Low Power V.22 bis Modem
CMX868
1.5.7 Rx Modem Filterering and Demodulation
When the receive part of the CMX868 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, 1200 and
2400 bps FSK, DPSK and QAM 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 7b-e:
10
0
10
0
-10
-20
-30
-40
-50
-60
-10
-20
-30
-40
-50
-60
dB
dB
0
0.5
1
1.5
2
2.5
3
3.5
4
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
kHz
Figure 7b QAM/DPSK Rx Filters
Figure 7c V.21 Rx Filters
10
0
10
0
-10
-20
-30
-40
-50
-60
-10
-20
-30
-40
-50
-60
dB
dB
0
0.5
1
1.5
2
2.5
3
3.5
4
0
0.5
1
1.5
2
2.5
3
3.5
4
kHz
kHz
Figure 7d Bell 103 Rx Filters
Figure 7e 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 QAM/DPSK demodulator
depending on the selected modem type.
ã 2004 CML Microsystems Plc
15
D/868/9
CMLMICRO [ CML MICROCIRCUITS ]
