Universal digital filter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Note to the student
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It's not that hard to put together a digital filter that provides bass or treble pass, and band pass or notch. All this is achieved by a simple combination of logic gates, flip-flops and inverters. Because this circuit is pulsed, it provides near-perfect filtering of the square wave signal.
The cutoff frequency of the low-pass or high-pass filter (A) (see figure) is determined by the values of R1 and C1. The same components set the pulse duration t of the one-shot, which is equal to half the period To of the cutoff frequency:
The first positive edge of the input square-wave voltage triggers the one-shot. The single vibrator pulse opens the gate G1 and imposes a ban on the gate C2.
If the frequency of the square wave is higher than fo, the gate G1 gives an output pulse that flips flip-flops FF1 and FF2. The output voltage of the flip-flop FF1 opens the valve G3, and the input signal is allowed to pass to the output of the circuit. During this time, valve G2 remains closed.
When the input frequency falls below fo, the gate G1 is closed, and the gate G2 is opened and the signal from its output resets the flip-flop FF1. This closes G2 and stops the passage of a rectangular voltage to the output of the circuit.
The FF2 trigger prevents the FF1 trigger from firing incorrectly at high frequencies. This trigger is reset at the start of the one shot pulse.
A slight circuit change turns the high-pass filter into a low-pass filter. To do this, you only need to connect the inverter between the output FF1 and the input G3. The cutoff frequency remains the same.
A band pass digital filter (B) can be obtained by applying the input signal and the signals from the outputs of the low-pass filter and the high-pass filter to a three-input AND gate. In this case, the cutoff frequency of the low-pass filter f1 must be higher than the cutoff frequency of the high-pass filter f2.
A bandstop filter (C) can be obtained in a similar way: the outputs of the low-pass and high-pass filters must be connected to the NAND gate, and the two-input AND gate should be turned on at the output of the circuit. In this case, the cutoff frequency of the low-pass filter f1 must still be higher than the frequency cutoff of the high pass filter f2.
These digital filters can process any periodic signals, as long as the latter have been previously formed into a rectangular voltage sufficient to operate the logic circuits of amplitude. The frequency range of each filter is limited only by the allowable start frequency of the single vibrator and the speed of the logic circuits.
The motorola MC790R half-wave multivibrator IC used in the main filter circuit operates normally with a large duty cycle and, after starting, produces a pulse of strictly constant duration. The other components of the circuit are the MC9713P two-input gate ICs, the MC790 RS flip-flop, and the MC789P circuit (all Motorola circuits) containing six inverters. For resistance R1 and capacitance C1, a wide choice of values is possible depending on the speed of the logic circuits and the quality factor of the capacitor.
Author: N. McKinley, Hollyburton Research Center (Duncan, Oklahoma); Publication: N. Bolshakov, rf.atnn.ru
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