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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Tunable notch filter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur In sound reproducing or radio receiving equipment, narrow-band interference or interference, for example, network interference, sometimes appears. The author of the article tells how to significantly weaken them. A notch filter is the most effective way to deal with such interference. It suppresses the interference signal and passes the rest of the signals. As a frequency-setting element, it most often uses a double T-shaped filter or a Wien bridge. As practice has shown, a device that uses a double T-filter (Fig. 1) is simple and reliable, since it can be made passive. In this variant, signals with a frequency above the rejected one pass with low losses through the C1R1C2 chain, and below - through R2C3R3. Signals with a rejected frequency pass through both chains, acquire the opposite phase shift, cancel each other out at the output, and are significantly attenuated.
If a filter with a tunable notch frequency is required, the device will have to be complicated, since it is necessary to change the parameters of at least three elements synchronously. It is easier to assemble another filter (Fig. 2), known as a differential-bridge filter (see, for example, Horowitz P., Hill W. The Art of Circuitry. - M .: Mir, 1998, p. 296), which has the same frequency properties. All capacitors must be of the same capacitance, and the resistance of the resistor R1 must be six times the resistance of the resistor R2.
A distinctive feature of the filter is the ability to change the notch frequency with one variable resistor. The filter tuning frequency is determined by the formula Frezh = 1/2 Cv3R2' R2", where R2' and R2" are the resistances of the left and right (according to the diagram) parts of the variable resistor, respectively; C is the capacitance of each capacitor. On the basis of this chain, it is possible to perform a notch filter with acceptable parameters (Fig. 3).
To eliminate the influence of the signal source and load on the filter parameters, buffer stages on field-effect transistors are installed at the input and output of the circuit. With the ratings of the elements indicated in the diagram, the filter can be tuned in the frequency range of 30 ... 120 Hz with a variable resistor R5. The tuned resistor R2 achieves maximum suppression of the rejected signal. On fig. Figure 4 shows the experimental frequency response of the filter for two tuning frequencies - 50 Hz (1) and 100 Hz (2). The overall signal attenuation by the device is 6...7 dB, and the relative attenuation at the tuning frequency is 40...43 dB. In other words, the interference signal in relation to the useful signal is attenuated by at least 100 times. The frequency response of the filter near the rejection frequency has smooth slopes.
The steepness of the slopes can be significantly increased (curves 3 and 4) if the engine of the resistor R5 is disconnected from the common wire and connected to the source of the transistor VT5. In this case, the loss in the passband and the depth of the notch are reduced. To change the filter tuning frequency, you need to install capacitors C2 - C4 of a different capacity - it is calculated according to the above formula. In addition to those indicated in the diagram, it is permissible to use transistors KP307A, KP307B. Capacitors C1 - C4 - series K73 or similar, and capacitors C2, C3, C4 must be selected with the same capacity to within a few percent; C5, C6 - series K50, K52, K53. Variable and tuned resistors - SP, SPL, SP4, constants - MLT, S2-33. Most of the parts are placed on a printed circuit board (Fig. 5) made of one-sided foil fiberglass, which is then installed in a metal or plastic metallized case of suitable dimensions. The metal or plating of the body is connected to a common wire. Variable and tuning resistors are placed on the front wall of the case.
The maximum input voltage for this filter is approximately 1 V. To increase it, you need to use op-amp amplifiers instead of source followers - each of them must be non-inverting with a gain of 1. For stereo amplifiers, it is necessary to make two filters and install separate variable and trimmer resistors, since dual resistors will introduce a mismatch and simultaneously adjusting the frequency of the filters will be ineffective. Author: I. Nechaev, Kursk
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