5
ATR4251C [DATASHEET]
9258E–AUDR–11/14
3. Functional Description
The Atmel® ATR4251C 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 (150kHz to 30MHz) that are
usually used for AM transmission, and FM means any of the frequency bands used world-wide for FM radio broadcast
(70MHz to 110MHz).
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 signals 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 provide 50Ω 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 100pF) dramatically reduces the signal level at the tuner input.
In order to overcome this problem, Atmel ATR4251C provides an AM buffer amplifier with low input capacitance (less than
2.5pF) and low output impedance (5Ω). 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 (0dB), but the insertion gain that is achieved when the buffer amplifier is inserted between antenna
output and cable may be much higher (35dB). The actual value depends, of course, on antenna and cable impedance.
The input of the amplifier is connected by an external 4.7MΩ 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 attenuators 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 guarantees 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.
3.2 AM AGC
The IC is equipped with an AM AGC capability to prevent overdriving of the amplifier in case the amplifier operates near
strong antenna signal level, for example, transmitters.
The AM amplifier output AMOUT1 is applied to a resistive voltage divider. This divided signal is applied to the AGC level
detector input pin AGCAMIN. The rectified signal is compared against an internal reference. The threshold of the AGC can
be adjusted by adjusting the divider ratio of the external voltage divider. If the threshold is reached, pin AGCAM opens an
external transistor which controls PIN diode currents and limits the antenna signal and thereby prevents overdriving the AM
amplifier IC.
3.3 FM Amplifier
The FM amplifier is realized with a single NPN transistor. This allows use of an amplifier configuration optimized on the
requirements. For low-cost applications, the common emitter configuration provides good performance at reasonable bills of
materials (BOM) cost(1). For high-end applications, common base configuration with lossless transformer feedback provides
a high IP3 and a low noise figure at reasonable current consumption(2). In both configurations, gain, input, and output
impedance can be adjusted by modification of external components.
The temperature compensated bias voltage (VREF1) for the base of the NPN transistor is derived from an integrated band
gap reference. The bias current of the FM amplifier is defined by an external resistor.
Notes: 1. See test circuit (Figure 8-1 on page 11)
2. See application circuit (Figure 9-1 on page 12)