Low Power V.32 bis Modem
CMX869
5.2
FSK and QAM Modulators
Serial data from the USART is fed to the FSK modulator if V.21, V.23, Bell 103 or Bell 202 mode has been
selected, or to the QAM modulator for V.22bis, V.22, V.32bis and V.32 modes.
The FSK modulator generates one of two frequencies according to the transmit mode and the value of
current transmit data bit.
In V.22bis and V.22 modes, QAM modulation is applied to a carrier of 1200Hz (Low Band, Calling modem)
or 2400Hz (High Band, Answering modem).
In V.32bis and V.32 modes, QAM modulation is applied to a carrier of 1800Hz, using Trellis encoding for
most bit rates.
5.3
Tx Filter and Equaliser
The FSK or QAM modulator output signal is fed through the Transmit Filter and Equaliser block which
limits the out-of-band signal energy to acceptable limits. The appropriate equalisation for the particular
operating mode is determined and selected automatically, by the device.
5.4
DTMF/Tone Generator
In DTMF/Tones mode this block generates DTMF signals or single or dual frequency tones. In V.22 bis
modem mode it is used to generate the optional 550Hz or 1800Hz guard tone.
5.5
Tx Level Control and Output Buffer
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 and to reduce harmonic
distortion of the signal.
In normal (non loopback) mode, the output from the Rx Input Amplifier is fed to the Rx Gain Control block.
This provides the ability to attenuate the received signal by up to 10.5dB, depending on the value
programmed into the Receive Mode Register. The output from the Rx Gain Control block is routed either
to the Modem functions or to the Tone Detectors.
5.6
Rx DTMF/Tones Detectors
In Rx Tones Detect mode the received signal, after passing through the Rx Gain Control block, is fed to
the DTMF / Tones / Call Progress / Answer Tone Detector. The user may select any of four separate
detectors:
The DTMF Detector detects standard DTMF signals and identifies the transmitted character in the Status
Register. A valid DTMF signal will set bit 5 of the Status Register to 1 for as long as the signal is detected.
The Programmable Tone Pair Detector includes two separate tone detectors (see Figure 10). The first
detector will set bit 6 of the Status Register for as long as a valid signal is detected, the second detector
sets bit 7, and bit 10 of the Status Register will be set when both tones are detected.
The Call Progress Detector measures the amplitude of the signal at the output of a 275Hz - 665Hz
bandpass filter and sets bit 10 of the Status Register to 1 when the signal level exceeds the measurement
© 2004 CML Microsystems Plc
14
D/869/4
CMLMICRO [ CML MICROCIRCUITS ]
