Data Sheet
4-Level FSK Modem Data Pump Page 38 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: 336 744 5054 Doc. # 20480170.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.
2. Any ac coupling at the receive input will transform any step in the voltage at the discriminator output to a
slowly decaying pulse which can confuse the modem's level measuring circuits. As illustrated in Figure 25
below, the time for this step to decay to 37% of its original value is 'RC' where:
network) RC the offrequency off-cut (3dB2
1
=RC
π
which is 32ms, or 153 symbol times at 4800 symbols/sec (9600bps) for a 5Hz network.
37%
T = RC
100%
Step Input
to RC Circuit
Output of
RC Circuit
Figure 25: Decay Time - AC Coupling
In general, it is best to DC couple the receiver discriminator to the modem and ensure that any AC coupling to
the transmitter's frequency modulator has a -3dB cut-off frequency of no higher than 5Hz for 4800
symbols/sec (9600bps).