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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Multiprikativnye hindrances from sources of secondary power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Radio reception is often accompanied by a strong hum of alternating current, heard when tuned to a radio carrier. Even if there is no carrier, such as when receiving SSB signals, speech becomes distorted and unintelligible. Telegraph signals take on a hoarse tone. The effect is especially pronounced in transistor receivers with whip antennas, often making it impossible to power them from the AC mains. The effect is equally manifested during transmission, distorting the station signal and expanding its spectrum. The reasons for this phenomenon and the means of combating it are described in the proposed material. The article discusses such conversions of radio signals, in which, as a result of the passage of radio frequency currents through the diodes of a working secondary power source containing a rectifier, unwanted modulation of radio signals with harmonic frequencies of the supply network occurs. This effect is most pronounced, for example, when a portable AM broadcast radio receiver with a whip antenna is powered from an alternating current network through a rectifier. The AC hum is heard only when the receiver is tuned to the frequency of the working station, and is not audible at all if there is no station signal. The intensity of the background increases with the signal level, so the background is most noticeable when receiving local radio stations [1]. Along with the background of alternating current, which is clearly distinguished during pauses in the transmission, significant distortions of speech and music are heard. In contrast to the additive background, which may be caused, for example, by poor filtering of the supply voltage and which is heard at the output of the receiver regardless of whether it is tuned to any station or not, this background is rightly called the multiplicative background (MF) [2], those. resulting from the functional multiplication of signal oscillations and noise. The process can proceed as follows: if a piece of wire is used as an antenna, then the wires of the mains supply inevitably enter the antenna system participating in the reception process as a counterweight, in which, as in the antenna wire, under the influence of the electromagnetic field of the radio station, EMF is also induced radio frequencies (Fig. 1).
In this case, the secondary power source (PS) also acts as a radio signal modulator with an alternating current background, since rectifier diodes are connected to the receiver antenna system (Rx) circuit, as shown in Fig. 2.
Each diode of a working rectifier is a parametric element for relatively low radio frequency voltages (i.e., a linear element, the parameters of which change significantly with time at a frequency of 50 Hz under the action of a relatively large voltage from the secondary winding of the transformer). The radio frequency current I in the antenna system circuit, which enters the receiver input, is defined as the product of the useful signal voltage on the diodes, which is proportional to the EMF induced in the antenna system, and the variable conductivity of the diodes. The useful signal is thus multiplied by the conductance change function of the diodes, while receiving parasitic modulation by the alternating current background. Due to the fact that under the influence of changes in the reverse voltage with closed diodes, their capacitance changes, the signal generally receives not only amplitude, but also phase (frequency) modulation [3]. Similar phenomena can occur not only during reception, but also during transmission. In this case, the source of radio frequency currents in the wires of the network is a transmitter powered from the network through a rectifier. An antenna system involving network wires emits a signal with a parasitic background modulation, and this multiplicative background will interfere with everyone who receives the signal from this transmitter. If the radio station in the reception and transmission modes uses the same antenna and is powered by the same rectifier, then the multiplicative background detected during reception indicates that during transmission there may also be spurious modulation of the signal by the background. The area of manifestation of the effect under consideration is by no means limited to portable radio equipment. In stationary installations with the simplest antennas, it would seem that currents should go along the ground wire, bypassing the power source. However, grounding in this sense is of little use, because effective high-frequency grounding, as is known in [4, 5], is practically unfeasible. Common-mode radio frequency currents in the network wires can be induced in the transmission mode and in the presence of a full-fledged (even symmetrical) antenna with a feeder. This occurs when the antenna itself is not sufficiently removed from the wires of the network or in the presence of the antenna effect of the feeder [6]. It was mentioned above that both the amplitude and the phase (frequency) of the signal undergo parasitic modulation. In practice, the initial spurious frequency modulation by the background is insignificant, however, if the modulation of the received (or transmitted) signal by the background is even purely amplitude, then the inevitable distortions in the frequency response of the transmission-reception path lead to the appearance of frequency modulation by the background and interference will be detected by receivers not only AM, but also FM signals. The considered multiplicative interference leads to a serious deterioration in the quality of broadcasting and communication signals. Reception of telegraph and single-sideband signals, as well as conventional broadcasting, is accompanied by a characteristic hoarseness. It is noted in [2] that a multiplicative background on a TV set "may be one of the reasons for the appearance of moving horizontal stripes on the screen, within which the image has a weakened or enhanced contrast and brightness." This happens when using simple indoor or built-in antennas. Often the cause of interference is the modulation of a radio signal in a rectifier connected to the same network, which is functionally (and even galvanically!) not connected to the receiver or transmitter of this signal. A detailed analysis of multiplicative noise is given in the book [3]. If the effect of additive interference that is added to the signal can be weakened by filtering, compensation, and even simply increasing the level of the useful signal, then the most realistic way to deal with multiplicative interference is to eliminate its causes and, in particular, in the power source. In the literature, one can find a number of ways to attenuate the multiplicative background [1, 2, 7 - 10], however, all the literary sources found touch upon the problem only from one side - with radio reception. Our goal is not only to show that the area of possible negative manifestations of the considered transformations is somewhat wider, but also to make a comparative