GMSK Modem Data Pump
Page 30 of 37
MX909A PRELIMINARY INFORMATION
5.4 AC Coupling
For a practical circuit, when AC coupling from the modem's transmit output to the Frequency Modulator and
between the receiver's Frequency Discriminator and the receive input of the modem may be desired. There
are, however, two problems:.
1. AC coupling of the signal degrades the Bit Error Rate performance of the modem (at 8kbits/sec, without
FEC, for different degrees of AC coupling). See Figure 17
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 shown in Figure 18,
the time for this step to decay to 37% of its original value is 'RC' when:
RC = 1/( 2 x π x the 3dB cut-off frequency of the RC network ) and is 8 ms - or 64 bit times at
8kbits/sec for a 20Hz network.
10-1
10-2
10-3
Tx and Rx dc coupled
Tx 5Hz, Rx dc coupled
Tx 5Hz, Rx 10Hz
10-4
Tx 5Hz, Rx 30Hz
Tx 5Hz, Rx 100Hz
10-5
4
5
6
7
8
9
10
11
12
13
SIGNAL-TO-NOISE RATIO (dB) (noise in 8kHz bandwidth)
Figure 17: Typical Bit Error Rates (at 8kbits/sec, without FEC, for different degrees of AC decoupling)
Step Input
to RC Circuit
100%
Output of
RC Circuit
37%
T = RC
Figure 18: Decay Time - AC Coupling
Note: For these reasons the maximum 3dB cut-off frequencies would appear to be approximately 5Hz in the
Tx path and 20Hz in the Rx path at 8kbits/sec.
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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