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
AUX I/O 2
from the
discriminator
RXIN 1
audio call
tone output
BPF
LPF
CTCSS encode
audio tone encode
1
0
CTCSS decoders
voltage
ref
subaudio
output
Vbias
Figure 34: Example Internal Audio Tone Encoder Path
6.4.3 Level Control
The Level Control section, shown in Figure 35, combines TX audio and subaudio signals using a summer,
selectable switch paths, digitally controlled gains, and 0°/180° phase selection to support a variety of radio TX
architectures.
Synthesized radio transmitters can attenuate subaudio tone levels in a manner related to the subaudio tone
frequency. This section, described below, supports several approaches to manage this aspect of designs
based on synthesizers.
• Use an FM modulator having flat subaudio response. This modulator can be driven by the composite sum
of audio and subaudio signals to perform ‘single point modulation.’
• Apply audio signal to the synthesizer VCO input and subaudio signal to the synthesizer reference
oscillator voltage control input. This modulator requires separate audio and subaudio output signals with
their relative levels trimmed to perform ‘two point modulation.’ Some oscillators are reverse acting and so
must have their driving signal inverted before it is applied. Applications requiring summed audio and
subaudio to be applied to both the modulator and synthesizer reference oscillator are also supported.
• Use an FM modulator with attenuating subaudio response. This modulator must be driven by constant
audio levels but subaudio frequency dependent subaudio levels. This driving signal is obtained by varying
the subaudio tone level, according to subaudio tone frequency, and then summing it with the audio signal.
audio
signal
+
subaudio
signal
Vbias
Vbias
0°/180°
phase
0°/180°
phase
Figure 35: Level Control
TXMOD1 21
TXMOD2 20
2003 CML Microsystems Plc
52
D/838/8
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