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车载收音机信号放大器,用车载收音机信号放大及阻抗匹配。
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Features
• 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
CREG
NC*
NC*
Paddle = GND
AGCCONST
AMOUT1
GND1
VREF4
VS
NC*
AGC1
VREF2
AMIN
AGC2
GND
NC*T
CONST
AGC
AM
AGC
(AM)
AGC
AMIN
VREF1
FM
IN
AGC
IN
FM
GAIN
FM
OUT
GND2
FM
amplifier
81012119
5
3
4
1
2
6
14
16
15
18
17
13
7
23 21 19202224
BAND
GAP
AM
AGC
* Pin must not be connected to any other pin or supply chain except GND.
Low-noise,
High-dynamic-
range AM/FM
Antenna
Amplifier IC
ATR4251
4913I–AUDR–03/08
2
4913I–AUDR–03/08
ATR4251
Figure 1-2. Block Diagram SSO20 Package
FMIN
VREF1
GND
AGC1
AGC2
AGCAMIN
CREG
AMIN1
VREF2
FMGAIN
2
3
4
5
6
10
9
8
7
1
FMOUT
AGCIN
VS
AGCCONST
VREF4
AGCAM
TCONST
GND1
AMOUT1
GND2
19
18
17
16
AGC
(AM)
Band
gap
AGC
SSO20
FM
amplifier
15
11
12
13
14
20
AM
3
4913I–AUDR–03/08
ATR4251
2. Pin Configuration
Figure 2-1. Pinning QFN24
NC
GND
AGC1
AGC2
VREF2
AMIN
NC
VS
AGCCONST
VREF4
AMOUT1
GND1
VREF1
FMIN
FMGAIN
GND2
FMOUT
AGCIN
NC
CREG
AGCAMIN
AGCAM
TCONST
NC
24 23 22 21 20 19
7 8 9 10 11 12
18
17
16
15
14
13
1
2
3
4
5
6
Table 2-1. Pin Description QFN24
Pin Symbol Function
1 NC Pin must not be connected to any other pin or supply chain except GND.
2 GND Ground FM
3 AGC1 AGC output for pin diode
4 AGC2 AGC output for pin diode
5 VREF2 Reference voltage for pin diode
6 AMIN AM input, impedance matching
7 NC Pin must not be connected to any other pin or supply chain except GND.
8 CREG AM - AGC time constant capacitance 2
9 AGCAMIN AM - AGC input
10 AGCAM AM - AGC output for pin diode
11 TCONST AM - AGC - time constant capacitance 1
12 NC Pin must not be connected to any other pin or supply chain except GND.
13 GND1 Ground AM
14 AMOUT1 AM output, impedance matching
15 VREF4 Bandgap
16 AGCCONST FM AGC time constant
17 VS Supply voltage
18 NC Pin must not be connected to any other pin or supply chain except GND.
19 AGCIN FM AGC input
20 FMOUT FM output
21 GND2 Ground
22 FMGAIN FM gain adjustment
23 FMIN FM input
24 VREF1 Reference voltage 2.7V
Paddle GND Ground Paddle
4
4913I–AUDR–03/08
ATR4251
Figure 2-2. Pinning SSO20
FMGAIN
FMIN
VREF1
GND
AGC1
AGC2
VREF2
AMIN1
CREG
AGCAMIN
GND2
FMOUT
AGCIN
VS
AGCCONS
T
VREF4
AMOUT1
GND1
TCONST
AGCAM
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
Table 2-2. Pin Description SSO20
Pin Symbol Function
1 FMGAIN FM gain adjustment
2 FMIN FM input
3 VREF1 Reference voltage 2.7V
4 GND FM ground
5 AGC1 AGC output for PIN diode
6 AGC2 AGC output for PIN diode
7 VREF2 Reference voltage for PIN diode
8 AMIN1 AM input, impedance matching
9 CREG AM AGC constant capacitance 2
10 AGCAMIN AM input, AM AGC
11 AGCAM AM AGC output for PIN diode
12 TCONST AM AGC constant capacitance 1
13 GND1 AM ground
14 AMOUT1 AM output, impedance matching
15 VREF4 Band gap 6V
16 AGCCONST FM AGC constant
17 VS Supply voltage
18 AGCIN FM AGC input
19 FMOUT FM output
20 GND2 FM ground
5
4913I–AUDR–03/08
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.
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