FX909AP4 View Datasheet(PDF) - CML Microsystems Plc
Part Name
Description
MFG CO.
'FX909AP4' PDF : 47 Pages View PDF
Wireless Modem Data Pump
FX909A
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 π 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.
© 1996 Consumer Microcircuits Limited
38
D/909A/4
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