Tunable balance network could replace proliferating SAW filters

February 22, 2016 // By Graham Prophet
LTE handsets must operate on an ever-growing list of frequency bands, and require a corresponding matrix of of SAW (surface-acoustic-wave) filters to provide matching, band selection and T/R switching isolation. Belgian nanoelectronics research centre imec and Vrije Universiteit Brussel (VUB) are proposing an alternative of a frequency division duplex (FDD) balance network, capable of dual-frequency impedance tuning for all LTE bands in the 0.7-to-1GHz range.

When integrated into an electrical-balance duplexer (EBD), the balance network enables FDD duplexing with antennas in real-world environments, paving the way to high-performance, low-power, low-cost solutions for mobile communication. The researchers say that they have built the first such Electrical Balance Duplexers supporting all LTE Bands from 0.7-1GHz.

An electrical balance duplexer is a tunable RF front-end concept that seeks to address several key challenges of 4G and 5G mobile systems. It balances an on-chip tunable impedance – the so-called balance network – with the antenna impedance, to provide transmit-to-receive (TX-to-RX) isolation and avoid unwanted frequency components in the received signal. It is a promising alternative to the fixed frequency surface-acoustic wave (SAW) filters implemented in today’s mobile phones as more and more SAW duplexers would be needed to support the ever growing amount of bands adopted by operators, increasing size and cost of these devices. Unlike filter-based front-ends, electrical-balance duplexers provide signal cancellation, which could help enable in-band full-duplex for double capacity and increased network density, among other benefits, for next-generation standards.

Imec and VUB’s dual-frequency balance network is the first FDD balance network that allows balancing the on-chip tunable impedance profile with the impedance profile of an antenna at two frequencies, simultaneously. This is crucial, because in real-world situations, the frequency-dependent impedance of an antenna varies over environmental conditions and limits the achievable isolation bandwidth. The balance network can generate, for any LTE band within 0.7-1GHz, a simultaneous transmit-frequency impedance and receive-frequency impedance to provide high Tx-to-Rx isolation at both frequencies. It is fabricated in a 0.18 µm partially depleted RF SOI CMOS technology, which allows it to better withstand the large voltages present in the EBD during full-power Tx operation. The active area of the balance network, which consists of 19 switched capacitors and 10 inductors, is 8.28 mm ². The balance network is tuned by an in-house developed custom algorithm, which can optimise the tuning