5

4913J–AUDR–10/09

ATR4251

3.

Functional Description

The ATR4251 is an integrated AM/FM antenna impedance matching circuit. It compensates

cable losses between the antenna (for example windshield, roof, or bumper antennas) and the

car radio which is usually placed far away from the antenna.

AM refers to the long wave (LW), medium wave (MW) and short wave (SW) frequency bands

(150 kHz to 30 MHz) that are usually used for AM transmission, and FM means any of the fre-

quency bands used world-wide for FM radio broadcast (70 MHz to 110 MHz).

Two separate amplifiers are used for AM and FM due to the different operating frequencies and

requirements in the AM and FM band. This allows the use of separate antennas (for example,

windshield antennas) for AM and FM. Of course, both amplifiers can also be connected to one

antenna (for example, the roof antenna).

Both amplifiers have automatic gain control (AGC) circuits in order to avoid overdriving the

amplifiers under large-signal conditions. The two separate AGC circuits prevent strong AM sig-

nals from blocking FM stations, and vice versa.

3.1

AM Amplifier

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

applications must be short compared to the wavelength. Therefore these antennas do not pro-

vide 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, ATR4251 provides an AM buffer amplifier with low input

capacitance (less than 2.5 pF) and low output impedance (5

Ω). The low input capacitance of the

amplifier reduces the capacitive load at the antenna, and the low impedance output driver is able

to drive the capacitive load of the cable. The voltage gain of the amplifier is close to 1 (0 dB), but

the insertion gain that is achieved when the buffer amplifier is inserted between antenna output

and cable may be much higher (35 dB). The actual value depends, of course, on antenna and

cable impedance.

The input of the amplifier is connected by an external 4.7 M

Ω resistor to the bias voltage (pin 7,

SSO20) in order to achieve high input impedance and low noise voltage.

AM tuners in car radios usually use PIN diode attenuators at their input. These PIN diode atten-

uators attenuate the signal by reducing the input impedance of the tuner. Therefore, a series

resistor is used at the AM amplifier output in the standard application. This series resistor guar-

antees a well-defined source impedance for the radio tuner and protects the output of the AM

amplifier from short circuit by the PIN diode attenuator in the car radio.