rfPIC12F675F datasheet(49/136 Pages) MICROCHIP

49 / 136 page

 2003-2013 Microchip Technology Inc.

Preliminary

DS70091B-page 49

rfPIC12F675

9.0

UHF ASK/FSK TRANSMITTER

9.1

Transmitter Operation

The transmitter is a fully integrated UHF ASK/FSK

transmitter consisting of crystal oscillator, Phase-

Locked Loop (PLL), Power Amplifier (PA) with open-

collector output, and mode control logic. There are 3

variations of this device to optimize its performance for

the most commonly used frequency bands.

TABLE 9-1:

FREQUENCY BANDS

The internal structure of the transmitter is shown in

Figure 9-1.

Colpitts

oscillator

generates

the

reference frequency set by the attached crystal. The

voltage controlled oscillator (VCO) converts the voltage

on the LF pin to a frequency. This frequency is divided

by 32 and compared to the crystal reference. If the

frequency or phase does not match the reference, the

charge pump corrects the voltage on the LF pin. The

VCO output signal is also amplified by the PA, whose

single ended output drives the user’s antenna.

The external components required are a crystal to set

the transmit frequency, a supply bypass capacitor, and

two to seven biasing/impedance matching components

to get maximum power to the antenna. The two control

signals from the microcontroller are connected exter-

nally for maximum design flexibility. The rfPIC12F675

is capable of transmitting data by Amplitude Shift

Keying (ASK) or Frequency Shift Keying (FSK).

The rfPIC12F675 is a radio frequency (RF) emitting

device. 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.

FIGURE 9-1:

TRANSMITTER BLOCK

DIAGRAM

9.2

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

microcontroller

. Both VSSRF pins should be tied to the

ground plane with the shortest possible traces. The

microcontroller ground should be tied to the same RF

ground potential. However, the VDDRF supply can be at

a different potential than the microcontroller as long as

the RFEN and DATA input levels are within specifica-

tion limits.

Device

Frequency

Modulation

rfPIC12F675K

290-350 MHz

ASK/FSK

rfPIC12F675F

390-450 MHz

ASK/FSK

rfPIC12F675H

850-930 MHz

ASK/FSK

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 required to place a ground plane on the PCB

to reduce unwanted radio frequency emissions.

Layout Considerations - Provide low impedance

power and ground traces to minimize spurious

emissions. 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 both VSSRF pins to the ground plane using

multiple PCB vias adjacent to the VSSRF pads. Do not

share these PCB vias with other ground traces. Filter

the VDDRF with an RC filter if the microcontroller noise

spurs exceed regulatory limits.

Clock

Divider

Voltage

Controlled

Oscillator

RF Power

Amplifier

Crystal

Oscillator

Phase/Freq

Detector

Charge

Pump

FSK Switch

REFCLK

Control

Logic

FSKOUT

VDDRF

VSSRF

RFEN

ANT

RFXTAL

DATAASK

Divide

by 32

DATAFSK

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