v.23 Compatible Modem
6
MX604 PRELIMINARY INFORMATION
4.2
Mode Control Logic
M1
0
0
1
1
M0
0
1
0
1
[1] If enabled.
Rx Mode
1200bps
off
1200bps
'Zero-Power'
Tx Mode
75bps
1200bps
off
Data Retime
[1]
Rx
Tx
Rx
-
The MX604's operating mode is determined by the logic levels applied to the M0 and M1 input pins:
Note: On applying power to the device the mode must be set to 'ZP', i.e. M0=1, M1=1, until V
DD
has
stabilized.
In the 'Zero-Power' mode, power is removed from all internal circuitry. When leaving 'Zero-Power' mode there
must be a 20ms delay before any Tx data is passed to, or Rx data read from, the device to allow the bias
level, filters and oscillator to stabilize.
4.3
Rx Input Amplifier
The Rx Input Amplifier is used to adjust the signal received to the correct amplitude for the FSK receiver and
Energy Detect circuits (see section 5.1).
4.4
Receive Filter and Equalizer
The Receive Filter and Equalizer is used to attenuate out of band noise and interfering signals, especially the
locally generated 75bps transmit tones which might otherwise reach the 1200bps FSK Demodulator and
Energy Detector circuits. This block also includes a switchable equalizer section. When the RXEQ pin is low
the overall group delay of the receive filter is flat over the 1200bps frequency range. If the RXEQ pin is high
the receive filter's typical overall group delay will be as shown in Figure 3.
0.025
Delay/ms
0
-0.025
-0.05
-0.075
-0.1
-0.125
-0.15
500
1000
1500
Frequency/Hz
2000
2500
Figure 3: Rx Equalizer Group Delay (RXEQ = 1) with respect to 1700Hz
1998
MXCOM, INC.
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CMLMICRO [ CML MICROCIRCUITS ]
