MLX72013 Datasheet, PDF (5/21 Pages) Melexis Microelectronic Systems – FSK/ASK Transmitter

MLX72013
433MHz
FSK/ASK Transmitter
7. Functional Description
7.1 Crystal Oscillator
A Colpitts crystal oscillator with integrated functional capacitors is used as the reference oscillator for the PLL
synthesizer. The equivalent input capacitance CRO offered by the crystal oscillator input pin ROI is about
18pF. The crystal oscillator is provided with an amplitude control loop in order to have a very stable
frequency over the specified supply voltage and temperature range in combination with a short start-up time.
7.2 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
fmin= fc - ∆f (FSK switch is closed)
fmax= fc + ∆f (FSK switch is open)
7.3 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
L1
crystal as a function of the effective load f_max
capacitance. This capacitance changes in
C1
C0
accordance with the logic level of FSKDTA
around the specified load capacitance. The
f_c
R1
figure illustrates 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
CX1 CRO CL
(CX1+CX2) CRO
capacitance could be chosen in order to reduce
CX1+CRO
CX1+CX2+CRO
the frequency drift caused by the tolerances of
the chip and the external pulling capacitors.
Fig. 3: Crystal pulling characteristic
CL _eff
CL _eff
3901072013
Rev. 007
Page 5 of 21
Data Sheet
Oct/14