GMSK Packet Data Modem
CMX909B
Figure 17 Typical Bit Error Rates (at 8kbits/sec, without FEC, for different degrees of ac coupling)
Secondly, 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 below, the time for this step to decay to 37% of its original value is ‘RC’ where
RC = 1/( 2 x p x the 3dB cut-off frequency of the RC network )
and is 8 msec - or 64 bit times at 8kbits/sec for a 20Hz network. See Figure 18.
Figure 18 Decay Time - AC Coupling
For these reasons the maximum 3dB cut-off frequencies would seem to be around 5Hz in the Tx
path and 20Hz in the Rx path at 8kbits/sec.
ã 2001 Consumer Microcircuits Limited
41
D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
