V.23 Compatible Modem

FX604

© 1996 Consumer Microcircuits Limited

7

D/604/3

1.5.5

Energy Detector

This block operates by measuring the level of the signal at the output of the Receive Filter, and comparing it

against a preset threshold.

The DET output will be set high when the level has exceeded the threshold for sufficient time. Amplitude and

time hysteresis are used to reduce chattering of the DET output in marginal conditions.

Note that this circuit may also respond to non-FSK signals such as speech.

See section 1.7.1 for definitions of Teon and Teoff

Figure 4 FSK Level Detector Operation

1.5.6

FSK Demodulator

This block converts the 1200bits/sec FSK input signal to a logic level received data signal which is output via

the RXD pin as long as the Data Retiming function is not enabled (see section 1.5.8). This output does not

depend on the state of the DET output.

When the Rx 1200bits/sec mode is 'off' or in 'ZP' the DET and RXD pins are held low.

Note that in the absence of a valid FSK signal, the demodulator may falsely interpret speech or other

extraneous signals as data. For this reason it is advised that the RXD pin is read only when data is expected.

1.5.7

FSK Modulator and Transmit Filter

These blocks produce a tone according to the TXD, M0 and M1 inputs as shown in the table below, assuming

data retiming is not being used:

M1

M0

TXD = '0'

TXD = '1'

1

1

-

1

0

0

0

450Hz

390Hz

0

1

2100Hz

1300Hz

Note:

[1]

TXOP held at approx V

When modulated at the appropriate baud rates, the Transmit Filter and associated external components (see

section 1.6.1) limit the FSK out of band energy sent to the line in accordance with Figures 5a and 5b,

assuming that the signal on the line is at -6dBm or less.