As we are probably aware, pulses passing down a digital transmission line suffer attenuation and are badly distorted by the frequency characteristic of the line. A regenerative repeater amplifies and reconstructs such a badly distorted digital signal and develops a nearly perfect replica of the original at its output. Regenerative repeaters are an essential key to digital transmission in that we could say that the "noise stops at the repeater."
Figure 6.11 is a simplified block diagram of a regenerative repeater and shows typical waveforms corresponding to each functional stage of signal processing. As illustrated in the figure, at the first stage of signal processing is amplification and equalization. With many regenerative repeaters, equalization is a two-step process. The first is a fixed equalizer that compensates for the attenuation-frequency characteristic (attenuation distortion), which is caused by the standard length of transmission line between repeaters (often 6000 ft or 1830 m). The second equalizer is variable and compensates for departures between nominal repeater section length and the actual length as well as loss variations due to temperature. The adjustable equalizer uses automatic line build-out (ALBO) networks that are automatically adjusted according to characteristics of the received signal.9
The signal output of the repeater must be precisely timed to maintain accurate pulse width and space between the pulses. The timing is derived from the incoming bit stream. The incoming signal is rectified and clipped, producing square waves that are applied to the timing extractor, which is a circuit tuned to the timing frequency. The output of the circuit controls a clock-pulse generator that produces an output of narrow pulses that are alternately positive and negative at the zero crossings of the square-wave input.
The narrow positive clock pulses gate the incoming pulses of the regenerator, and the negative pulses are used to run off the regenerator. Thus the combination is used to control the width of the regenerated pulses.
Regenerative repeaters are the major source of timing jitter in a digital transmission system. Jitter is one of the principal impairments in a digital network, giving rise to pulse distortion and intersymbol interference. Jitter is discussed in more detail in Section 6.9.2.
9Line build-out is the adding of capacitance and/or resistance to a transmission line to look "electrically" longer or shorter than it actually is physically.
Most regenerative repeaters transmit a bipolar (AMI) waveform (see Figure 6.10). Such signals can have one of three possible states in any instant in time—positive, zero, or negative (volts)—and are often designated +, 0, —. The threshold circuits are gates to admit the signal at the middle of the pulse interval. For instance, if the signal is positive and exceeds a positive threshold, it is recognized as a positive pulse. If it is negative and exceeds a negative threshold, it is recognized as a negative pulse. If it has a (voltage) value between the positive and negative voltage thresholds, it is recognized as a 0 (no pulse).
When either threshold is exceeded, the regenerator is triggered to produce a pulse of the appropriate duration, polarity, and amplitude. In this manner the distorted input signal is reconstructed as a new output signal for transmission to the next repeater or terminal facility.
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