V.23 Compatible Modem
FX604
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
1
1
M0
1
0
TXD = '0'
TXD = '1'
-
0Hz[1]
0
0
450Hz
390Hz
0
1
2100Hz
1300Hz
Note: [1]
TXOP held at approx VDD/2.
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.
ã 1996 Consumer Microcircuits Limited
7
D/604/3
CMLMICRO [ CML MICROCIRCUITS ]
