Micro-reflections

What are micro-reflections?

A phenomenon that has been observed on many upstream links is micro-reflections. Micro-reflections are caused by impedance mismatches at both the source and load in a transmission line, and they create many small reflections between devices in the coaxial plant, such as between amplifiers and taps, or between power inserters and splitters, or between taps etc.
Micro-reflections were predicted on the downstream path, but were relatively benign because the high attenuation of the downstream cable damps out the reflections between devices. In the upstream however, the cable loss is so low that the micro-reflections have become an observable phenomena as well as a problem. If the vector diagram of Figure 1 were showing micro-reflections, the vector sum would comprise several smaller echoes, each with a different magnitude and rate of rotation with frequency.

Linear distortion is a problem for digital transmissions because it creates a problem called inter-symbol interference (or ISI). Figure 2 is a plot of two symbols that were transmitted one symbol period apart. The first symbol goes positive, and the second goes negative. The plot axes are voltage versus time.

Normally, on an oscilloscope you would see only one trace, but the single trace has been decomposed to show the two individual symbols. The symbols are sin(x)/(x) waveforms (impulse response of a “brick wall” filter), and the timing ticks at the top of the plot show the correct sampling times. Small circles on the plot also note correct sampling time.

Note that the first symbol component hits a positive peak just to the left of center, and the second symbol peak hits a negative peak just to the right of center, as noted by “x.” Observe that while any symbol is going through a peak, the other symbol is going through zero. If the channel has linear distortion, the waveforms will be “smeared” and no longer cross through zero at the sampling instant. A composite plot of many symbols that have been laid one atop the other is called an “eye” diagram, and will be described in detail later. A measure of inter-symbol interference is modulation error ratio (MER), which is a combination of linear distortions plus any additive interference, such as random noise or ingress, in the channel. MER also will be described in detail later.

A good place to observe echoes is on a magnitude plot. An echo’s amplitude can be calculated from the peak-to-peak excursions in the magnitude response that are caused by the echo. As an example, if the amplitude varies by plus and minus 7 percent, the peak-to-valley ripple will be 20*log(1.07/0.93)= 1.22 dB. The echo is down 20*log(.07)=23.09 dB relative to a carrier. One problem with observing echoes is that many conventional sweep systems have a frequency resolution that is much too coarse to display ripple.