Datasheet > TH72005_12

TH72005_12

更新时间： 2022-02-26 12:04:55

品牌	Logo	应用领域
迈来芯 - MELEXIS		/
页数	文件大小	规格书
20页	850K
描述
FSK/ASK Transmitter

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中文翻译

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数据手册

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TH72005_12 数据手册

TH72005

315MHz

FSK/ASK Transmitter

2.2 FSK Modulation

FSK modulation can be achieved by pulling the

CMOS-

VCC

crystal oscillator frequency.

Fig. 2: Crystal pulling circuitry

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_cto 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

ROI

XTAL

CX1

FSKSW

CX2

VEE

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.

FSKDTA

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 f₀at a given load

capacitance CL and within the specified

calibration tolerance. The only way to pull the

oscillation frequency is to vary the effective load

capacitance CL_effseen by the crystal.

XTAL

Figure 3 shows the oscillation frequency of a

crystal as a function of the effective load

capacitance. This capacitance changes in

accordance with the logic level of FSKDTA

around the specified load capacitance. The

figure illustrates the relationship between the

external pulling capacitors and the frequency

deviation.

max

eff

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

eff

CX1 CRO CL

CX1+CRO

(CX1+CX2) CRO

CX1+CX2+CRO

band FSK applications,

higher load

capacitance 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

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

1 2 345 6 7...20

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