EVB72015-315-ASK-A MELEXIS [Melexis Microelectronic Systems], EVB72015-315-ASK-A Datasheet - Page 4

• Manufacturer Part Number: EVB72015-315-ASK-A
• Description: 433MHz FSK/ASK Transmitter Evaluation Board Description
• Manufacturer: MELEXIS [Melexis Microelectronic Systems]

2.2 FSK Modulation
FSK modulation can be achieved by pulling the
crystal
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
justed independently. At FSKDTA = 0, CX2 is
connected in parallel to CX1 leading to the low-
frequency component of the FSK spectrum
(f
and the XOSC is set to its high frequency f
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.
2.3 Crystal Pulling
A crystal is tuned by the manufacturer to the
required oscillation frequency 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
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.
For ASK applications CX2 can be omitted. Then CX1 has to be adjusted for center frequency.
39012 72015 01
Rev. 007
min
); while at FSKDTA = 1, CX2 is deactivated
oscillator
eff
seen by the crystal.
frequency.
0
at a given load
A
c
to be ad-
CMOS-
max
.
Page 4 of 14
f
f
max
min
Fig. 2:
Fig. 3:
f
c
f
FSKDTA
CX1 CRO
CX1+CRO
0
1
Crystal pulling circuitry
Crystal pulling characteristic
433MHz FSK/ASK Transmitter
Evaluation Board Description
CL
f
f
min
max
= f
= f
XTAL
c
(CX1+CX2) CRO
CX1
CX1+CX2+CRO
c
- Δf (FSK switch is closed)
+ Δf (FSK switch is open)
Description
EVB72015
CX2
C1
L1
XTAL
R1
FSKSW
ROI
EVB Description
C0
VCC
VEE
June/07
CL
CL
eff
eff