Antenna design for outstanding antenna amplifiers

Car radio antennas are becoming a standard component in automotive design. Atmel's antenna amplifier chip specifically addresses this challenging small and micro antenna design trend while maintaining the reception of car radios.

The development prospects of modern car antennas have raised some technical requirements and constraints that are more difficult to deal with. Among them, the most important and most challenging requirement is to reduce the size of the antenna; secondly, the length of the connecting cable is long (especially in the SUV (Sport Utility Vehicle) sports type). Other requirements include multi-antenna design and a variety of design issues for different antenna types (mounted on windshield, roof or front bumper).

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Antenna physics knowledge

The traditional AM/FM frequency range has been extended and AM broadcasts have now been extended to 30 MHz to enable car radios to receive DRM (World Digital Broadcasting) signals. The frequency range of the FM broadcast starts at the 78 MHz Japan band and ends at the band III with a maximum frequency of 240 MHz, which can be used to receive DAB signals in Europe.

Since AM broadcasts use vertically polarized waves to transmit signals, the car antenna should also be polarized in the vertical direction. In FM radio, most of the transmitted signals are polarized in the horizontal direction; this seems to conflict with the AM receiving requirements.

Fortunately, the metal body of a car can change the electromagnetic field, so the vertical antenna required by AM broadcasting can also be used for FM broadcasting. Figure 1 shows the effect of the vehicle body on the electromagnetic field; it can be seen that there is only a vertical electromagnetic field component at the horizontal plane of the vehicle body (for example, the roof or the trunk). The horizontal component is curved in the vertical direction.

In addition, the opening formed by the window causes the metal surface of the vehicle body to be discontinuous, so that the field strength is increased. This discontinuous surface structure determines the most suitable mounting position for the car antenna. The middle of the roof is usually not the best place to install the antenna.

Since the physical structure of the vehicle body has a reinforcing effect on the electromagnetic field, it is feasible to use a small dipole antenna (one of the antenna forms) for receiving AM and FM broadcasts.

Due to design considerations, today's roof antennas are much shorter than the signal wavelength. Thus, the antenna impedance is very small and has a large capacitive reactance component. If such an antenna is connected to the cable, the final cable capacitance can easily reach 150 pF even with a dedicated low capacitance (eg 20 ? C 30 pF/m) cable. In this case, the cable capacitance acts as a capacitive divider, reducing the effective signal strength at the input of the car's radio tuner.

In this case, it is best to add an antenna amplifier, especially an isolation amplifier for the AM signal between the antenna and the cable. Atmel's monolithic integrated antenna amplifier, the ATR4251, has a very low input capacitance (2.45 pF) in the AM band and a very low impedance (5 ohms) at the AM isolation amplifier output, which is good for driving cable capacitance. A solution with an AM isolation amplifier can increase the gain by 35 dB compared to a passive antenna directly connected to the cable. Of course, the specific amount of boost depends on the antenna and cable impedance.

Due to the different distances from the transmitting station and the shielding effect caused by multipath interference, the difference in the receiving environment may be large. The antenna LNA (Low Noise Amplifier) ​​must be tuned to receive full AM or FM broadcasts and must have excellent large signal performance, with second order intercept point (IP2) and third order intercept point (IP3) being very high.

In addition, for very large signals, automatic gain control (AGC) must be used to suppress signal peaks and maintain a constant maximum level at the car radio tuner input. On the other hand, the antenna amplifier must also have good small signal performance parameters, especially the noise figure and stability factor (k).

The basic principle of the ATR4251

The ATR4251 is a monolithically integrated AM/FM antenna matching amplifier circuit. Due to the different operating frequencies and frequency band requirements, two separate amplifiers can be used for the AM and FM bands, respectively. In this way, separate antennas can be used separately, but the two amplifiers can also be connected to a single antenna (such as a roof antenna).

To avoid over-excitation in the case of large signals, both amplifiers are equipped with an automatic gain control circuit (AGC). Two independent AGC circuits, the thresholds can be adjusted separately to prevent the AM strong signal from blocking the weak FM station and vice versa. The ATR4251 also integrates a level detector to provide high linearity and excellent large signal characteristics (FM input crossover intercept point of 142 dBμV and AM of 146 dBμV). (See Figure 2: Functional Block Diagram of the ATR4251).

The AM tuner for car radios typically uses a PIN diode attenuator at the input. These PIN diode attenuators reduce the signal from the antenna by reducing the input impedance of the tuner. For standard applications, the AM amplifier output of the ATR4251 is also connected in series with a resistor of approximately 33 ohms. The series resistors are used on the one hand to ensure that the radio tuner has a defined source impedance; on the other hand to protect the output stage of the AM antenna amplifier from short circuiting of the PIN diode attenuator in the car radio.

All pins of the FM amplifier's NPN transistor can be optimally mated from the outside according to customer requirements. For low-cost applications, a common emitter connection can be used; this connection material is cost effective and performs well. For high-end applications, a common base connection with lossless transformer feedback can be used; this connection has a high intercept point (OIP3) of 148 dBμV, a low noise figure (2.8 dB), and reasonable current consumption. In both connections, the amplifier's gain, input and output impedance can be adjusted by changing the external components.

The ATR4251 chip is also equipped with two independent AGC circuits for the AM and FM bands, respectively, to prevent the antenna amplifier from over-exciting under very strong antenna signals. The amplifier output is connected to a voltage divider (resistive in the AM circuit and capacitive in the FM). The separated signal is applied to the corresponding AGC level detector (which is specially optimized for distortion). The rectified signal is compared to an internal reference level. For specific applications, the AGC threshold can be easily adjusted by changing the voltage divider ratio of the external divider. When the signal reaches the threshold, the AGC output turns on an external transistor; this transistor controls the PIN diode current, which reduces the level of the amplifier's input signal.

The built-in bandgap voltage reference circuit maintains the overall specification without changing with temperature and operating voltage, even extending to the automotive temperature range of -40 to +115 °C. The reason for considering such a high ceiling temperature is because the antenna amplifier is mounted on the roof.

Summary and outlook

Atmel offers highly integrated antenna amplifiers for AM, FM, DRM and HD radios. Excellent antenna performance is achieved with this antenna amplifier. Atmel's antenna amplifiers have a large dynamic range, so the radio is clear, and even when the car is driving, the antenna is not distorted under extremely difficult conditions where the signal strength changes rapidly. This long cable, small size, stylish antenna can cause signal loss, and this loss can be compensated by an impedance matched amplifier.

Atmel has plans to introduce an enhanced version of the ATR4251 to further improve performance and reduce component cost of the application.

Atmel has 30 years of experience in designing wireless broadcast solutions, such as AM/FM front-end devices or active antenna solutions, one of which is automotive radio. Atmel has deep radio technology know-how and meets industry standards (ISO 9001 and TS 16949) to meet the specific quality and performance requirements of the automotive market. Atmel offers complete solutions for antenna amplifiers and tuner front ends, and a variety of development tools, including software and specialized reference designs, to support the implementation of the design.

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