EVB72016
433MHz FSK/ASK Transmitter
Evaluation Board Description
2.2
FSK Modulation
Fig. 2:
Crystal pulling circuitry
ROI
VCC
FSK modulation can be achieved by pulling the
crystal oscillator frequency. A CMOS-
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 f
c
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
(f
min
); while at FSKDTA = 1, CX2 is deactivated
and the XOSC is set to its high frequency f
max
.
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.
XTAL
FSKSW
CX2
CX1
VEE
FSKDTA
0
1
Description
f
min
= f
c
-
Δf
(FSK switch is closed)
f
max
= f
c
+
Δf
(FSK switch is open)
2.3 Crystal Pulling
A crystal is tuned by the manufacturer to the
required oscillation frequency f
0
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 CL
eff
seen by the crystal.
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
XTAL
L1
f
max
C1
R1
C0
CL
eff
f
c
f
min
CX1 CRO
CX1+CRO
CL
(CX1+CX2) CRO
CX1+CX2+CRO
CL
eff
Fig. 3:
Crystal pulling characteristic
For ASK applications CX2 can be omitted. Then CX1 has to be adjusted for center frequency.
3901272016 01
Rev. 004
Page 4 of 12
EVB Description
June/07
MELEXIS [ Melexis Microelectronic Systems ]
