![]() ![]() An adaptive equalizer can compensate for in-channel amplitude- and phase-versus-frequency impairments.Īdaptive equalizers use sophisticated algorithms to derive coefficients for an equalizer solution “on the fly”-in effect creating a digital filter with essentially the opposite complex frequency response of the impaired channel. True frequency response is a complex entity that has two components: amplitude-versus-frequency, and phase-versus-frequency. The cable industry has long used the term frequency response to describe amplitude (or magnitude)-versus-frequency-that is, what is seen on the display of test equipment used to sweep outside plant. Adaptive means the equalizer can change its characteristics as channel conditions change.Īn adaptive equalizer is a digital circuit that compensates for a digital signal’s in-channel complex frequency response impairments. Rather than equalizing the entire downstream or upstream RF spectrum, it deals with just a single channel. Equalization in the coaxial distribution networkĪdaptive equalization performs a function similar to that of a cable amplifier’s fixed-value plug-in equalizer. The equalizer is in effect a broadband filter that cancels the tilted response in the operating bandwidth, resulting in a flat amplitude-versus-frequency spectrum at the second amp’s internal gain stages.įigure 1. The equalizer is a small passive circuit that has the opposite amplitude-versus-frequency response of the length of coaxial cable preceding the amp. In order to get the tilted spectrum flat again, we have to install a fixed-value plug-in equalizer at the second amp. By the time that flat group of signals passes through coax to the next amplifier, it will be tilted because of greater attenuation at higher frequencies than at lower frequencies. For instance, if the all downstream signals in the, say, 50 MHz to 870 MHz spectrum have the same amplitude at the output of an amplifier, we say the overall frequency response-technically speaking, amplitude (or magnitude)-versus-frequency-is flat. As you know, in a given length of coaxial cable higher frequencies are attenuated more than lower frequencies. ![]() Let’s first look at the concept of equalization from the perspective of a cable distribution network (refer to Figure 1). How do they work and how does adaptive equalization relate to PNM? Read on to find out. Adaptive equalizers are most often used for this purpose. Furthermore, distortions causing ISI can change over time, so the equalizer must somehow be adjustable to compensate for changes in channel conditions. However, in a typical cable network the signal path between the headend and each cable modem (or digital set-top box) is unique, so a one-size-fits-all fixed equalizer is impractical. If the channel characteristics are known and don’t change over time, fixed-value equalizers can be used. One way to compensate for or reduce ISI is to incorporate an equalizer in the receiver or transmitter. The presence of distortion in the channel results in something called inter-symbol interference (ISI), which can cause data transmission errors. Digital communications systems are designed to transmit high-speed data through band-limited channels-for example, 6 MHz-wide downstream channels or 6.4 MHz-wide upstream channels, which are susceptible to various distortions. ![]()
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