V.22 bis Modem with AT Commands
CMX866
For V.22 bis 2400bps QAM the transmit data stream is divided into groups of 4 consecutive data
bits. The first two bits of each group are encoded as a phase quadrant change and the last two
bits define one of four elements within a quadrant:
First two bits of group
(left-hand bit is the
Phase quadrant change
first of the pair)
00
01
11
10
+90° (e.g. quadrant 1 to 2)
0° (no change of quadrant)
+270° (e.g. quadrant 1 to 4)
+180° (e.g. quadrant 1 to 3)
Figure 7 V.22 bis Signal Constellation
1.5.7 Tx Filter and Equaliser
The FSK or QAM/DPSK modulator output signal is fed through the Transmit Filter and Equaliser block
which limits the out-of-band signal energy to acceptable limits. In 1200 and 2400 bps FSK, DPSK and
QAM 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 bit 1 of the S24 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.
1.5.8 DTMF/Tone Generator
In DTMF/Tones mode this block generates DTMF signals or single or dual frequency tones. In
QAM/DPSK modem modes it is used to generate the optional 550 or 1800Hz guard tone (see &Gn
command and S23 register).
1.5.9 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 at low values of VDD and to
reduce harmonic distortion of the signal. The output level is adjusted by bits 0-2 of the S25 register.
ã 2004 CML Microsystems Plc
32
D/866/4
CMLMICRO [ CML MICROCIRCUITS ]
