3

9194CS–AUDR–10/10

Atmel ATR4253

3.

Functional Description

The Atmel

and functions. It compensates cable losses between the antenna and the car radio, which is

usually placed far away from the antenna. The device can be used in the frequency range of

50 up to 900MHz (i.e. FM, DAB (+), DMB, DVB-T, ISDB-T, etc.) and has the flexibility to

address different antenna types (e.g., glass, windscreen, roof antennas).

A separate automatic gain control (AGC) function is integrated in order to avoid overdriving the

RF amplifier at large signal conditions.

The integrated PIN diode drivers help to reduce the external component cost and to minimize

the board space.

The external component cost can be further reduced as the voltage regulation stage is also

integrated. This stage provides overvoltage protection and current limitation. An external tran-

sistor is used as power driver for this stage.

3.1

RF Amplifier Stage

The RF amplifier is realized with a high-performance NPN transistor. This allows the use of an

amplifier configuration which is optimized for the individual application requirements. For

low-cost applications, the common emitter configuration enables good performance at reason-

able BOM cost. For high-end applications, a common base configuration with transformer

feedback provides high IP3 and low noise figures at reasonable current consumption. In both

configurations, gain, input and output impedance can be adjusted by modifying the external

components.

The bias voltage (BIAS) for the base of the NPN transistor is derived from an integrated volt-

age reference and has a PTAT (proportional to absolute temperature) behavior. The bias

current of the RF amplifier is defined by an external resistor and is kept constant over

temperature.

3.2

AGC

The IC is equipped with an AGC functionality to prevent overdriving the amplifier in case the

amplifier is operated at strong antenna signals, e.g., near transmitters. It is possible to realize

an additional antenna amplifier path with integrated AGC and external RF transistor. The

bandwidth of the integrated AGC circuit is 900MHz.

The amplifier output (RFC) is connected to a capacitive voltage divider and the divided signal

is applied to the AGC level detector at pin DET. This level detector input is optimized for low

distortion. The rectified signal is compared against an internal reference. The divider ratio of

the external voltage divider defines the AGC’s threshold. If the threshold is reached, pin PD1

drives a positive current through one or two external pin diodes in shunt configuration. In case

the limiting range of the shunt configuration is not sufficient, an additional pin diode in serial

configuration can be added and controlled via pin PD2. The current from PD2 through the

serial pin diode is opposite to the current of PD1. This helps to limit the amplifier input signal

and thus to prevent the FM amplifier from signal overdrive.

The drivers required for the external pin diode(s) are built-in into the ATR4253 IC, which

reduces the BOM cost and the application size.