Rf Receiver; Pcb Layout Guidelines - Motorola M68HC08 Designer Reference Manual

RF Front End

2.4 RF Receiver

2.5 PCB Layout Guidelines

DRM055
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Freescale Semiconductor, Inc.
rate wireless mouse application is increased from ±2.5 kHz to ±4.5 kHz which
is controlled by the crystal characteristics.
The gain of the RF amplifier in the final stage should be adjusted to
compensate the gain loss at the oscillator stage with extend bandwidth
operation. Two stages of RF amplifier are used in this reference design to
maintain the performance in communication distance.
The RF receiver is implemented by using a single-chip solution (3361
compatible part) which includes:
• A frequency downward conversion mixer
• A local oscillator circuit
• A baseband FSK quadrature demodulation unit
The RF input signal from the antenna, is frequency down converted into an IF
signal at 455 kHz by the mixer and oscillator circuits. The IF frequency value is
equal to the RF input frequency plus or minus the LO input frequency. The
higher frequency components should be filtered out by using a passive IF filter.
The bandwidth should be increased from 15 kHz to 20 kHz to match the data
rate change on the transmitter side.
However, the image frequency component would not be filtered out by the IF
filter. This should be considered in the PCB layout in order to prevent any noise
component at image frequency to be injected into the mixer input. An example
of this would be a noise pattern generated from the MCU.
The frequency used for a 3361 mixer local oscillator is selected by the crystal
connected at the oscillator base input pin. There are two frequency channels
that can be selected to match the transmitter channel.
Care should be taken in PCB layout in order to avoid any noise generated from
MCU coupling into the RF stage. For example:
• The power supply traces used for digital and analog circuit blocks should
be separated.
• The location of decoupling capacitors should be as close as possible to
device's supply input pins (V
• The V to V ground loop area should be reduced to minimize the
DD SS
magnetic coupling effect.
• The PCB trace loop formed by any input/output (I/O) signal pin should
be kept to a minimum.
• The RF receiver uses a loop antenna formed by using a PCB trace line.
RF Front End
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/V or V /GND).
DD SS CC
High Data Rate Wireless USB Optical Mouse Solution
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