Digital PMR Radio Processor
CMX7131/CMX7141
4.1 Recommended External Components
R1 100k
R2 100k
R3 100k
R4 100k
R5 See note 2
R6 100k
R7 See note 3
R8 100k
R9 See note 4
R10 100k
C1 18pF
C2 18pF
C3 10nF
C4 not used
C5 1nF
C6 100pF
C7 100nF
C8 100pF
C9 100pF
C10 not used
C11 not used
C12 100pF
C13 See note 5
C14 100pF
C15 See note 5
C16 200pF
C17 10µF
C18 10nF
C19 10nF
C20 10µF
C21 10nF
C22 10nF
C23 10nF
C24 10µF
C25
C26
C27
C28
X1 6.144MHz
See note 1
Resistors 5%, capacitors and inductors 20% unless otherwise stated.
Notes:
1. X1 can be a crystal or an external clock generator; this will depend on the application. The tracks
between the crystal and the device pins should be as short as possible to achieve maximum stability
and best start up performance. By default, a 19.2MHz oscillator is assumed (in which case C1 and
C2 are not required), other values could be used if the various internal clock dividers are set to
appropriate values.
2. R5 should be selected to provide the desired dc gain of the discriminator input, as follows:
GAIN
= 100k / R5
DISC
The gain should be such that the resultant output at the DISCFB pin is within the DISC input signal
range specified in 6.13.2. For 4-FSK modulation, this signal should be dc coupled from the
limiter/discriminator output.
3. R7 should be selected to provide the desired dc gain (assuming C13 is not present) of the alternative
input as follows:
GAIN = 100k / R7
ALT
The gain should be such that the resultant output at the ALTFB pin is within the alternative input
signal range specified in 6.13.
4. R9 should be selected to provide the desired dc gain (assuming C15 is not present) of the
microphone input as follows:
GAIN = 100k / R9
MIC
The gain should be such that the resultant output at the MICFB pin is within the microphone input
signal range specified in 6.13.1. For optimum performance with low signal microphones, an additional
external gain stage may be required.
5. C13 and C15 should be selected to maintain the lower frequency roll-off of the MIC and ALT inputs as
follows:
C13 1.0µF GAIN
ALT
C15 30nF GAIN
MIC
6. ALT and ALTFB connections allow the user to have a second discriminator or microphone input.
Component connections and values are as for the respective DISC and MIC networks. If this input is
not required, the ALT pin should be connected to AV
.
SS
7. AUDIO output is only used in this Function ImageTM when SPI-Codec mode has been selected. It
may also be used by analogue Function ImagesTM which may also be used on this device.
8. A single 10µF electrolytic capacitor (C24, fitted as shown) may be used for smoothing the power
supply to both VDEC pins, providing they are connected together on the pcb with an adequate width
power supply trace. Alternatively, separate smoothing capacitors should be connected to each VDEC
pin. High frequency decoupling capacitors (C3 and C23) must always be fitted as close as possible to
both VDEC pins.
2014 CML Microsystems Plc
Page 13
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CMLMICRO [ CML MICROCIRCUITS ]
