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MTC20454

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.)

DAC

276kHz

AAF

DAC

DAC

DAC

276kHz

AAF

276kHz

AAF

276kHz

AAF

1.1 mHz

HCDS

L0AGCtx

L0AGCrx

L0AACrx

L1AGCrx

L1AACrx

L2AGCrx

L2AACrx

L3AGCrx

L3AACrx

L1AGCtx

L2AGCtx

L3AGCtx

1.1 mHz

HCDS

1.1 mHz

HCDS

1.1 mHz

HCDS

Tuning circuit

L/V - Ref

CTRL/TST

ifce

ADC

Digital ifce

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