U4256BM
Rev. A5, 06-Oct-00
Preliminary Information
2 (14)
Pin Description
1
2
3
4
5
6
7
8
13
14
15
16
17
18
19
20
EN
DATA
CLK
SWO1
SWO2
SWO3
SWO4
GNDan
OSCOUT
OSCIN
MX2LO
VS
PD
PDO
DAC2
V5
9
11
12
FMOSCIN
DAC1
GND
10
DAC3
U4256BM
Figure 2. Pinning
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
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. The special design allows to build
software-controlled tuner alignment systems. Two pro-
grammable 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 channel 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 frequency 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 oscillat-
ing ability depends very strongly on the used crystal
oscillator. Initializing the oscillator should be established
without switching any additional capacities to guarantee
that the oscillator 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 re-
duce tracking range or use another crystal oscillator)
The U4256BM has a very fast response time of maximum
800 s (at 2 mA, fref = 100 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.
