RFHCS362F datasheet(26/54 Pages) MICROCHIP

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rfHCS362G/362F

DS41189B-page 26

6.0

UHF ASK/FSK TRANSMITTER

6.1

Transmitter Operation

The transmitter is a fully integrated UHF ASK/FSK

transmitter consisting of crystal oscillator, Phase-

Locked Loop (PLL), open-collector differential-output

Power Amplifier (PA), and mode control logic. External

components consist of bypass capacitors, crystal, and

PLL loop filter. The rfHCS362G is capable of Amplitude

Shift Keying (ASK) modulation. The rfHCS362F is

capable of ASK or Frequency Shift Keying (FSK) mod-

ulation by employing an internal FSK switch to pull the

transmitter crystal via a second load capacitor.

Figure 2-1 shows the internal structure of the transmit-

ter. Transmitter connections are independent from the

encoder to provide for maximum design flexibility.

Example application circuits for ASK or FSK modula-

tion are presented in Section 1.2.

The rfHCS362G/362F are radio frequency (RF) emit-

ting devices. Wireless RF devices are governed by a

country’s regulating agency. For example, in the United

States it is the Federal Communications Committee

(FCC) and in Europe it is the European Conference of

Postal

and

Telecommunications

Administrations

(CEPT). It is the responsibility of the designer to ensure

that their end product conforms to rules and regulations

of the country of use and/or sale.

6.2

Supply Voltage (VDDRF, VSSRF)

Pins VDDRF and VSSRF supply power and ground

respectively to the transmitter. These power pins are

separate from power supply pins VDD and VSS to the

encoder.

6.3

Crystal Oscillator

The transmitter crystal oscillator is a Colpitts oscillator

that provides the reference frequency to the PLL. It is

independent from the encoder oscillator. An external

crystal or AC coupled reference signal is connected to

the XTAL pin. The transmit frequency is fixed and

determined by the crystal frequency according to the

formula:

Due to the flexible selection of transmit frequency, the

resulting crystal frequency may not be a standard off-

the-shelf value. Therefore, for some carrier frequencies

the designer will have to consult a crystal manufacturer

and have a custom crystal manufactured. Crystal

parameters are listed in Table 6-1. For background

information on crystal selection see Application Note

AN588, PIC® Microcontroller Oscillator Design Guide,

and AN826 Crystal Oscillator Basics and Crystal Selec-

tion for rfPIC™ and PIC MCU Devices.

The crystal oscillator start time (ton) is listed in

Table 10-7, Transmitter AC Characteristics.

TABLE 6-1:

CRYSTAL PARAMETERS

RF devices require correct board level implementa-

tion in order to meet regulatory requirements. Layout

considerations are listed at the end of each subsec-

tion. It is best to place a ground plane on the PCB to

reduce radio frequency emissions and cross talk.

Layout Considerations - Provide low impedance

power and ground traces to minimize spurious emis-

sions. A two-sided PCB with a ground plane on the

bottom layer is highly recommended. Separate

bypass capacitors should be connected as close as

possible to each of the supply pins VDD and VDDRF.

Connect VSS and VSSRF to the ground plane using

separate PCB vias. Do not share a PCB via with mul-

tiple ground traces.

XTAL

transmit

Sym

Characteristic

Min

Max

Units

Conditions

fXTAL

Crystal Frequency

9.69

MHz

Parallel Resonant Mode

Load Capacitance

Shunt Capacitance

—

ESR

Equivalent Series Resistance

—

These values are for design guidance only.

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