TH72036
868/915MHz
FSK/ASK Transmitter
2.2 FSK Modulation
FSK modulation can be achieved by pulling the
VCC
crystal oscillator frequency.
A
CMOS-
Fig. 2: Crystal pulling circuitry
compatible data stream applied at the pin
FSKDTA digitally modulates the XOSC via an
integrated NMOS switch. Two external pulling
capacitors CX1 and CX2 allow the FSK devia-
tion Δf and the center frequency fc to be ad-
justed independently. At FSKDTA = 0, CX2 is
connected in parallel to CX1 leading to the low-
frequency component of the FSK spectrum
(fmin); while at FSKDTA = 1, CX2 is deactivated
ROI
XTAL
CX1
FSKSW
CX2
VEE
and the XOSC is set to its high frequency fmax
.
An external reference signal can be directly AC-
coupled to the reference oscillator input pin
ROI. Then the transmitter is used without a
crystal. Now the reference signal sets the car-
rier frequency and may also contain the FSK (or
FM) modulation.
FSKDTA
Description
0
1
fmin= fc - Δf (FSK switch is closed)
fmax= fc + Δf (FSK switch is open)
2.3 Crystal Pulling
A crystal is tuned by the manufacturer to the
required oscillation frequency f0 at a given load
capacitance CL and within the specified calibra-
tion tolerance. The only way to pull the oscilla-
tion frequency is to vary the effective load ca-
pacitance CLeff seen by the crystal.
f
XTAL
L1
Figure 3 shows the oscillation frequency of a
crystal as a function of the effective load ca-
pacitance. This capacitance changes in accor-
dance with the logic level of FSKDTA around
the specified load capacitance. The figure illus-
trates the relationship between the external
pulling capacitors and the frequency deviation.
It can also be seen that the pulling sensitivity
increases with the reduction of CL. Therefore,
applications with a high frequency deviation
require a low load capacitance. For narrow
band FSK applications, a higher load capaci-
tance could be chosen in order to reduce the
frequency drift caused by the tolerances of the
chip and the external pulling capacitors.
f
max
C0
CL
C1
R1
eff
f
c
f
min
CL
eff
CX1 CRO CL
CX1+CRO
(CX1+CX2) CRO
CX1+CX2+CRO
Fig. 3: Crystal pulling characteristic
For ASK applications CX2 can be omitted. Then CX1 has to be adjusted for center frequency.
3901072036
Rev. 005
Page 4 of 16
Data Sheet
June/07
MELEXIS [ Melexis Microelectronic Systems ]
