MC33493 Motorola, MC33493 Datasheet - Page 17

• Manufacturer Part Number: MC33493
• Description: PLL tuned UHF Transmitter for Data Transfer Applications
• Manufacturer: Motorola

Table 6 gives the measured FSK deviations respective to C3 and C4 capacitor values for three deviations.
Crystal reference is NDK NX8045GB/CSJ S1-40125-8050-12.
Another crystal reference, NDK NX1255GA (see table 4), enables to reach higher deviation as mentioned on
table 7. These results are due to the higher crystal motional capacitor.
RECOMMENDATIONS FOR FSK MODULATION
FSK deviation is function of total load capacitance presented to the crystal. This load capacitance is constituted
by various contributors:
The schematic given in figure 16 shows a typical FSK application using serial capacitor configuration, where
device pads and PCB track capacitances are mentioned.
Device pad capacitance is defined by the package capacitance and by the internal circuitry. Typical capacitance
values for these pads are given in table 8.
Some realistic assumptions and measurements have been made concerning track parasitic capacitances for a
0.8mm FR4 double side application PCB. They are given in table 8 and the corresponding PCB layout is shown
in figure 15.
To achieve large deviations, this total load capacitance need to be lowered. For a given crystal, the PCB must be
very carefully laid out in order to reduce as much as possible the capacitance of the tracks wired to XTAL0,
Figure 15: Application PCB layout for FSK modulation, serial configuration, 434MHz frequency band
Carrier frequency
Carrier frequency
- the crystal characteristic, especially its static capacitance,
- the external load capacitors (C3, C4 as defined in figure 14 and table 6),
- the device internal capacitance of pins XTAL0, XTAL1, CFSK,
- the PCB track capacitance.
MOTOROLA
(MHz)
(MHz)
Table 6 : Crystal pulling capacitor values versus carrier frequency total deviation -1-
Table 7 : Crystal pulling capacitor values versus carrier frequency total deviation -2-
434
868
434
868
total deviation (kHz)
total deviation (kHz)
Carrier frequency
Carrier frequency
Freescale Semiconductor, Inc.
100
140
200
150
300
45
70
90
For More Information On This Product,
Go to: www.freescale.com
MC33493 Technical Data
C3 capacitor
C3 capacitor
value (pF)
(pF)
4.7
2.2
4.7
2.2
1
1
1
1
RECOMMENDATIONS FOR FSK MODULATION
C4 capacitor
C4 capacitor
value (pF)
(pF)
6.8
6.8
10
15
10
15
27
27
R_off value (kΩ)
R_off value (kΩ)
Recommended
Recommended
10
22
10
22
-
-
-
-
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