V.22 bis Modem with AT Commands
CMX866
1.5.11 Rx Modem Filterering and Demodulation
When the receive part of the CMX866 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 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 fixed compromise line equaliser may be
enabled or disabled by bit 1 of the S24 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 8b-e:
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Figure 8b QAM/DPSK Rx Filters
Figure 8c V.21 Rx Filters
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Figure 8d Bell 103 Rx Filters
Figure 8e 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 is automatically handled by the on-
chip µController.
The output of the Receive Modem Filter and Equaliser is also applied to the FSK or QAM/DPSK
demodulator depending on the selected modem type.
The FSK demodulator recognises individual frequencies as representing received ‘1’ or ‘0’ data bits:
ã 2004 CML Microsystems Plc
34
D/866/4
CMLMICRO [ CML MICROCIRCUITS ]
