4-Level FSK Modem Data Pump
Page 41 of 50
MX929B PRELIMINARY INFORMATION
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:
1
RC =
25(3dB cut - off frequency of the RC network)
which is 32ms, or 153 symbol times at 4800 symbols/sec (9600bps) for a 5Hz network.
Step Input
to RC Circuit
100%
Output of
RC Circuit
37%
T = RC
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).
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