DB929B datasheet(41/51 Pages) CMLMICRO

41 / 51 page

4-Level FSK Modem Data Pump

Page 41 of 50

MX929B PRELIMINARY INFORMATION

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Doc. # 20480171.003

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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)

the

frequency

off

cut

(3dB

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).

DB929B Datasheet(PDF) 41 Page - CML Microcircuits