1
Features
Reference Oscillator up to 15 MHz (Tuned)
Oscillator Buffer Output (for AM Up/Down Conversion)
Two Programmable 16-bit Dividers
Fine-tuning Steps Possible
Fast Response Time due to Integrated Loop Push-pull Stage
3-wire Bus (Enable, Clock and Data; 3 V and 5 V Microcontrollers Acceptable)
Four Programmable Switching Outputs (Open Drain)
Three DACs for Software Controlled Tuner Alignment
Low-power Consumption
High S/N Ratio
Integrated Band Gap only One Supply Voltage Necessary
Description
The U4256BM-R is a synthesizer IC for FM receivers and an AM up-convertion
system in BICMOS technology. Together with the AM/FM IC T4258 or U4255BM, it
performs a complete AM/FM car radio front-end, which is recommended also for RDS
(Radio Data System) applications. It is controlled by a 3-wire bus and also contains
switches and Digital to Analog Converters (DACs) for software-controlled alignment of
the AM/FM tuner. The U4256BM-R is the pin-compatible succesor IC of U4256BM-N.
Pin Configuration
Figure 1. Pinning SSO20
1
2
3
4
5
6
7
8
13
14
15
16
17
18
19
20
EN
DAT
A
CL
K
SWO
1
SWO
2
SWO
3
SWO
4
GNDa
n
OSCOUT
OS
CIN
MX
2LO
VS
PD
PDO
DAC2
V5
9
11
12
FMOS
C
I
N
DAC1
GND
10
DAC3
U
4256B
M-R
Frequency
Synthesizer for
Radio Tuning
U4256BM-R
Preliminary
Rev. 4562CAUDR08/04
2
U4256BM-R
4562CAUDR08/04
Figure 2. Block Diagram
Pin Description
Pin
Symbol
Function
1
PDO
Phase detector output
2
PD
Pulsed current output
3
DAC1
Digital-to-analog converter 1
4
DAC2
Digital-to-analog converter 2
5
DAC3
Digital-to-analog converter 3
6
VS
Supply voltage analog part
7
SWO1
Switching output 1
8
SWO2
Switching output 2
9
SWO3
Switching output 3
10
SWO4
Switching output 4
11
GND
Ground, digital part
12
OSCOUT
Reference oscillator output
13
OSCIN
Reference oscillator input
14
V5
Capacitor band gap
15
MX2LO
Oscillator buffer output
16
DATA
Data input
17
CLK
Clock
18
EN
Enable
19
FMOSCIN
FM-oscillator input
20
GNDan
Ground, analog part
FM-
preamp
N-
divider
Phase
detector
Current
sources
DAC1
AM/FM
DAC2
DAC3
3-bit
V
Ref
SWO1 SWO2 SWO3
SWO4
Switching outputs
R-
divider
Oscillator
OSC
buffer
3W-
bus
interface
16
17
18
13
12
15
19
7
8
9
10
OSCIN
OSCOUT
MX2LO
CLK
DATA
EN
FMOSCIN
6
1
2
5
4
3
V
Ref
14
11
20
GNDan
V5
GND
DAC3
DAC2
DAC1
VS
PDO
PD
Tuning
DAC
Bandgap
3
U4256BM-R
4562CAUDR08/04
Functional
Description
For a tuned FM-broadcast receiver, the following parts are needed:
Voltage-Controlled Oscillator (VCO)
Antenna Amplifier Tuned Circuit
RF Amplifier Tuned Circuit
Typical modern receivers with electronic tuning are tuned to the desired FM frequency
by the frequency synthesizer IC U4256BM-R. The special design allows the user to
build software-controlled tuner alignment systems. Two programmable DACs (Digital-to-
Analog Converter) support the computer-controlled alignment. The output of the PLL is
a tuning voltage which is connected to the VCO of the receiver IC. The output of the
VCO is equal to the desired station frequency plus the IF (10.7 MHz). The RF and the
oscillator signal (VCO) are both input to the mixer that translates the desired FM chan-
nel signal to the fixed IF signal. For FM, the double-conversion system of the receiver
requires exactly 10.7 MHz for the first IF frequency, which determines the center fre-
quency of the software-controlled integrated second IF filter.
