TH72005_12 Datasheet, PDF (4/20 Pages) Melexis Microelectronic Systems – FSK/ASK Transmitter

TH72005
315MHz
FSK/ASK Transmitter
2.2 FSK Modulation
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
deviation ∆f and the center frequency f_c to be
adjusted 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
carrier frequency and may also contain the FSK
(or FM) modulation.
Fig. 2: Crystal pulling circuitry
VCC
ROI
XTAL
FSKSW
CX2
CX1
VEE
FSKDTA
0
1
Description
f_min= f_c - ∆f (FSK switch is closed)
f_max= f_c + ∆f (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
f
capacitance CL and within the specified
calibration tolerance. The only way to pull the
oscillation frequency is to vary the effective load
XTAL
capacitance CL_eff seen by the crystal.
Figure 3 shows the oscillation frequency of a
crystal as a function of the effective load ^fmax
capacitance. This capacitance changes in
L1
C1
C0
CL _eff
accordance with the logic level of FSKDTA
R1
around the specified load capacitance. The ^fc
figure illustrates the relationship between the
external pulling capacitors and the frequency
deviation.
f _min
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
capacitance could be chosen in order to reduce
CX1 CRO CL
CX1+CRO
(CX1+CX2) CRO
CX1+CX2+CRO
CL _eff
the frequency drift caused by the tolerances of
the 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.
3901072005
Rev. 011
Page 4 of 20
Data Sheet
Jul/12