CMX867 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'CMX867' PDF : 45 Pages View PDF
Low Power V.22 Modem
CMX867
1.5.7 Rx Modem Filterering and Demodulation
When the receive part of the CMX867 is operating as a modem, the received signal is fed to a bandpass
filter to attenuate unwanted signals and to provide fixed compromise line equalisation for 600 and
1200bps FSK and DPSK modes. The characteristics of the bandpass filter and equaliser are determined
by the chosen receive modem type and frequency band. The line equaliser may be enabled or disabled
by bit 10 of the General Control Register and compensates for one quarter of the relative amplitude and
delay distortion of ETS Test Line 1.
The responses of these filters, including the line equaliser and the effect of external components used in
Figures 4a and 4b, are shown in Figures 6b-e:
10
10
0
0
-10
-10
-20
-20
-30
dB
-40
-30
dB
-40
-50
-50
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
Figure 6b DPSK Rx Filters
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
Figure 6c V.21 Rx Filters
10
10
0
0
-10
-10
-20
-20
-30
dB
-40
-30
dB
-40
-50
-50
-60
-60
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
Figure 6d Bell 103 Rx Filters
Figure 6e V.23/Bell 202 Rx Filters
The signal level at the output of the Receive Modem Filter and Equaliser is measured in the Modem
Energy Detector block, compared to a threshold value, and the result controls bit 10 of the Status
Register.
The output of the Receive Modem Filter and Equaliser is also fed to the FSK or DPSK demodulator
depending on the selected modem type.
© 2004 Consumer Microcircuits Limited
15
D/867/5
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