RFPIC12C509AF datasheet(35/104 Pages) MICROCHIP

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Preliminary

DS70031A-page 33

rfPIC12C509AG/509AF

7.0

UHF ASK/FSK TRANSMITTER

7.1

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 rfPIC12C509AG is capable of

Amplitude

Shift

Keying

(ASK)

modulation.

The

rfPIC12C509AF is capable of ASK or Frequency Shift

Keying (FSK) modulation by employing an internal FSK

switch to pull the transmitter crystal via a second load

capacitor.

Figure 7-1 shows the internal structure of the transmit-

ter. Transmitter connections are independent from the

PICmicro microcontroller unit (MCU) to provide for

maximum design flexibility. Example application cir-

cuits for ASK or FSK modulation are presented at the

end of this section.

The rfPIC12C509AG/509AF are radio frequency (RF)

emitting devices. Wireless RF devices are governed by

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

United States it is the Federal Communications Com-

mittee (FCC) and in Europe it is the European Confer-

ence

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.

7.2

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

PICmicro MCU.

7.3

Crystal Oscillator

The transmitter crystal oscillator is a Colpitts oscillator

that provides the reference frequency to the PLL. It is

independent from the PICmicro 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 7-1. For background

information on crystal selection see Application Note

AN588, PICmicro® Microcontroller Oscillator Design

Guide.

The crystal oscillator start time (ton)islisted in

Table 11-10, Transmitter AC Characteristics.

TABLE 7-1:

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 emmissions 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

—

Ω

These values are for design guidance only.

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