of communication system improvement, broadband wireless communication has become a conspicuous development trend but the congested spectrum has turned into one of the system bottlenecks. Therefore, shifting into higher frequency bands, that is, wavelengths of millimeter scale would be a solution to suffice the escalating consumer demand, and Radio-over-Fiber (RoF) is the key for successful system deployment. Under RoF structure, Radio Frequency (RF) signals can be directly distributed from central station to base stations via optical fiber, as a result, size of base station can be implemented into a palm-size package, and more importantly, lower unit cost of base stations crucial due to high volume use.
In this work, we started with the design of an optical receiver as the first step of transceiver integration, and targeted at 40 GHz or above. Different from the widespread digital optical receiver, optical nature of RoF transmission is analog signal, and consequently its receiver demands higher qualification standards. Noise, intermodulation distortion, nonlinearities and other aspects are all required to be validated.
Putting the cost factor into consideration, we used Miniature Hybrid Microwave Integrated Circuit (MHMIC) technology to implement our analog optical receiver. Design and simulation of the 40 GHz receiver was mainly carried out by Agilent Advanced Design System (ADS), and the bondwire interconnection is identified as a major potential bandwidth degradation factor of the receiver.
After the circuit fabrication, the S-parameter results showed the receiver bandwidth is limited to 30 GHz due to certain fabrication error caused by bondwires. The bandwidth evaluation is further verified from Error Vector Magnitude (EVM) results by transmitting Ultra-wideband (UWB) signal centered at 30.31 GHz through a 20 KM long optical fiber. In back-to-back characterization of the receiver, the 1-dB compression point is found as 11.7 dBm (referred to input) and the SFDR based on two sets of two-tone frequencies (4 GHz with 6 GHz, and 13 GHz with 14 GHz) is 107.45 dB/Hz2/3. Responsivity of the receiver is 0.325 A/W at 1550 nm.