4-Level FSK Modem Data Pump

FX929A

© 1996 Consumer Microcircuits Limited

37

D/929A/4

S/N dB (Noise in 20 to 9600Hz band)

1E-4

1E-3

1E-2

1E-1

4

5

6

7

8

9

10

11

12

13

14

Tx & Rx DC coupled

Tx 5Hz, Rx DC

Tx 5Hz, Rx 5Hz

Tx 5Hz, Rx 10Hz

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.

37%

T=RC

100%

Step input

to RC circuit

Output of

RC circuit

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).