ATR4251-PFQY Atmel, ATR4251-PFQY Datasheet - Page 5

• Manufacturer Part Number: ATR4251-PFQY
• Description: IC AMP AM/FM ANT HI RANGE 24-QFN
• Manufacturer: Atmel

Specifications of ATR4251-PFQY

• Current - Supply: 11mA ~ 17mA
• Frequency: AM, FM
• Gain: 6dB
• Noise Figure: 2.8dB
• Package / Case: 24-VQFN Exposed Pad, 24-HVQFN, 24-SQFN, 24-DHVQFN
• Rf Type: Broadcast Radio
• Voltage - Supply: 8 V ~ 11 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• P1db: -
• Test Frequency: -

3. Functional Description
3.1
4913J–AUDR–10/09
AM Amplifier
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.
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 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 ). 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 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.
ATR4251
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