Differential interfaces Improves performance in RF transceiver

Published:2012/3/21 2:53:00 Author:Ecco From:SeekIC

The traditional transceivers use 50-Ω single-ended interfaces. When they are used in RF and IF circuits, the 50-Ω output and input impedances should be matching. However, the modern transceivers use differential interfaces to get better performance in IF circuits, but they require designers to solve several common issues, including impedance matching, common-mode voltage matching, and difficult gain calculations.

The interfacing solutions are used for three cases: a ZIF receiver, a superheterodyne receiver, and a transmitter. These three architectures are widely used in wireless remote radio units (RRU), digital repeaters, and other wireless instruments. Differential interfaces have three main advantages. First, the external interference and ground noise could be suppressed. Second, they suppress even-order output distortion components. Third, the output voltage can be twice that of single-ended output, thus improving output linearity by 6 dB on a given power supply.

Typical demodulators may provide optimum performance when driving 200-Ω to 450-Ω loads, and analog to digital converter (ADC) drivers generally have input impedance other than 50-Ω, so interfaces systems with dc-coupled circuits is both critical and difficult. Figure 1 shows a ZIF receiver configuration using two ADL5523 low-noise amplifiers (LNAs), an ADF4350 wideband synthesizer as a local oscillator (LO), an ADL5380 400-MHz to 6000-MHz quadrature I/Q demodulator and an AD8366 two-channel digitally programmable variable-gain amplifier (VGA). Table 1 shows the relevant ADL5380 interface and gain parameters.

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