Designing an AM/FM Active Antenna LNA Solution Using the MAX2180 LNA

This article is a reference design (RD) for an automotive AM/FM active antenna. This RD demonstrates the flexibility of the MAX2180 active antenna low-noise amplifier (LNA) and describes how to set the AM and FM gain and automatic gain control. Single and dual antenna mechanisms are described in detail, including input and output matching circuits. Using this design in conjunction with the data sheet and device evaluation kit (EV), it is easy to develop an antenna topology that can meet a variety of active antenna needs.

Introduction
This application note presents a reference design (RD) for an AM/FM automotive antenna. The design features the MAX2180, a highly integrated AM/FM low-noise amplifier (LNA) for active antenna modules. The LNA incorporates Maxim's automatic gain control technology with user-selectable set points on both the AM and FM signal paths. The maximum gain of both the AM and FM signal paths can also be varied to meet a wide range of user requirements. The RD details the flexibility and performance of this integrated automotive solution.

Overview
Automotive antennas require smaller, more integrated solutions while maintaining the high performance required by modern AM/FM wireless communication devices. Some solutions require automatic gain control (AGC), while others use fixed gain LNAs to minimize cost. Some solutions provide a regulated supply voltage for active antennas, but most are still battery powered. The challenge for antenna solution providers is how to meet the diverse industry requirements without using discrete solutions that require repeated design or expensive ICs (which still require external PIN diodes and regulators). With limited resources and space, the ideal solution for antenna suppliers must be a high-performance, low-cost, and very flexible IC that can easily meet various requirements without redesign, BOM changes, and board changes.

There are a few vendors that offer integrated AM/FM solutions for active antennas. Unfortunately, they require external PIN diodes to provide AGC. These solutions also require a regulated power supply, or an external pass transistor when powered by batteries. External components increase the cost and footprint of the solution. If AGC is not required, the solution is usually discrete to achieve the lowest cost. The problem with discrete designs is that any change in gain, supply voltage, or footprint will result in a redesign, which will take up more design resources, which are very scarce in most cases.

Optimal Antenna Solution and MAX2180
Maxim Integrated Products has developed an AM/FM antenna solution that integrates all active components to meet today's demanding automotive antenna requirements. The antenna uses the MAX2180 LNA. The MAX2180 uses an internal high-voltage CMOS process to integrate the AM and FM AGCs, as well as high-voltage regulators, in a small 4mm x 4mm TQFP package. This design avoids all PIN diodes and external voltage regulation circuits or pass transistors, while operating from a battery or regulated supply. The MAX2180 achieves maximum AM and FM gain with variable AGC set points. It also includes antenna monitoring and draws 15mA under fault conditions.

The LNA’s on-chip voltage regulator operates from 7V to 24V. To prevent thermal damage, an integrated temperature sensor limits the maximum junction temperature by folding back the current. This ensures that the amplifier remains operational regardless of environmental conditions.

The AM input is high impedance and the output is low impedance, and the FM amplifier provides 50Ω input and output impedance. The maximum AM gain can be set from 0dB to 6dB by changing the external resistor. The maximum FM gain can be varied from 5.8dB to 8.5dB (R1 = 0Ω). To improve the noise figure, R1 should be 390Ω, which increases the gain range to 10.0dB to 10.8dB. Both signal paths use Maxim's patented AGC circuit with a gain control range of 30dB. In addition, the AGC set point is variable to provide the desired maximum output level for the host.

When designing with the MAX2180, the design can be simplified by selecting the desired signal path gain and AGC setpoints using the tables in the data sheet. This custom range of parameter values ​​allows one design to meet multiple requirements without re-layout of the board. As shown in Figure 1, the MAX2180 provides a higher level of integration than competing solutions while providing the flexibility to meet a variety of requirements. Figure 2 shows the application schematic for a single antenna solution.

Figure 1. Comparison of the MAX2180 highly integrated solution (A) and competing AM/FM active antenna solutions (B).


Figure 2. Application schematic of a single-antenna solution using the MAX2180.