Section 2. Rf Front End; Introduction; Functional Description; Rf Transmitter - Motorola M68HC08 Designer Reference Manual

Designer Reference Manual — DRM055

2.1 Introduction

2.2 Functional Description

2.3 RF Transmitter

High Data Rate Wireless USB Optical Mouse Solution
MOTOROLA
Freescale Semiconductor, Inc.
A high data rate 27-MHz RF link is designed as the wireless communication
media for this application. The RF frequency is determined by the crystal
frequency used at the oscillator circuit stage and there are two selectable
channels for transmission. The transmission data rate is 4.8 kbps.
The high-frequency carrier signal on the transmitter side is modulated by the
digital encoded data from the QY4 using a FSK modulation scheme. The
modulated RF signal is propagated through free-air space and received by an
integrated chip, the 3361, on the receiver side which includes all mixer, local
oscillator, and demodulator circuits. The demodulated data output is received
by the JB12 for decoding and processing. The data will then be converted to
the USB mouse report format and sent to the host.
The RF transmitter consists of three parts:
• The crystal type oscillator
• The FSK modulation switching circuit
• The RF amplifier
The crystal oscillator works with a crystal frequency at half of the target channel
frequency and the second harmonic frequency is filtered out by the RF amplifier
together with a high Q-factor antenna. For example, a 13.5225-MHz crystal is
used for a frequency channel at 27.045-MHz.
The FSK modulation is achieved by changing the loading capacitance at the
crystal with a transistor switching circuit controlled by the encoding data
generated from the MCU. The maximum data rate for a particular FSK
transmission is limited by the RF bandwidth of the system and controlled by the
frequency deviation which represents the logic "0" and logic "1" data. In
general, the frequency deviation should be adjusted proportionally to the
change on the required transmission data rate. Higher data rates require more
bandwidth / frequency deviation. The frequency deviation for this 4.8-kbps data
RF Front End
For More Information On This Product,
Go to: www.freescale.com

Section 2. RF Front End

DRM055
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