CMX867(2000) Ver la hoja de datos (PDF) - CML Microsystems Plc
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CMX867 Datasheet PDF : 45 Pages
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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.
The FSK demodulator recognises individual frequencies as representing received ‘1’ or ‘0’ data bits:
© 2000 Consumer Microcircuits Limited
14
D/867/1
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