RFPIC12F675 Datasheet, PDF (49/136 Pages) Microchip Technology – FLASH-Based Microcontroller with ASK/FSK Transmitter

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
Device
rfPIC12F675K
rfPIC12F675F
rfPIC12F675H
Frequency
290-350 MHz
390-450 MHz
850-930 MHz
Modulation
ASK/FSK
ASK/FSK
ASK/FSK
The internal structure of the transmitter is shown in
Figure 9-1. A 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.
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.
FIGURE 9-1: TRANSMITTER BLOCK
DIAGRAM
Clock
Divider
Crystal
Oscillator
Phase/Freq
Detector
Divide
by 32
Charge
Pump
PS
DATAASK
RFEN
DATAFSK
Voltage
Controlled
Oscillator
RF Power
Amplifier
RF
Control
Logic
FSK Switch
REFCLK
RFXTAL
LF
ANT
VDDRF
VSSRF
VSSRF
FSKOUT
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.
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.
 2003-2013 Microchip Technology Inc.
Preliminary
DS70091B-page 49