Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
Two analogue mode audio filters are provided, one for 25kHz channel spacing and the other for 12.5kHz channel
spacing. These are designed for use in ETS-300-086 and/or TIA/EIA-603 compliant applications. Both filters
attenuate sub-audio frequencies below 250Hz by more than 33dB wrt the signal level at 1kHz.
The filter characteristics of the 25kHz channel filter fits the filter template shown in Figure 12. This filter
facilitates implementation of systems compliant with ETS-300-086 (25kHz channel spacing).
10
reference 0dB at 1kHz
+0.5dB/-2dB wrt ref.
3kHz
0
-10
-20
-14dB/octave
-30
-33dB
-40
-50
-60
10
100
1000
10000
100000
Frequency (Hz)
Figure 12 25kHz Channel Audio Filter Response Template
The filter characteristics of the 12.5kHz channel filter fits the filter template shown in Figure 13 (solid outline).
This filter facilitates implementation of systems compliant with ETS-300-086 (12.5kHz channel spacing) and
TIA/EIA-603 ‘A’ and ‘B’ bands. To achieve attenuation above 3kHz of better than - 100dB/decade for TIA/EIA-
603 ‘C’ bands (dashed outline), additional external circuitry is required, such as suggested in section 1.4.
10
reference 0dB at 1kHz
+0.5dB/-2dB wrt ref.
3kHz
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-60dB/decade wrt ref.
-33dB
(-50dB)
20kHz
-100dB/decade
(-82.5dB)
20kHz
10
100
1000
10000
100000
Frequency (Hz)
Figure 13 12.5kHz Channel Audio Filter Response Template
The CMX880 provides selectable pre-emphasis filtering of +6dB per octave from 300Hz to 3000Hz, matching the
template shown in Figure 14.
ã 2001 Consumer Microcircuits Limited
28
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