assessment of possible ways to suppress MF and give reasonable arguments in favor of one of the directions in the fight against this phenomenon. A multiplicative background, both during transmission and reception, occurs when two conditions coincide: the presence of a significant connection between the receiver (transmitter) and the network wires, i.e., the significant participation of the network wires in the operation of the antenna system and the presence of modulating parametric elements (rectifier diodes ) in the circuit of the antenna system, including the transmitter (receiver). Consequently, the fight against the multiplicative background can be carried out in at least one of two ways, respectively: by weakening the connection between the transmitter (receiver) and the wires of the network or by weakening the modulating action of the diodes. Any of these methods to weaken the MF may be sufficient. The most popular way of suppressing the multiplicative background belongs to the second method. It consists in shunting the rectifier diodes with capacitors [2, 8-10]. The path of the RF currents becomes shorter through the low-resistance line capacitors rather than through the diodes, and with a sufficiently large capacity of the shunt capacitors, a significant attenuation of the interference can be obtained. Since about the end of the 70s, shunting of rectifier diodes by capacitors has been used by many domestic and foreign manufacturers of secondary power supplies for radio equipment. Capacitors are installed both in bridge and in full-wave rectifiers with a tap from the middle of the secondary winding, and even in single-wave rectifiers. We have not been able to trace the root cause and find out the purpose of installing capacitors, however, a number of (few) comments found on this matter indicated that this was done to "smooth out high-frequency noise coming from the mains." In any case, the effect of modulation by the alternating current background is markedly reduced. Capacitors also contribute to the reduction of impulse noise from transients in the diodes themselves during the operation of the rectifier [5]. Another way to exclude rectifier diodes from the circuit for common-mode radio frequency currents is more accessible: you can simply connect the network wires to the common wire (case) of the radio device at a high frequency [1, 7]. This is done, for example, in all interference-sensitive measuring instruments and signal generators. Both wires of the network are connected to the body of the device with capacitors of 10 ... 100 nF. In this case, the ungrounded case of the device may be under dangerous voltage, so protective grounding (or grounding) of the case is mandatory. Note that as a result of shunting the capacitors of the diodes or the rectifier as a whole, various kinds of interference penetrating both from the side of the network (to the receiver) and towards the network (from the transmitter) do not decrease, but, on the contrary, increase, since the resistance on their way. Thus, by weakening the multiplicative background that occurs in our rectifier using the second method, we do not eliminate, but, on the contrary, increase the radio frequency currents in the network wires. There remains a powerful potential source of interference - the power grid as an active part of the antenna system. In this way, as experience shows, it is practically impossible to effectively suppress the MF in real networks in the presence of non-linear or parametric elements in neighboring devices connected to the same network, in particular secondary power supply devices. It is much better in this respect not to facilitate the path of radio frequency currents through the rectifier, but, on the contrary, to exclude the cause of these currents or close this path for them, following the first of the methods indicated above. One way is to install shut-off chokes [2]. They are included in the power circuits (primary and / or secondary) in the vicinity of the object (receiver or transmitter), without requiring intervention in the rectifier circuit. Chokes serve to exclude or to limit the participation of network wires in the antenna system of the radio device. They protect the receiver not only from interference from their rectifier, but also from interference from all other rectifiers and other sources associated with the network. After all, parasitic modulation can also occur on the diodes of a "foreign" rectifier. Chokes in the network wires are installed in almost all modern television receivers with switching secondary power supplies, although their main purpose is to close the path for the harmonics of the frequency converter and horizontal scan generator to the network wires. Another way [2] is to shield the secondary winding of the power transformer from the primary. Ideal shielding involves the complete elimination of capacitive coupling between the transformer windings. However, this is impossible due to the practical impossibility of effectively grounding the shield by radio frequency. And for transformerless power supplies, this method, of course, is not suitable at all. Another way to combat the multiplicative background is to weaken the electromagnetic connection between the antenna and the network wires. This can be achieved by removing, as far as possible, the antenna wires from the network wires, avoiding their parallel arrangement, as well as preventing or weakening the antenna effect of the feeder [6], which is achieved, for example, using balancing devices and shut-off chokes (line insulators) in the feeder. For the most effective suppression and prevention of multiplicative interference, it is possible and necessary to use all available methods in combination. However, in most descriptions of amateur secondary power units, unfortunately, no means of combating MF were found. We emphasize that the methods of the first method, while not being necessary for the narrow purpose of suppressing multiplicative interference from power sources, may turn out to be highly desirable and even necessary to combat other types of interference of other types (additive), while the methods of the second method taken separately can aggravate interference situation with respect to these other interferences. Therefore, the preferred use of the first method alone or in combination with the second seems to us more than appropriate. The foregoing is illustrated by oscillograms obtained using computer simulation (Electronics Workbench v.5.12). The simulation scheme is shown in fig. 3.
The bridge rectifier, powered by an AC voltage source G1, is loaded with the R2C7 circuit. Bridge diodes VD1 - VD4 are close in parameters to domestic diodes KD204B. The radio frequency current of 150 kHz through the rectifier is created by the action of the EMF of the generator G2. To indicate it, a current-to-voltage converter (current-controlled voltage generator) U1 is used. Capacitors C3 - C6 and / or inductor L1 serve as elements of suppression of the MF. Elements C1, C2, R1 represent a model (equivalent) of some antenna system with network participation. Author: D.Avdonin, A.Grechikhin
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