TH72036 Datasheet, PDF (4/16 Pages) Melexis Microelectronic Systems – 868/915MHz FSK/ASK Transmitter

TH72036
868/915MHz
FSK/ASK Transmitter
2.2 FSK Modulation
FSK modulation can be achieved by pulling the
crystal oscillator frequency. A CMOS- Fig. 2: Crystal pulling circuitry
VCC
compatible data stream applied at the pin
FSKDTA digitally modulates the XOSC via an
ROI
integrated NMOS switch. Two external pulling
capacitors CX1 and CX2 allow the FSK devia-
tion Δf and the center frequency f_c to be ad-
justed independently. At FSKDTA = 0, CX2 is
XTAL
FSKSW
connected in parallel to CX1 leading to the low-
frequency component of the FSK spectrum
(f_min); while at FSKDTA = 1, CX2 is deactivated
and the XOSC is set to its high frequency f_max.
CX2
CX1
VEE
An external reference signal can be directly AC-
PRELIMINARY ^{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
0
1
2.3 Crystal Pulling
A crystal is tuned by the manufacturer to the
required oscillation frequency f₀ at a given load
f
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 CL_eff seen by the crystal.
Figure 3 shows the oscillation frequency of a
crystal as a function of the effective load ca- ^fmax
pacitance. This capacitance changes in accor-
Description
f_min= f_c - Δf (FSK switch is closed)
f_max= f_c + Δf (FSK switch is open)
XTAL
L1
C1
C0
CL _eff
dance with the logic level of FSKDTA around
R1
the specified load capacitance. The figure illus- ^fc
trates the relationship between the external
pulling capacitors and the frequency deviation.
It can also be seen that the pulling sensitivity f_min
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
CX1 CRO CL
CX1+CRO
(CX1+CX2) CRO
CX1+CX2+CRO
CL _eff
chip and the external pulling capacitors.
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