CMX838 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX838' PDF : 71 Pages View PDF
FRS/PMR446/GMRS Family Radio Processor
CMX838
6.2 Basic FRS Radio Architecture
An FRS radio transmits a baseband voice signal using RF FM modulation in the UHF band. A form of
selective calling is highly desirable in FRS applications to help coordinate the use of the available RF
channels. The most popular technique divides the available audio spectrum into two frequency bands, audio
and subaudio, to allow simultaneous transmission of voice in the audio band and a control signal in the
subaudio band.
A transmitting radio combines audio voice and subaudio tone signals into a composite signal and FM
modulates it into RF with properly adjusted frequency deviation and bandwidths. A receiving radio
demodulates the RF signal to recover the baseband composite signal and decodes the embedded subaudio
tone to control the loudspeaker signal path. This technique is used to enable loudspeaker operation only
when the appropriate subaudio control signal is received with voice. An acronym for this technique is
CTCSS – continuous tone controlled selective squelch.
The advanced tone processing functions of the CMX838 support the use of subaudio tones to selectively call
different groups of receivers. For example, a group call feature would allow selective calling of ‘parents,’
‘children’ and ‘entire family’ groups to better coordinate radio use.
FRS radios usually support multiple RF channels via synthesized radio techniques to optimize cost and size.
Figure 23 and Figure 24 are conceptual diagrams that identify the audio processing, subaudio (encoder and
decoder), modulation, baseband clock, and synthesizer functions described above. Note that because FRS
radios are half duplex, several of the functions shown serve both TX and RX modes of operation.
IN
Voice (audio) Input Signal
OUT
baseband crystal
REF clock from TCXO
RF input signal
Audio Processing
+
Subaudio Tone
Encoder
XTAL Oscillator & Baseband
Clock Generation
clocks
Level
Control
RF Synthesizer and charge pump
PA
VCO +
Figure 23: Basic FRS Radio Tx Architecture
IN
LNA
Rx input
signal
Discriminator
Audio Processing
speaker
driver
ceramic
IF
Subaudio Decoder
Loudspeaker control
baseband crystal
XTAL Oscillator & Baseband
Clock Generation
clocks
REF clock from TCXO
RF input signal
RF Synthesizer and charge pump
VCO +
2003 CML Microsystems Plc
Figure 24: Basic FRS Radio Rx Architecture
46
OUT
D/838/8
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