Prepping Residential Gateways for WAN Access

Posted on Feb 6, 2014

Much of the focus around residential gateways designs has focused on distributing communication connectivity within a home or residence. But, in order to distribute content, gateways must first have an effective WAN interface that provides access to cable modem, fixed broadband wireless, or DSL broadband pipes. In this article, we`ll explore someof the design challenges

Prepping Residential Gateways for WAN Access
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that engineers will face when building the WAN portion of a residential gateway design. Specifically, we`ll look at the challenges of building an ADSL modem/router into a gateway without jeopardizing the performance of the ADSL signals. The interface to the ADSL line is composed of a set of discrete circuits as well as an ADSL chip set. In general, the analog chip, termed the analog front end (AFE) [See Figure 1], incorporates all the analog-to-digital conversion, digital-to-analog conversion, and filtering functions required to acquire and create the characteristic discrete multi-tone (DMT) signal. For many ADSL modem designs, the receive side (Rx) of the AFE will first filter the DMT signal coming from the line to the AFE chip using a plain old telephone system (POTS) high-pass (HP) filter for attenuation of speech and POTS signaling. This method also prevents the ADSL signal from leaking into the POTS bandwidth. A channel filter is also used for attenuation of the echo signal to improve the characteristics of the Rx path, or the maximum amount of useful amplitude allowed within the chip. The signal is amplified by a low noise gain stage (-15 to +31 dB) and is follow by a low-pass signal that is filtered to avoid anti-aliasing and to ease further digital processing by removing unwanted high-frequency, out-of-band noise. A 13-bit analog-to-digital (ADC) converter samples the data at 8. 832 Mbps, transforms the signal into...

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