Digital PMR Radio Processor
CMX7131/CMX7141
Figure 5 CMX7131 Power Supply and Decoupling
Component Values as per Figure 3
Notes:
It is important to protect the analogue pins from extraneous inband noise and to minimise the impedance
between the CMX7131/CMX7141 and the supply and bias de-coupling capacitors. The de-coupling
capacitors C3, C7, C18, C19, C21, C22 and C24 should be as close as possible to the
CMX7131/CMX7141. It is therefore recommended that the printed circuit board is laid out with separate
ground planes for the AV and DV supplies in the area of the CMX7131/CMX7141, with provision to
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make links between them, close to the CMX7131/CMX7141. Use of a multi-layer printed circuit board will
facilitate the provision of ground planes on separate layers.
VBIAS is used as an internal reference for detecting and generating the various analogue signals. It must be
carefully decoupled, to ensure its integrity, so apart from the decoupling capacitor shown, no other loads
should be connected. If VBIAS needs to be used to set the discriminator mid-point reference, it should be
buffered with a high input impedance buffer.
The single-ended microphone input and audio output must be ac coupled (as shown), so that their return
paths can be connected to AV
advised.
without introducing dc offsets. Further buffering of the audio output is
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The crystal, X1, may be replaced with an external clock source.
2014 CML Microsystems Plc
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