Microwave Tech - Choose the right amplifier for active antenna design

The automotive industry is transitioning to the use of remote and shark fin antenna modules on a large scale to achieve unified ground and satellite communications. Due to the compact antenna structure and the remote location of the radio unit, the shark fin module requires high-performance, highly integrated and low-noise amplifiers (LNAs) to optimize the antenna performance. Before the popularity of shark fin antenna, the mainstream technology was glass antenna (planar antenna structure printed on window glass). The glass antenna will still be widely used, usually in the rear window or side window of the car. Therefore, these antennas, like shark fin antennas, are located at the far end of the radio unit and usually use local LNA to improve performance. Because LNA is used in the design of shark fin and glass antenna, active antenna has become a very popular technology in modern vehicles.

Remote antenna problem

When the antenna is located at the far end, it will have different effects on the performance according to the specific application. In FM band, the antenna is usually matched with 50 Ω or 75 Ω impedance RF cable to support power transmission. However, the noise figure increases in proportion to the loss of the cable between the antenna and the receiver. For longer cables, the added value of noise figure may exceed 1dB, resulting in the same degree of sensitivity reduction. Placing the LNA between the antenna and the cable can greatly reduce this impact.

In the AM band, the influence of the far end position of the antenna on the performance is different, although the final result is to reduce the sensitivity. The source impedance of typical AM antenna is very high, which is often modeled as serial capacitance. The capacitance value is between 3pf and 100pF. The specific capacitance value is related to the structure. The parallel parasitic capacitance in the cable connecting the antenna and the receiver forms a capacitive voltage divider with the source capacitance. The parallel parasitic capacitance of longer cables may be as high as 100pF, which may greatly attenuate the signal. Placing LNA with high impedance input and low impedance output between antenna and cable can improve signal transmission performance. In the AM and FM operating frequency bands, increasing the signal level at the antenna through the remote LNA can greatly reduce the sensitivity to the environmental noise picked up by the cable and make the radio scheme more reliable.

Common active antenna schemes

The performance and functional requirements of active antennas depend on the specific application. Some active antenna schemes require automatic gain control (AGC), while others use fixed gain LNA to obtain cost; Some schemes provide a regulated power supply voltage for active antennas, but most still work with batteries; Some designs require particularly high gain, while others may be particularly sensitive to AGC threshold. Therefore, the challenge of antenna power supply is how to meet a variety of industry requirements without redesigning the split scheme or using expensive IC (external active and passive components are still required).

A few manufacturers provide integrated AM / FM solutions for active antennas. Unfortunately, these often require external pin diodes and regulated power supplies for AGC. If batteries are used, external transmission transistors are required (Fig. 1).Choose the right amplifier for active antenna design

The external components increase the cost and increase the size of the scheme, and its competitiveness is slightly better than the split design. In addition to the large physical size, another disadvantage is that if the required gain, supply voltage or overall size changes, the circuit board may need to be redesigned. This requires more design resources that are already in shortage. In the case of limited resources and space, the ideal solution for antenna suppliers must be high-performance, low-cost and very flexible IC, and can easily meet various requirements without redesign, BOM change or circuit board change.

Ideal active antenna scheme

Max2180a is an example of this kind of active antenna design scheme. It is an AM / FM low noise amplifier with optional application configuration. Max2180a adopts exclusive high-voltage CMOS process, integrates am, FM, AGC and high-voltage regulator. The max2180a eliminates external pin diodes and external regulators or transmission transistors, reducing the overall size of the antenna scheme. Max2180a adopts highly integrated, 4mm x 4mm, TQFP package, which is easy to fit the antenna module.

The AM and FM signal paths include adjustable gain and AGC threshold to improve flexibility and support seamless connection of downlink tuners. These gain values and AGC trigger points can be set through external pins to support rapid implementation of different configurations on the same PC board (Figure 2).

In addition, the max2180a supports an operating voltage as low as 6V (Figure 3), making it ideal for vehicles with start stop technology. At this time, there is a large voltage transient drop when the engine starts. Previously, voltage sag was an insignificant problem, because once the car was started, the electrical system would maintain a relatively constant 14V supply voltage. If the new start stop technology is used, the engine will shut down automatically when the car does not move. The engine must be restarted frequently, and the radio unit must work normally during repeated startup and device power supply voltage transient drop.

This paper focuses on the challenges faced by the rapid deployment of active antennas in automotive applications under the environment of continuous reduction of area and diversified requirements. An ideal solution to meet such challenges is introduced: integrating all active and multiple passive components in the solution. The scheme uses the selection pin to set the main internal parameters, which improves the flexibility, and the working voltage supports the vehicle with new start stop technology.