FSK Modem and DTMF Codec
CMX865
5.2
FSK Modulator
Serial data from the USART is fed to the FSK modulator if V.23, Bell 103 or Bell 202 mode has been
selected.
The FSK modulator generates one of two frequencies according to the transmit mode and the value of
current transmit data bit.
5.3
Tx Filter and Equaliser
The FSK modulator output signal is fed through the Transmit Filter and Equaliser block which limits the
out-of-band signal energy to acceptable limits. In 1200bps modem modes this block includes a fixed
compromise line equaliser which is automatically set for the particular modulation type and frequency
band being employed. This fixed compromise line equaliser may be enabled or disabled by b10 of the
General Control Register. The amount of Tx equalisation provided compensates for one quarter of the
relative amplitude and delay distortion of ETS Test Line 1 over the frequency band used.
5.4
In DTMF/Tones mode this block generates DTMF signals or single or dual frequency tones.
5.5 Tx Level Control and Output Buffer
DTMF/Tone Generator
The outputs (if present) of the Transmit Filter and DTMF/Tone Generator are summed then passed
through the programmable Tx Level Control and Tx Output Buffer to the pins TXA and TXAN. The Tx
Output Buffer has symmetrical outputs to provide sufficient line voltage swing at low values of VDD and to
reduce harmonic distortion of the signal.
© 2005 CML Microsystems Plc
12
D/865/3
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