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One-shot provides frequency discrimination

-May 01, 2003

You use a frequency discriminator to compare one signal frequency with another one. A functional feature, retriggering, of a monostable, one-shot 74xx123 multivibrator can yield frequency discrimination. Figure 1 shows a frequency discriminator that determines the relation of input-pulse frequency to a reference frequency. The external components, R1 and C1, set the reference frequency. These values determine the 74xx123's reference frequency as follows: fR=1/tW, and tW=kR1C1. The multiplication factor k depends on C1's value and the power-supply voltage. The rising edge of the input pulse starts the one-shot, whose output switches high for the interval tW. The same pulse edge sets the 74xx174 flip-flop to the same state as the output of the one-shot. If the interval between pulses is longer than tW, the next pulse arrives after the one-shot returns to its initial state. The one-shot's output is low, and the rising edge of the input pulse sets the flip-flop low. The low flip-flop output indicates that the input-pulse frequency, fIN, is lower than fR.

If the interval between input pulses is shorter than tW, the next pulse arrives before the one-shot completes its cycle and returns to its initial state. The one-shot's output is high, and the rising edge of the input pulse sets the flip-flop high. A high flip-flop output indicates that the input-pulse frequency, fIN, is higher than fR. Doubling the circuit in Figure 1 implements frequency discrimination with a "window" characteristic (Figure 2). Two pairs of R and C values determine the lower and upper reference frequencies. An exclusive-OR circuit takes the outputs of the upper and lower flip-flops. The exclusive OR's output is high when fIN is between fRL and fRH. When fIN is outside the frequency band fRL to fRH the exclusive OR's output is low. Figure 3 shows the frequency-discrimination characteristic. With R and C values as in Figure 2, and the use of a 74LS123 one-shot, fRL=16 kHz, and fRH=46 kHz. Other types of one-shots could produce different results.

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