Selecting High-Linearity Mixers for Wireless Base Stations

Today’s communication systems make strong demands on both receiver sensitivity and large-signal performance (dynamic range). When selecting components for a new design, therefore, it behooves the designer to focus on the performance of each one with regard to these requirements. In particular, this article deals with mixers and the basic parameters specified in their data sheets. The communication standards for wireless base stations—e.g., GSM, UMTS, and (now) LTE, define minimum specifications for various parameters, including receiver sensitivity and performance in the presence of large signals. These key requirements make heavy demands on every functional block of the radio in a wireless base station, and are affected by every single component. In the receive signal path, the mixer performance has a major impact on the receiver’s overall sensitivity and large signal performance, so by understanding mixer performance issues and parameters, you can select the best mixer for your receive channel. To start, we first analyze the block diagram of a typical receiver used in wireless base stations (Figure 1). Such receivers are referred to as dual-conversion superheterodyne receivers, because the received signal undergoes two consecutive down-conversions from the operating frequency to lower IF frequencies. As shown, the signal is received by the antenna and then filtered by RF filter #1, which is normally used to reject out-of-band “trash” signals that can cause overload or interference. This filter output is then amplified by a low-noise amplifier (LNA), which normally has a very low noise figure. The amplified signal is again filtered, this time by RF filter #2, which limits the frequency range while removing any remaining unwanted out-of-band signals that can limit the mixer’s performance. The amplified and band-limited signal is then fed to the first mixer, where it is down-converted to an IF frequency by mixing with a local oscillator signal. This article discusses the key parameters of mixer performance to assist in the selection of an appropriate device for wireless base station design