RFHCS362F Datasheet, PDF (29/60 Pages) Microchip Technology – Code Hopping Encoder with UHF ASK/FSK Transmitter

rfHCS362G/362F
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.
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.
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.
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.
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:
f_transmit = f _XTAL × 32
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, PICmicro^® Microcontroller Oscillator Design
Guide, and AN826 Crystal Oscillator Basics and Crys-
tal Selection for rfPIC™ and PICmicro^® Devices.
The crystal oscillator start time (t_on) is listed in
Table 10-7, Transmitter AC Characteristics.
TABLE 6-1: CRYSTAL PARAMETERS
Sym
Characteristic
fXTAL
Crystal Frequency
CL
Load Capacitance
CO
Shunt Capacitance
ESR
Equivalent Series Resistance
These values are for design guidance only.
Min Max
9.69 15
10
15
—
7
—
60
Units
MHz
pF
pF
Ω
Conditions
Parallel Resonant Mode
© 2002 Microchip Technology Inc.
Preliminary
DS41189A-page 29