In traditional transceiver designs, 50-Ω single-ended interfaces are widely used in RF and IF circuits. When circuits are interconnected, they should all see matching 50-Ω output and input impedances. In modern transceiver designs, however, differential interfaces are frequently used to obtain better performance in IF circuits, but implementing them requires designers to confront several common issues, including impedance matching, common-mode voltage matching, and difficult gain calculations. An understanding of differential circuits in transmitters and receivers is helpful for optimizing gain matching and system performance.Differential Interface Advantage
Differential interfacing has three main advantages. First, differential interfacing can suppress external interference and ground noise. Second, even-order output distortion components can be suppressed. This is very important with zero-intermediate-frequency (ZIF) receivers because even-order components appearing in the low-frequency signal cannot be filtered out. Third, the output voltage can be twice that of single-ended output, thus improving output linearity by 6 dB on a given power supply.
This article discusses interfacing solutions 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.
ZIF Receiver Interface Design and Gain Calculation
In zero-IF (ZIF) receiver designs, the IF signal is complex, with dc and very low frequency signals providing useful information. 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 interfacing 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 ADL5380 400-MHz to 6000-MHz quadrature I/Q demodulator, an ADF4350 wideband synthesizer as a local oscillator (LO), and an AD8366 two-channel digitally programmable variable-gain amplifier (VGA). Table 1 shows the relevant ADL5380 interface and gain parameters.
Figure 1. ZIF receiver block diagram.