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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING On the noise immunity of household equipment. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer With the development of the television and FM radio broadcasting network and the increase in the number of VHF transmitters operating in large cities, interference with radio receivers located nearby (in the HF band), bass amplifiers and other household radio equipment has increased. Entering the input of a broadcasting receiver operating in the HF band, the signals of VHF FM transmitters, including television (sound), along with the useful signal, participate in frequency conversion, are detected, amplified and listened to with strong distortion. Video signals from television transmitters are heard as 50 Hz hum. For example, when tuning the receiver to frequencies of 12,089 and 11,856 MHz (range 25 m), 9,392 and 9,578 MHz (range 31 m), interference from the transmitter of the first television broadcast channel (carrier frequency 49,75 MHz) is involved in the frequency conversion, respectively, with the fourth and fifth harmonics local oscillator. In low-frequency amplifiers, interference from FM transmitters induced in the installation wires can be detected by transistor junctions. As a result, FM broadcasting programs are heard at the output of the amplifier. To attenuate interference of this kind, it is sometimes enough to use a small capacitor (100 ... 1000 pF) with short leads to shunt the detecting transition of the transistor or the inputs of stages that "receive" and amplify the interference before detection. In this way, it is possible to identify cascades that are sensitive to interference. By the way, the input circuits are most susceptible to interference from VHF transmitters, therefore, in all cases, it is advisable to shunt the input, for example, of a low-frequency amplifier with a capacitor of the specified capacitance. For the same reason, it is desirable to connect the antenna in the receiver not to the tap of the loop coil, but to the capacitor of the input loop, as shown schematically (without matching elements with the antenna) in Fig. 1. In this case, most of the currents induced by the electromagnetic interference field will pass through the capacitor C1 (for them, its resistance is small), practically bypassing the coil L1. In other words, the interference voltage induced in the coupling coil L2 will be significantly reduced.
In the event of careless installation, interference voltages can also occur in the field connections. To avoid this, the wires connecting the emitter and base of the transistor to the signal source (for example, to the coupling coil) should be kept to a minimum length. The interference induced in the field connections can be further weakened by shielding the cascades sensitive to pickups or even the entire apparatus. However, it should be borne in mind that these measures to reduce interference from VHF transmitters are effective only if the common wires of the device are connected correctly. An example of an unsuccessful installation is the connection diagram shown in fig. 1. Here, an interference current Ip flows through the "grounded" section of the signal wire AB, which is the sum of two currents. One of them arises due to the connection of the antenna dipole poles - "ground" with this wire (the dipole can also be formed by other elements of the device, for example, the chassis of the device, long external signal wires, power circuit wires, etc.), and the other is induced in the loop ABC, which includes the wire AB. As a result, the interference current Ip creates an interference voltage Up on the wire inductance, which is added to the signal voltage at the emitter junction of the transistor V1. You can get rid of the interference that has arisen for this reason if the common wires are connected at one point, choosing it so that the interference currents do not fall into the signal wires. In the case under consideration (Fig. 1), it is enough to break the extra connection to the right of point B or, by disconnecting the lower (according to the diagram) output of the coil L2 from point A, connect it with a separate wire to the emitter of the transistor V1 (this is shown by a dashed line in the diagram). Interference voltages can occur in long connecting wires. For example, if a signal is supplied to the input of the LF amplifier via a long cable, then with an ungrounded signal source a dipole is formed, and with a grounded signal source, a loop is formed in which high-frequency pickups occur, creating an interference voltage in the common wire. To attenuate them at the input of the amplifier, it is recommended to turn on a neutralizing transformer, the functions of which can be performed by a ferrite ring (for example, brands 150НН1, 100НН, etc.) put on the cable. Even better, wind several turns of cable around such a ring (see Fig. 2).
