TH71221 Datasheet, PDF (17/44 Pages) Melexis Microelectronic Systems – 27 to 930MHz FSK/FM/ASK Transceiver

TH71221
27 to 930MHz
FSK/FM/ASK Transceiver
3.3.5 FSK Modulation
• FSK modulation via crystal pulling
FSK modulation can be achieved by pulling the crystal oscillator
frequency. A CMOS-compatible data stream applied at pin
IN_DTA digitally modulates the XOSC via an integrated NMOS
switch. Two external pulling capacitors CX1 and CX2 allow the
FSK deviation Δf and center frequency f_c to be adjusted inde-
pendently. At IN_DTA = LOW CX2 is connected in parallel to CX1
leading to the low-frequency component of the FSK spectrum
(f_min); while at IN_DTA = HIGH CX2 is deactivated and the XOSC
is set to its high frequency, leading to f_max.
IN_DTA
0
1
Description
f_min = f_c - Δf (FSK switch is closed)
f_max = f_c + Δf (FSK switch is open)
VCC
36pF
RO
I_RO
36pF
XTAL
CX2
VEE
FSKSW
CX1
Fig. 9: Crystal Pulling Circuit
An external reference signal can be directly AC-coupled to the reference oscillator input pin RO. Then the
transceiver is used without a XTAL. Now the reference signal sets the carrier frequency and has to contain
the FSK (or FM) modulation
• FSK modulation via direct VCO modulation
Alternatively FSK or FM can be achieved by injecting the modu-
lating signal into the loop filter to directly control the VCO fre-
quency. Fig. 10 shows a circuit proposal for direct VCO modula-
tion. This circuit is recommended for data rates in excess of about
20 kbps NRZ. An external VCO tuning varactor should be added
for narrow-band applications, for example at channel spacings of
25 kHz. For details please refer to the application notes “TH7122
and TH71221 High Speed Data Communication” and “TH7122
and TH71221 Used In Narrow Band FSK Applications” as well as
to the “TH7122 and TH71221 Cookbook”
IN_DTA
VCC
CB6
CM1
RF
L0
RM1
CF1
CF2
17 FS0/SDEN
18 VEE_DIG
19 FS1/LD
20 VCC_PLL
21 TNK_LO
22 VEE_PLL
23 LF
24
Fig. 10: Circuit schematic for direct VCO modulation
3.3.6 Crystal Tuning
A crystal is tuned by the manufacturer to the re-
f
quested oscillation frequency f₀ for a certain load
capacitance CL within the specified calibration
tolerance. The only way to tune this oscillation
frequency is to vary the effective load capacitance
f _max
CL_eff seen by the crystal.
Figure 8 shows the oscillation frequency of a crys-
tal in dependency on the effective load capaci-
f_o
tance. This capacitance changes in accordance
with the logic level of IN_DTA around the specified
load capacitance. The figure illustrates the rela- ^fmin
tionship between the external pulling capacitors
and the frequency deviation.
XTAL
L1
C1
C0
R1
CL _eff
Fig. 11: Crystal Tuning Characteristic
CX1 CRO CL
CX1+CRO
(CX1+CX2) CRO
CX1+CX2+CRO
CL _eff
39010 071221
Rev. 005
Page 17 of 44
Data Sheet
June/07