High Speed GMSK Modem 4k to 64kbps
Page 16 of 20
MX589
5.2
AC Coupling of Tx and Rx Signals
In practical applications, it is possible to arrange AC coupling between the MX589 Tx Output and the
frequency modulator to cut-off at a very low frequency, such as 5.0Hz. AC coupling between the receive
discriminator and the input of the MX589 may need a shorter time-constant to avoid problems from voltage
steps at the output of the discriminator when changing channels or when the distant transmitter turns on.
For these reasons, as well as to maintain reasonable BER, the optimum –3dB cut-off frequencies are around
5.0Hz in the Tx path and 20.0Hz in the Rx path.
Figure 13 shows the typical static Bit-Error-Rate performance of the MX589 operating under nominal
conditions for various degrees of AC coupling at the Rx input and the Tx output.
Data Rate = 8kbps
VDD = 5.0V
TAMB = 25C
Tx BT = 0.3
10-1
10-2
BER
10-3
10-4
10-5
TX and RX DC coupled
TX 5Hz, RX DC coupled
TX 5Hz, RX 10Hz
TX 5Hz, RX 30Hz
TX 5Hz, RX 100Hz
4
5
6
7
8
9
10
11
12
13
S/N (dB) (noise in 8kHz bandwidth)
Figure 13: Effect of AC Coupling on Typical Bit-Error Rate
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 14,
the time for this step to decay to 37% of its original value is ‘RC’ where:
1
RC =
2π(the 3dB cutoff frequency of theRCnetwork)
which is 32ms, or 256 bit times at 8kbps, for a 5Hz network.
Figure 14: Decay time-AC Coupling
2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 204800103.011
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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