MTC20454 ST Microelectronics, Inc., MTC20454 Datasheet - Page 7

• Manufacturer Part Number: MTC20454
• Description: Quad Integrated Adsl CMOS Analog Front End Circuit
• Manufacturer: ST Microelectronics, Inc.

ANALOG TX/RX SIGNALS
The reference impedance for all power calculations is 100 Ohm.
DMT Signal
A DMT signal is basically the sum of N independently QAM modulated signals, each carried over a distinct
carrier. The frequency separation of each carrier is 4.3125 KHz with a total number of 256 carriers (ANSI).
For a large N, the signal can be modelled by a gaussian process with a certain amplitude probability den-
sity function. Since the maximum amplitude is expected to arise very rarely, the signal is clipped to trade-
off the resulting SNR loss against AD/DA dynamic range.
A clipping factor (Vpeak/Vrms = “crest factor”) of 5 is used resulting in a maximum SNR of 75 dB. ADSL
DMT signals are nominally sent at -40 dBm/Hz +/- 3 dB (-3.65 dBm/carrier) with a maximal power of 100
mW for downlink transmitter and 4.5 mW for uplink transmitter. The minimum SNR+D needed for DMT
carrier demodulation is about (3*N+20) dB with a minimum of 38 dB where N is the constellation size of a
carrier (in bits).
Block Diagram
The transformer has a 1:2 ratio. The termination resistors are 12.5 Ohm in case of 100 Ohm lines. The
hybrid bridge resistors should be < 2.5 kOhm for low-noise. An HP filter must be used on the TX path to
reduce DMT sidelobes and out-of- band noise influence on the receiver. On the RX path, a LP filter must
be used in order to reduce the echo signal level and to avoid saturation of the input stage of the receiver.
The POTS filter is used in both directions to reduce cross talk between ADSL signals and POTS speech
and signalling.
Figure 4. AFE Block Diagram (for detailed schematics see MTB20450-EBC reference design.)
L0AGCrx
L0AACrx
276kHz
AAF
1.1 mHz
HCDS
+
+
-
-
DAC
+
+
-
-
L0AGCtx
L1AGCrx
L1AACrx
276kHz
AAF
1.1 mHz
HCDS
+
+
-
-
DAC
+
+
-
-
L1AGCtx
Digital ifce
ADC
L2AGCrx
L2AACrx
276kHz
AAF
1.1 mHz
HCDS
+
+
-
-
DAC
+
+
-
-
L2AGCtx
L3AGCrx
L3AACrx
276kHz
AAF
1.1 mHz
HCDS
+
+
-
-
Tuning circuit
DAC
CTRL/TST
L/V - Ref
ifce
+
+
-
-
L3AGCtx
MTC20454
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