V23 / Bell 202 Modem
CMX624
1.5.2
Rx Input Amplifier
This amplifier, with suitable external components, is used to adjust the received signal to the correct
amplitude for the FSK receiver and Energy Detect circuits and may also form part of a 2-wire or 4-wire
hybrid circuit; see Section 1.6.1.
1.5.3
Receive Filter and Equaliser
This block includes a bandpass filter whose characteristics are set by bits 4 and 5 of the FSK MODE
Register according to the receive operating mode (Call Progress, 75/150bps FSK or 1200bps FSK). It is
used to attenuate out of band noise and interfering signals, especially the locally generated transmit FSK
signal which could otherwise interfere with the received FSK signal when the modem is operating in 2-wire
duplex mode. When receiving 1200bps FSK data an optional equaliser section, enabled by setting bit 6 of
the FSK MODE Register, compensates for one-half of the ETS Test Line 1 characteristics shown in Figure
3b.
0
-10
dB -20
1200bps
75/150 bps
Call Progress
-30
-40
100
1000
Hz
10000
Figure 3a Rx Frequency Responses with Line Interface,
see section 1.6.1 (equaliser disabled)
5
4
3
dB
ms
2
dB wrt 800Hz
ms wrt 1700Hz
1
0
0
500
1000
1500
2000
Hz
2500
3000
3500
4000
Figure 3b ETS 300 114 Test Line 1 Characteristics (Normalised)
2003 CML Microsystems Plc
7
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
