FX929A Datasheet, PDF (37/44 Pages) CML Microcircuits – 4-Level FSK Modem Data Pump

4-Level FSK Modem Data Pump
1E-1
FX929A
1E-2
1E-3
1E-4
4
Tx & Rx DC coupled
Tx 5Hz, Rx DC
Tx 5Hz, Rx 5Hz
Tx 5Hz, Rx 10Hz
5
6
7
8
9
10
11
12
13
14
S/N dB (Noise in 20 to 9600Hz band)
Figure 18 Effect of AC Coupling on BER
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 in Figure 19
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 )
which is 32 msec, or 153 symbol times at 4800 symbols/sec, for a 5Hz network.
Step input
to RC circuit
Output of
RC circuit
100%
37%
T=RC
Figure 19 Decay Time - AC Coupling
In general, it will be best to dc couple the receiver discriminator to the modem, and to ensure that any ac
coupling to the transmitter's frequency modulator has a -3dB cut-off frequency of no higher than 5Hz (for 4800
symbols/sec).
© 1996 Consumer Microcircuits Limited
37
D/929A/4