TH72001

315MHz

FSK Transmitter

3901072001

Page 4 of 20

Data Sheet

Rev. 007

June/07

f

eff

CL

eff

CL

R1

C1

C0

L1

XTAL

CL

CX1 CRO

CX1+CRO

(CX1+CX2) CRO

CX1+CX2+CRO

2

Functional Description

2.1

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 fre-

quency over the specified supply voltage and temperature range in combination with a short start-up time.

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 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

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.

Fig. 2:

Crystal pulling circuitry

FSKDTA

Description

0

1

2.3

Crystal Pulling

A crystal is tuned by the manufacturer to the

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-

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.

Fig. 3:

Crystal pulling characteristic

CX2

VCC

XTAL

CX1

ROI

FSKSW

VEE