U4256BM
Rev. A5, 06-Oct-00
Preliminary Information
10 (14)
OSCIN, OSCOUT
A crystal resonator (up to 15 MHz) is connected between
OSCIN and OSCOUT in order to generate the reference
frequency. By using the U4256BM in connection with
U4255BM the crystal frequency must be 10.25 MHz. The
complete application circuit is shown in figure 15. If a
reference is available, it can be applied at OSCIN. The
minimum voltage should be 100mVrms. In this case, pin
OSCOUT has to be open.
V
S
OSCIN
V
S
OSCOUT
Figure 11.
Application Information
Function of DAC1, 2 in FM Mode
For automatic tuner alignment, the DAC1 and 2 of the
U4256BM can be controlled by setting gain of VPDO and
offset values. The following figure shows the principle of
the operation. The gain is in the range of 0.7 to 2.15. The
offset range is +0.5 V to –0.6 V. For alignment, DAC1
and 2 are connected to the varicaps of the preselection
filters. For alignment, offset and gain is set for having the
best tuner tracking.
PDO (FM)
Offset
DAC1,2
+/–
Gain
Vref (AM)
Bit 34
Figure 12.
Function of DAC1, 2 in AM Mode
If Bit 94 = 1, the DAC1, 2 can be used as standard DAC
converters. The resolution of 8 bit is controlled via the
gain bits in a range of approximately 0.5 V to 7 V,
depending on the offset value.
FMOSCIN Sensitivity
0
50
100
150
0 20 40 60 80 100 120 140 160
Frequency (MHz)
Vi (mV
rms
on 50 )
Figure 13.
