U4253BM
AM/FM Antenna Impedance Matching IC
Description
The U4253BM is an integrated AM/FM antenna
impedance matching circuit in BICMOS technology. The
device is designed in particular for car application and is
suitable for windscreen and roof antennas.
Features
D
D
D
D
High dynamic range for AM and FM
Integrated AGC for FM
High intercept point 3rd order for FM
FM amplifier adjustable to various cable impedance
D
High intercept point 2nd order for AM
D
Low noise output voltage
D
Low power consumption
Block Diagram
FMIN
GND1
FMGAIN
AGC
VS
1
2
3
4
14
5
I
AGC
FM
15
FMOUT
AGC
13
AGCADJ
VREF2
VREF
12
VREF1
AMIN
8
AM
7
GND2
10
AMOUT
12389
Figure 1. Block diagram
TELEFUNKEN Semiconductors
Rev. A1, 08-Jul-97
1 (12)
U4253BM
Ordering and Package Information
Extended Type Number
U4253BM-AFP
U4253BM-AFPG3
Package
SO16
SO16
Remarks
Taping corresponding, ICE-286-3
Pin Description
Pin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Symbol
FMIN
GND1
FMGAIN
AGC
VREF2
NC
GND2
AMIN
NC
AMOUT
NC
VREF1
AGCADJ
VS
FMOUT
NC
Function
FM input
Ground for FM part
FM gain adjustment
AGC output
Reference voltage 2 output
Not connected
Ground for AM part
AM input
Not connected
AM output
Not connected
Reference voltage 1 output
Adjustment FM wide-band
AGC threshold
Supply voltage
FM output
Not connected
FMIN
1
GND1
2
FMGAIN
3
AGC
VREF2
NC
GND2
16
NC
15
FMOUT
14
VS
13
AGCADJ
12
VREF1
11
NC
10
AMOUT
9
12046
4
5
6
7
AMIN
8
NC
Figure 2. Pinning
2 (12)
TELEFUNKEN Semiconductors
Rev. A1, 08-Jul-97
U4253BM
Pin Description
FMIN
FMIN is the input of the FM amplifier. It is the base of a
bipolar transistor. A resistor or a coil is connected
between FMIN and VREF2. If a coil is used, noise perfor-
mance is excellent.
AGC
4
ESD
V
S
1
FMIN
ESD
12390
12392
Figure 5.
Figure 3.
GND1
To avoid crosstalk between AM and FM signals, the
circuit has two separate ground pins. GND1 is the ground
for the FM part.
AGCADJ
The threshold of the AGC can be adjusted by variing the
DC current at pin AGCADJ. If pin AGCADJ is connected
directly to GND1, the threshold is set to 96 dBµV at the
FM amplifier output. If a resistor is connected between
AGCADJ and GND1, the threshold is shifted to higher
values with increasing resistances. If AGCADJ is open,
the threshold is set to 106 dBµV.
FMGAIN
The DC current of the FM amplifier transistor is adjusted
by an external resistor which is connected between
FMGAIN and GND1. In order to influence the AC gain
of the amplifier, a resistor is connected in series to an
capacitor between FMGAIN and GND1. The capacitor
has to be a short at frequencies of 100 MHz.
65 k
W
ESD
13
AGCADJ
12397
Figure 6.
ESD
FMGAIN
3
12391
FMOUT
The FM amplifiers output is an open collector of a bipolar
RF-transistor. It should be connected to VS via a coil.
15
FMOUT
Figure 4.
AGC
DC current flows into the AGC pin at high FM antenna
input signals. This current has to be amplified via the cur-
rent gain of an external PNP transistor that feeds a
PIN-diode. This diode dampens the antenna input signal
and protects the amplifier input against overload. The
maximum current which flows in the AGC pin is approxi-
mately 1 mA. In low end applications, the AGC function
is not necessary and therefore the external components
can be omitted.
ESD
12398
Figure 7.
TELEFUNKEN Semiconductors
Rev. A1, 08-Jul-97
3 (12)
U4253BM
AMIN
The AM input has an internal bias voltage. The DC
voltage at this pin is V
Ref1
/2. The input resistance is about
470 k
W
. The input capacitance is less than 10 pF.
V
Ref1
2
470 k
W
8
VREF1
VREF1 is the stabilized voltage for the AM amplifier and
the AGC block. To achieve excellent noise performance
at LW frequencies, it is recommended that this pin is
connected to ground via an external capacitor of about
1
m
F.
V
S
AMIN
ESD
12394
12
VREF1
ESD
GND1
12396
Figure 8.
Figure 10.
AMOUT
The DC voltage at this pin is almost V
Ref1
/2. The AC
output resistance is about 200
W
. The output capacitance
is less than 10 pF.
VREF2
For the DC biasing of the FM amplifier a second voltage
reference circuit is integrated. Because of temperature
independence of the collector current the output voltage
has a negative temperature coefficient of about –1 mV/K.
To stabilize this voltage an external capacitor to ground
of a few nF is recommended.
V
Ref1
10
AMOUT
ESD
ESD
GND1
12395
5
VREF2
12393
Figure 11.
GND2
Figure 9.
GND2 is the ground for the AM amplifier.
4 (12)
TELEFUNKEN Semiconductors
Rev. A1, 08-Jul-97
U4253BM
Functional Description
The U4253BM is an integrated AM/FM antenna imped-
ance matching circuit. It compensates cable losses
between the antenna (for example windscreen, roof or
bumper antennas) and the car radio which is usually
placed far away from the antenna.
The FM amplifier provides excellent noise performance.
External components are used to adjust the gain and the
input-output matching impedance. Therefore it is
possible to adjust the amplifier to various cable
impedances (usually 50, 75 or 150
W
). To protect the
amplifier against input overload an
Automatic Gain
Control
(AGC) is included on the chip. The AGC
observes the AC voltage at the FM amplifier output,
rectifies this signal, and delivers DC current to dampen
the input antenna signal via an external PIN diode. The
threshold for the AGC is adjustable. Simple and
temperature compensated biasing is possible due to the
integrated voltage reference V
Ref2
.
The AM part consists of a buffer amplifier. The voltage
gain of this stage is approximately one. The input
resistance is 470 k
W
, the input capacitance less than
10 pF. The output resistance is 150
W
. An excellent
dynamic range is achieved due to a special push-pull
circuit technique.
Absolute Maximum Ratings
Reference point is ground (Pins 2 and 7)
Parameters
Supply voltage
Power dissipation, P
tot
at T
amb
= 85°C
Junction temperature
Ambient temperature
Storage temperature
Electrostatic handling (HBM)
Symbol
V
S
P
tot
T
j
T
amb
T
stg
V
ESD
Value
8.8
460
150
–40 to +85
–50 to +150
2000
Unit
V
mW
°C
°C
°C
V
"
Thermal Resistance
Parameters
Junction ambient
Symbol
R
thJA
Value
140
Unit
K/W
TELEFUNKEN Semiconductors
Rev. A1, 08-Jul-97
5 (12)
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