Features
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High Dynamic Range for AM and FM
Integrated AGC for AM and FM
High Intercept Point 3rd Order for FM
FM Amplifier Adjustable to Various Cable Impedances
High Intercept Point 2nd and 3rd Order for AM
Low Noise Output Voltage
Low Power Consumption
Low Output Impedance AM
1. Description
The ATR4251 is an integrated low-noise AM/FM antenna amplifier with integrated
AGC in BiCMOS2S technology. The device is designed in particular for car applica-
tions, and is suitable for windshield and roof antennas.
Figure 1-1.
Block Diagram QFN24 Package
FM
VREF1 IN
24
23
Low-noise,
High-dynamic-
range AM/FM
Antenna
Amplifier IC
ATR4251
FM
FM
GAIN GND2 OUT
22
21
20
AGC
IN
Paddle = GND
19
NC*
GND
AGC1
AGC2
VREF2
AMIN
1
FM
amplifier
BAND
GAP
AGC
18
NC*
VS
AGCCONST
VREF4
AMOUT1
GND1
2
17
3
16
4
15
5
AM
14
6
7
8
AGC
(AM)
9
10
11
12
13
T
NC*
NC* CREG AGC AGC
AMIN AM CONST
* Pin must not be connected to any other pin or supply chain except GND.
4913J–AUDR–10/09
ATR4251
2. Pin Configuration
Figure 2-1.
Pinning QFN24
VREF1
FMIN
FMGAIN
GND2
FMOUT
AGCIN
NC
GND
AGC1
AGC2
VREF2
AMIN
1
2
3
4
5
6
7 8
24 23 22 21 20 19
18
17
16
15
14
13
9 10 11 12
NC
VS
AGCCONST
VREF4
AMOUT1
GND1
Table 2-1.
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Paddle
Pin Description QFN24
Symbol
NC
GND
AGC1
AGC2
VREF2
AMIN
NC
CREG
AGCAMIN
AGCAM
TCONST
NC
GND1
AMOUT1
VREF4
AGCCONST
VS
NC
AGCIN
FMOUT
GND2
FMGAIN
FMIN
VREF1
GND
Function
Pin must not be connected to any other pin or supply chain except GND.
Ground FM
AGC output for pin diode
AGC output for pin diode
Reference voltage for pin diode
AM input, impedance matching
Pin must not be connected to any other pin or supply chain except GND.
AM - AGC time constant capacitance 2
AM - AGC input
AM - AGC output for pin diode
AM - AGC - time constant capacitance 1
Pin must not be connected to any other pin or supply chain except GND.
Ground AM
AM output, impedance matching
Bandgap
FM AGC time constant
Supply voltage
Pin must not be connected to any other pin or supply chain except GND.
FM AGC input
FM output
Ground
FM gain adjustment
FM input
Reference voltage 2.7V
Ground Paddle
NC
CREG
AGCAMIN
AGCAM
TCONST
NC
3
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 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
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 (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 (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 MΩ 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.
5
4913J–AUDR–10/09
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