Selection of Pulse Signals

the selection from a number of video pulses (signals), of only those pulses that have specified properties. Depending on which properties of the pulse or sequence of pulses are used, various types of selection are distinguished: selection on the basis of amplitude, duration, spacing, or code characteristics (seePULSE ENGINEERING).

Figure 1. Schematic of amplitude selection and corresponding signal diagrams: (u_in) input signals; (E_thresh) specified threshold level (selection threshold) for the limitation of amplitude from below; (u_out) output signals; (1), (2), and (3) puise numbers

Amplitude selection involves the selection of the pulse whose amplitude exceeds a specific level (the selection threshold), is below this level, or is within specified limits (Figure l). Such selection is carried out by an amplitude selector (seeAMPLITUDE DISCRIMINATOR).

When selection is done with respect to pulse duration, pulses are selected whose duration is comparable to, greater than, or less than a specified duration (Figure 2). A selector based on duration usually includes a device for the differentiation of the pulse (the identification of the leading and trailing edges), a delay line that produces a time delay equal to the selection level, and a logic element capable of such operations as logical multiplication or rejection.

Figure 2. Schematic of selection with respect to duration and corresponding signal diagrams: (RC) pulse differentiation device (reduction circuit), (DL) delay line, (LE) logic element with two inputs, (u_m) input signal, (i_ld) specified signal duration, (r_pd) pulse duration, (To) time delay produced by delay line

Selection with respect to pulse spacing involves the selection of pulses whose position in time relative to clock, or synchronizing, pulses is constant or varies according to a definite law. An example is the selection of signals reflected from ground environment or from a moving target in coherent pulse radars.

Figure 3. Schematic of selection of a coded train of pulses (occurring with specified spacing) and corresponding signa! diagrams: (DL) delay line; (LE) logic element with three inputs; (u,_in) input signal; (U_out) output signal; (T₂₁) and (T₃₂) spacing between pulses; (T_dl) and (T_d2) time delay of signals in DL₁ and DL₂; (1), (2) and (3) pulse numbers

In the selection of sequences of pulses—that is, the selection of pulses on the basis of pulse code characteristics—a pulse train is selected according to some property of its pulses. Thus, trains may be selected whose pulses have identical repetition frequencies. Alternatively, a specified pulse—such as the third—and every subsequent pulse of the input sequence may be selected. Another example is the selection of a group of pulses whose sequence corresponds to a specified code (Figure 3). Circuits for selectors of sequences vary widely. Such selectors are used primarily in control devices of various sampleddata systems. For example, the control unit of a digital computer is a selector of coded pulse trains.