U4256BM-R Datasheet, PDF (3/19 Pages) ATMEL Corporation – Frequency Synthesizer for Radio Tuning

Functional
Description
Input/Output
Interface Circuits
PDO (Pin 1)
PD (Pin 2)
U4256BM-R
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 µs (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.
PDO is the buffer amplifier output of the PLL. The bipolar output stage is a rail-to-rail
amplifier.
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 (µA)
B71
B70
25
0
0
100
0
1
500
1
0
2000
1
1
3
4562C–AUDR–08/04