Interference suppression on AF. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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sort of, in a school physics course they say "about interference", i.e. About. that two identical waves, but with different phases, adding up, give maxima in some places, and minima in others. There it is considered on the example of light waves (easy to observe). But sound is also waves, only much longer. And using the principles of interference, you can try to "crush" interference at audio frequencies by the phase method.

Let's take two dynamic microphones, connect their windings in parallel antiphase (Fig. 1).

Alternatively, a serial anti-phase connection of microphones is also possible, which is even somewhat more convenient when introducing an input signal level control (a variable resistor is connected in parallel with the “noise” microphone, and in a parallel circuit, the regulator must be connected in series with the microphone).

When acoustic vibrations with the same level and phase hit both microphones, ideally, there should be no signal at the UZCH input (the signals converted by microphones from acoustic to electric are mutually compensated). If one of the microphones (for example, BM2) is turned to the noise source (fan, etc.), and the other (BM1) is turned towards you and speaks into it, then the signal levels (useful and noise) at the microphone outputs will be different, and the task is to equalize the amplitudes of the interference signals from both microphones and prevent the useful signal from entering the additional microphone VM2. Therefore, a level regulator is needed in the BM2 circuit, as well as protecting it from getting a useful signal.

In [2], a diagram of a device based on transistors is presented, which allows you to suppress unwanted signals at the inputs, for example, loud-speaking warning systems in production, transceivers, etc. (Fig. 2).

According to the same block diagram (Fig. 1), a device for suppressing acoustic interference on an IC (Fig. 3) was also assembled, the prototype of which is described in [3].

Acoustic signals (useful and noise) come from electret microphones to different inputs of the operational amplifier (inverting and non-inverting, pins 2 and 3 DA1 in Fig. 3, respectively). At the output of the op-amp, the resulting signal (algebraically summed) is obtained. With the same signal levels at both inputs, the output signal of the op-amp (ideally) should be equal to zero. The task during the operation of this device is to ensure the greatest possible separation of the useful signal and the interference signal, which each must act on its own microphone and, if possible, not fall into the other (especially the useful signal into the interference microphone). The rest of the interference signal that penetrates the microphone input of the useful signal is compensated in the op-amp (provided that the interference signal from its microphone is set equal in amplitude to the useful one).

In the description of the device (Fig. 2) [2], this operation is recommended to be assigned to the resistor R2 of the voltage divider in the base circuit of the transistor VT1, which is fraught with distortion (the divider sets the operating point of the transistor for direct current). The distorted interference signal cannot be compensated, since the interference signal at the output of the useful signal is not distorted, i.e. differs in shape from that distorted in the interference channel. Such a scheme is more suitable for highlighting distortions, for example, when analyzing them.

In the device in Fig. 3, to preserve the phase-frequency characteristics of the amplifier and the gain of the op-amp, you should also use this recommendation. In order not to disturb the DC supply mode of electret microphones, an RC chain is connected in parallel with them, consisting of a variable resistor with a resistance of 10 ... 100 kOhm and a capacitor of a sufficiently large capacity (several microfarads) connected in series.

The acoustic interference suppression device only works effectively when the interfering signal in both channels coincides in time. The speed of propagation of sound waves in the atmosphere at normal pressure is approximately 330 m/s. As you can see, it makes no sense to attribute the interference microphone from the microphone of the useful signal, especially since the wavelength decreases with increasing frequency. Therefore, it is better to strengthen two directional microphones side by side coaxially, orienting them in different directions (for example, at an angle of 180 °). By pointing the auxiliary microphone at the source of the interfering signal, it is possible to significantly reduce the proportion of interference in the useful signal, and by using an elementary amplitude control in the interference channel, it can be almost completely suppressed.

Often a radio amateur is disturbed by the "tedious" hum of equipment cooling fans. Its noise can be reduced using the proposed device (Fig. 3). The device is placed on a printed circuit board made of one-sided 4-adjusted fiberglass with a thickness of 1...1,5 mm and dimensions of 35x17 mm, the drawing of which is shown in Fig. 4, and the location of the parts - in Fig. 5. The board can also be made of double-sided fiberglass, then the foil on the side of the parts serves as a screen.

As microphones, you can use any electret (for example, MKE-3, MKE-84-1) or dynamic (R1 and R2 are not needed for them), having at least some directionality. Their bodies are fastened to each other coaxially and directed in different directions (to signal and interference). It is desirable to swivel the microphones for a more accurate orientation to the source of interference. Microphone capsules are placed in a common shielding case. If necessary, decoupling capacitors (up to 1000 pF) or LC chains are connected in parallel with the capsules. If amplitude adjustment is required, a potentiometer is connected in parallel with the interference channel microphone. but this will change the frequency response of the interference channel as mentioned above.

The circuit uses both ordinary small-sized parts (MLT-0,125 resistors, capacitors with a distance between the leads of 5 mm), and SMD (R6, R7, C3). The latter are mounted from the side of the printed tracks. In the device, you can use the op-amp KR140UD708 or low-voltage KR140UD1208 (here you need to turn on a resistor with a resistance of 8 ... 180 kOhm from output 360 of the IC to a common wire). By changing the resistance R5, the gain of the op-amp is adjusted (when the gain of the op-amp is indicated in the diagram is 1).

Literature

1. V. Besedin. We fight interference. - KB and VHF, 2008, No. 8.9.
2. By Klaus Spies, WB9YBM and Rolf Spies, N9BRL. Noise Canceling with Electret Condenser Microphones. - QST. December 2000, pp. 38...39.
3. John Beech (G8SEQ). A Noise Canceling Microphone. - SPRAT 95.

Author: V.Besedin, UA9LAQ, Tyumen

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