MX589 Consumer Microcircuits Limited, MX589 Datasheet - Page 16

• Manufacturer Part Number: MX589
• Description: Mx589 High Speed Gmsk Modem
• Manufacturer: Consumer Microcircuits Limited

High Speed GMSK Modem 4k to 64kbps
5.2
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.
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:
which is 32ms, or 256 bit times at 8kbps, for a 5Hz network.
©2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
AC Coupling of Tx and Rx Signals
BER
10
10
10
10
10
-3
-2
-4
-5
-1
4
Data Rate = 8kbps
Figure 13: Effect of AC Coupling on Typical Bit-Error Rate
www.mxcom.com tel: [phone] 744 5050 fax: [phone]
5
RC
TX and RX DC coupled
TX 5Hz, RX DC coupled
TX 5Hz, RX 100Hz
TX 5Hz, RX 10Hz
TX 5Hz, RX 30Hz
=
2
6
π
Figure 14: Decay time-AC Coupling
(the
3dB
S/N (dB) (noise in 8kHz bandwidth)
V
7
DD
cutoff
= 5.0V
Page 16 of 20
frequency
8
All trademarks and service marks are held by their respective companies.
1
T
AMB
9
of
= 25C
the
RC
10
network)
Tx BT = 0.3
11
12
Doc. # 204800103.011
13
MX589