The neutralizing transformer does not affect the passage of the useful signal, since its currents flow along the connecting wires in opposite directions, and the magnetic fields created by them are mutually compensated in the magnetic circuit. High-frequency pickup currents from VHF transmitters flow in both wires in the same direction and are therefore attenuated. To prevent interference, it is desirable to include neutralizing transformers not only at the input, but also at the output of low-frequency devices, as well as in the power circuit (all wires connecting two blocks can be wound on one common ferrite ring). We will consider the practical application of the methods proposed here for combating interference from VHF transmitters using the example of a portable receiver VEF-202 (it is similar to the models VEF-12, VEF-201). Alternate shunting by capacitors of the inputs of the high-frequency stages of the receiver showed that interference penetrates into it mainly from the input of the RF amplifiers, made on the T3 transistor (according to the circuit diagram attached to the operating instructions). The connection diagram of the input circuits of the receiver is shown in fig. 3. The useful signal from the communication coil 1.4 to the emitter of the transistor T3 comes through the conductor connecting pin 5 of the drum range switch (hereinafter - pin 5B), pin 6 of the receiver circuit board (hereinafter - 6P) and capacitors C45, C48. At the same time, the signal received by the antenna An1 and received by the coil of the input circuit L3 passes through the same wire 5B-6P to a group of wires that form the counterweight of the antenna. This group includes the case of the KPE S3S40 block, the printed conductor of the power supply circuit of the local oscillator and the mixer and the common positive wire connected to it through the capacitor C45, as well as the screens of the IF filters and other conductors connected to the common wire through capacitors in the cascades of the IF and LF amplifiers. The wire 5B-6P is included in two more loops from the common wires: 5B-6P - printed conductor to the board contact 1 (1P) - the body of the KPE S3S40 block - contact 8 of the switch (8B) - S7-5B and 5B-6P-S3- 8B-S7-5B. It is hardly necessary to prove that with such an installation, high-frequency pickups in the 6B-6P signal wire are simply inevitable. To avoid this, it is necessary to cut the printed conductor connecting contacts 6P and 1P (it is superfluous, since these contacts are actually connected on the body of the KPE S3S40 block). As a result, the long "ground" wires of the input and heterodyne circuits will be disconnected and, in addition to attenuating high-frequency pickups, the mutual influence of the settings of these circuits will slightly decrease, which will improve the stability of the receiver's tuning.
To free the emitter circuit of the transistor T3 from the interfering currents of the second loop and the antenna counterweight, you need to solder the left (according to the diagram) output of the capacitor C48 from the printed conductor of the power supply and connect it with a separate wire to contact 5B (in Fig. 3 this is shown by a dashed line) Due to this, useful the signal to the emitter of transistor T3 will go to a new wire free from high-frequency interference currents. After such simple changes in the installation, the interference induced in the common wire circuit is significantly reduced and the interference that penetrates the receiver input from the antenna due to the inductive between coils L3 and L4 becomes more noticeable. This interference can be reduced by switching the antenna from the tap of the L3 coil to the junction point of the capacitors C2 and C6. To further reduce interference from VHF transmitters, it is useful to shunt the emitter junction of the T3 transistor with a capacitor with a capacity of approximately 100 pF. When powered from the mains, a neutralizing transformer must be connected to the power wires at the input or output of the rectifier. The described alteration of the VEF-202 receiver made it possible to reduce interference from VHF transmitters to such a level that it became difficult to find them where they had previously "clogged" the signals of even fairly powerful radio stations. Author: I. Egorov, Moscow Specialist commentary "In order to increase the noise immunity of high-frequency radio equipment, we, old radio amateurs, were taught to treat its installation with special attention: arrange parts on the board or chassis in rationally compact groups, connect them only with short straight wires, the entire apparatus sensitive to pickups, and, if possible, individual cascades There should be no common "grounding" wires at all - their functions may well be performed by a metal chassis, compartment walls or large sections of printed circuit board foil, covering individual current-carrying pads from all sides. These recommendations, by the way, are still carried out when developing communication equipment, allow, if not eliminate, then to a large extent to reduce interference with radio reception. Unfortunately, designers of consumer radio equipment do not always follow these recommendations: the metal parts of the devices often turn out to be "ungrounded". The KPI is placed in one corner of the device, and the loop coils are in the other, the connecting wires are often unnecessarily long, etc. The result, as expected in such cases, is deplorable: noise immunity is low, local oscillator radiation, and therefore interference other radio and television receivers are large, the selectivity of the input circuits (of course, if the signal comes not from the signal generator, but from the air) is very low. This is a very painful question! Another aspect of the same problem is the correct choice of the transistor mode. It is known that in a linear amplifying stage only limitation can take place, and by no means detection of high-frequency pickups. In other words, to reduce interference from radio stations, it is necessary to correctly set the operating mode of the amplifier stage transistor (by the way, in this case, non-linear distortions will also be minimal). The steepness of the transient response of the mixer should depend linearly on the voltage of the local oscillator and, therefore, change according to a purely sinusoidal law. Such a mixer does not mix on the harmonics of the local oscillator. However, the picture changes when the local oscillator voltage is excessively high, which often happens in household transistor receivers. As a result, the collector current takes on the character of short pulses (cutoff mode), and this contributes to the conversion at the local oscillator harmonics. It is not difficult to get rid of this phenomenon - it is enough to reduce the local oscillator voltage supplied to the mixer (halving it reduces the mixer's sensitivity to the fifth harmonic by about 25 times, i.e., by more than 30 dB!). Thus. It is possible to reduce interference (and, apparently, significantly) by minimizing the local oscillator voltage and choosing the mode of the mixing stage. To the above, we can only add that, in all likelihood, there are other ways to improve the noise immunity of household radio equipment. I would like to hope that the designers involved in its development will pay attention to the discussed problems and take measures to bring this parameter to acceptable values in a modern city. Commentary: V.T.Polyakov
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