GMSK Modem
CMX589A
5.2 AC Coupling of Tx and Rx Signals
In practical applications, it is possible to arrange AC coupling between the CMX589A 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 CMX589A 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 CMX589A 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 = 25°C
Tx BT = 0.3
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 =
2p(the3dB cutoff frequency of theRCnetwork)
which is 32ms, or 256 bit times at 8kbps, for a 5Hz network.
Figure 14: Decay time-AC Coupling
ã 2002 CML Microsystems Plc
17
D/589A/4
CMLMICRO [ CML MICROCIRCUITS ]
