4

9154AS–AUDR–09/09

ATR4252 [Preliminary]

3.

Functional Description

The ATR4252 is a highly integrated AM/FM antenna IC with lots of features and functions. In fact

the most important feature is the impedance matching on both the antenna input and the cable.

The ATR4252 compensates cable losses between the antenna (for example, windscreen, roof

or bumper antennas) and the car radio, which is usually placed far away from the antenna.

AM means long wave (LW), medium wave (MW) and short wave (SW) frequency bands

(150 kHz to 30 MHz) that are usually used for AM as well as for DRM transmissions, and FM

means any of the world wide used frequency bands for FM radio broadcast (70 MHz to

110 MHz).

Two separate amplifier chains are used for AM and FM due to the different operation frequen-

cies and requirements in the AM and FM band. This allows the use of separate antennas (e.g.,

windscreen antennas) for AM and FM. Of course, both amplifier chain inputs can also be con-

nected to one antenna (e.g., roof antenna).

The AM amplifier chain is separated into two amplifiers. The first one is an LNA that is optimized

for low noise figure and low input capacitance. The second amplifier (AM buffer) is optimized to

drive a possibly long antenna cable with high parasitic capacitance. Both amplifiers have out-

standing large signal performance. All input and output terminals of these two amplifiers are

accessible from outside so they can be connected together according to the application needs.

Additionally, a filter can be inserted between LNA output and buffer amplifier input.

For AM and FM amplifier chain, two separate automatic gain control (AGC) circuits have been

integrated in order to avoid overdriving the amplifiers in large signal conditions. The two sepa-

rate AGC loops prevent strong AM signals from blocking FM stations and vice versa.

The integrated PIN diode drivers reduce the external component cost and board space.

A voltage regulation stage is integrated in order to further reduce the external component costs.

This stage provides overvoltage protection and current limitation. An external transistor is used

as power driver for this stage.

3.1

AM Amplifier

Due to the long wavelength in AM bands, the antennas used for AM reception in automotive

applications are short compared to the wavelength. Therefore, these antennas do not provide

50

Ω output impedance, but have an output impedance of some pF. If these (passive) antennas

are connected to the car radio by a long cable, the capacitive load of this cable (some 100 pF)

dramatically reduces the signal level at the tuner input.

In order to overcome this problem, ATR4252 provides two AM amplifiers, one LNA and one AM

buffer amplifier. These two amplifiers can be used independently because all input/output termi-

nals and bias inputs are externally accessible for the application.

The AM LNA has low input capacitance (12 pF typically) to reduce the capacitive load at the

antenna and provides a voltage gain of typically 9 dB that can be varied from 0 to 15 dB depend-

ing on external application.