If this oscillator tuning feature is not used, the internal capacities have to be switched off
and the oscillator has to be operated with high-quality external capacities to ensure that
the operational frequency is exactly 10.250 MHz.
When dimensioning the oscillator circuit, it is important that the additional capacities
enable the oscillator to operate through its complete tracking range. The oscillating abil-
ity depends very strongly on the used crystal oscillator. Initializing the oscillator should
be established without switching any additional capacities to guarantee that the oscilla-
tor starts to operate properly. Due to the lower quality of the integrated capacities
compared to discrete capacities, the amount of the switched integrated capacities
should always be minimized. (If necessary reduce tracking range or use another crystal
oscillator.)
The U4256BM-R has a very fast response time of maximum 800 s (at 2 mA,
f
Step
= 50 kHz, measured on MPX signal). It performs a high signal to noise ratio. Only
one supply voltage is necessary, due to a integrated band gap.
Input/Output
Interface Circuits
PDO (Pin 1)
PDO is the buffer amplifier output of the PLL. The bipolar output stage is a rail-to-rail
amplifier.
PD (Pin 2)
PD is the current charge pump output of the PLL. The current can be controlled by
setting the Bits. The loop filter has to be designed corresponding to the choosen pump
current and the internal reference frequency. A recommendation can be found in the
application circuit.
The charge-pump current can be choosen by setting the Bits 71 and
70 as
following:
IPD (A)
B71
B70
25
0
0
100
0
1
500
1
0
2000
1
1
4
U4256BM-R
4562CAUDR08/04
Figure 3. Internal Components at PDO Connection
FMOSCIN (Pin 19)
FMOSCIN is the preamplifier input for the FM oscillator signal.
Figure 4. Internal Components at FMOSCIN
MX2LO (Pin 15)
MX2LO is the buffered output of the crystal oscillator. This signal can be used as a refer-
ence frequency for U4255BM or T4258.
The oscillator buffer output can be switched by the OSCB Bit as following (Bit 69)
Figure 5. Internal Components at MX2LO
PD
V
S
V
S
V
S
PDO
FMOSCIN
V
5
MX2LO AC Voltage
B69
ON
0
OFF
1
V5
V5
OSCIN
MX2LO
5
U4256BM-R
4562CAUDR08/04
Function of DAC1, 2 in
FM and AM Mode
(Pin 3 and Pin 4)
For automatic tuner alignment, the DAC1 and DAC2 of the U4256BM-R can be con-
trolled by setting gain of VPDO and offset values. The following figure shows the
principle of the operation. In FM Mode the gain is in the range of 0.69
V
(PDO)
to 2.16
V
(PDO)
. The offset range is +0.56 V to -0.59 V. For alignment, DAC1 and DAC2 are con-
nected to the varicaps of the preselection filters. For alignment, offset and gain is set for
having the best tuner tracking.
Figure 6. Principle Operation for Alignment
The DAC mode can be controlled by setting the Bit 34 as following:
If Bit 34 = 1 (AM Mode), the DAC1, DAC2 can be used as standard DAC converters.
The internal voltage of 3 V is connected to the gain- and offset-input of DAC1 and DAC2
(only in AM Mode). The gain is in the range of 0.46
3 V to 3.03
3 V. The offset range
is +1.46 V to -1.49 V.
Figure 7. Internal Components at DAC1,2 Output
DAC Mode
B34
FM
0
AM
1
PDO (FM)
Offset
DAC1,2
+/-
Gain
Vref (AM)
(3 V)
Bit 34
VS
DAC1,2
PDF 
ZIP