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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Does an attenuator improve dynamic range? Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices Let's talk about one of the simplest receiver nodes - the input attenuator. The complexity of its design really does not deserve special attention - three resistors or three capacitors dividing the signal. But the choice of attenuation of the attenuator is not as simple as its design. Sometimes in the literature they write that the attenuator expands the dynamic range of the receiver. But don't get too carried away by this. Not everyone is aware that there are two concepts of dynamic range that have nothing in common. First. The ability of the receiver to receive both the weakest and strongest useful signals that fall within the passband of the main selection filter and those that we wish to receive. Weak received signals are common among radio amateurs. Very, very strong received signals, when we want to exchange a couple of phrases with a friend living on a nearby street - a rare exception. In this case, the receiver can be overloaded even at the most minimal gain, and the reception of the neighbor's signals is accompanied by distortions or is not possible at all. As V. Drozdov accurately notes on this occasion, some people in this situation have to turn off the receiving antenna. Second. The ability of the receiver to receive the weakest useful signals against the background of very strong interference simultaneously acting over the entire range, or at least one powerful interference that does not fall into the reception band of the useful signal. This is a completely different case, and if you want to receive a weak DX station against the background of strong interference from neighboring stations on your continent, then turning off the receiving antenna will hardly help you. Hi! But will an attenuator help? In the first case - receiving a powerful useful signal - the attenuator will help out with a guarantee. The attenuation of the signal at the receiver input will allow reception at the middle position of the gain knob in the mode of good linearity. There is nothing to think about the noise of the receiver in this case, because they are thousands of times weaker than the signal. In the second case - the reception of a weak station against the background of strong interference - the situation is more complicated, and it is far from always possible to improve reception by turning on the attenuator. Powerful interference affecting the high frequency path overloads it and creates intermodulation products in it over the entire range. These products, although much weaker than the interference that generated them, are usually stronger than the useful signal and mask it. Intermodulation products depend non-linearly on the level of overload. And attenuation of interference, for example, by 2 - 3 times can cause attenuation of intermodulation products by 10 - 20 times or even eliminate them completely. It all depends on the degree of overload of the receiver input. With a slight overload, attenuation of interference even by a factor of two completely eliminates intermodulation. At high overloads, greater interference attenuation is needed to attenuate intermodulation. What is the benefit of an attenuator? The benefit lies in the non-linear dependence of intermodulation products on the interference that generated them. For example, we turned on the 6 dB attenuator - the useful signal was weakened by 6 dB. And the intermodulation products caused by these interferences are attenuated more, say by 20 dB. As we can see, the ratio of the received signal and the intermodulation product that masked it improved by 14 dB. In a favorable situation, the gain can be even greater. And everything would be fine if not for the receiver's own noise. When the signal is attenuated, the noise at the input is not attenuated by the noise of the receiver. And if the received signal before the attenuator was turned on was only slightly higher than the receiver's own noise (by 2–3 times), then after the attenuator was turned on, such a signal would be completely lost in the receiver noise and would not be received even with the complete elimination of intermodulation products. So you need to attenuate the signal at the input not roughly, by 20 dB in one fell swoop, but carefully, by 3 - 6 dB (no more). If you are lucky, then by maneuvering the attenuator links - 5 - 10 - 15dB, you will select a situation where both the attenuation of intermodulation will be sufficient, and the signal may still remain distinguishable against the background of the receiver's own noise. But such a situation is not always possible. If the received signal is barely distinguishable against the background of the receiver's own noise, then the inclusion of even a small attenuation at the input immediately lowers it below these noises, and reception becomes impossible. The same situation arises in the case of very large interference. To significantly attenuate intermodulation, a strong attenuation is required at the input. But at the same time, even quite decent signals, which were 3-5 times higher than the receiver's own noise, will disappear in the noise after the attenuator is turned on. So, the attenuator does not improve the dynamic range of the receiver by 1 dB. It only coordinates the capabilities of the receiver with the real situation on the air. And, in order not to lose sensitivity, this matching should be carried out smoothly. But in receivers, we see very often coarse attenuators with a step of 20 dB! In professional receivers, this is correct. There, no one pulls VK and ZL from under the neighbor, and the attenuator serves to match the dynamics of the received, and not the interfering signal. But as soon as a professional receiver gets into the hands of a radio amateur, the attenuator in it must be redone in steps of no more than 6 dB. You can use an adjustable UHF from a separate resistor. Under no circumstances should UHF be regulated from the AGU. Its maximum possible gain is entirely determined by the magnitude of the total interference on the band, which can create intermodulation, and not by the levels of the currently received useful signal. When you turn on the transceiver to operate on the low bands, where the greatest danger of overloads, the UHF control is set to the minimum gain position. Near the desired frequency is the cleanest section and some weak station. Then the UHF gain gradually increases. The signal of a weak station gradually increases and the reception becomes better. But at some threshold, extraneous signals begin to appear, which were not there before - an overload of the RF path has begun. The gain is removed a little back until the overload disappears and the handle does not move anymore. Sometimes, on your favorite section of the range, interference suddenly appears that was not there before. In order to determine if this is really working on the band of the station that is interfering with you or if this is a receiver malfunction, turn on the attenuator. If interference occurs at this frequency, it will be attenuated exactly in line with the damping of the attenuator. If the interference weakens much more than the attenuator attenuation or disappears altogether, then this interference is the result of intermodulation. Author: G.Gonchar (UC2LB); Publication: N. Bolshakov, rf.atnn.ru
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