Let's talk about one of the most simple nodes of the receiver on the input attenuator. The complexity of its design and really doesn't deserve the special attention of the three resistor or three capacitors, dividing the signal. But the choice of attenuation of the attenuator it is not as simple as his device. Sometimes in the literature they write that the attenuator extends the dynamic range of the receiver. But everything has limits. Not all are aware that there are two concepts about dynamic range, not having anything in common.
First. The ability of a receiver to take as the weakest and strongest signals within the passband of the main filter selection, and those that we wish to take.
Weak received signals is the radio pretty often. Very, very strong received signals, when we wish to exchange a few phrases with a friend living in the next street - a rare exception. In this case, the receiver can be overloaded even at the minimum gain, and the signal receiving neighbor is accompanied by distortion or impossible. How accurately observes on this occasion, V. Drozdov, some people in this situation it is necessary to disconnect the receiving antenna.
Second. The ability of a receiver to take the most useful weak signals in the presence of simultaneously operating across a range of very strong interference, or at least one strong interference outside the band of the desired signal. It's a completely different case, and if you want to hear weak DX station on the back of strong interference from adjacent stations on the continent, a disconnection of the receiving antenna you are unlikely to help. Hi!
But will the attenuator? In the first case - the powerful reception of a useful signal attenuator will help out with warranty. The weakening of the signal at the receiver input will allow admission with an average handle position gain in the mode of good linearity. About the noise of the receiver in this case I don't think, because they are thousands of times weaker signal. In the latter case, the reception of a weak station in the background of strong noise - the situation is more complicated, and to improve reception by turning on the attenuator can not always. Powerful disturbances to the path of high frequency, overload it and create intermodulation products across the range. These products are although much weaker and gave birth to their interference, but, as a rule, stronger than the desired signal and mask it. Intermodulation products depends on the level of congestion is not linear. And noise attenuation, for example, 2 to 3 times can cause the weakening of the intermodulation products in 10 - 20 times or even eliminate them completely.
All depends on the degree of overloading the input of the receiver. With a slight overload noise attenuation even twice completely eliminates intermodulation. Overloading to attenuate intermodulation need greater noise attenuation. What is the benefit of the attenuator? The benefit lies in the nonlinear dependence of intermodulation products generated from their interference. For example, we have included the attenuator 6 dB - 6 dB weakened the useful signal. And intermodulation products caused by these disturbances, weakened stronger, say 20 dB. At 14 dB, as you can see, has improved the ratio of the received signal and masked his intermodulation product. In favorable situations the gain could be even higher. And it would be well, if not for the intrinsic noise of the receiver.
With the weakening of the interference signal at the input of the receiver noise is not attenuated. And if the received signal to turn on the pad only slightly exceeded the intrinsic noise of the receiver (2 - 3 times), then after switching of the attenuator, the signal is generally lost in the noise of the receiver and will not be accepted even in the complete elimination of intermodulation products. So to weaken the input signal need not be roughly 20 dB in one fell swoop, and carefully, for a 3 - 6 dB (maximum). If you're lucky, maneuvering links attenuator- 5 - 10 - 15dB, you will choose a situation where the attenuation of the intermodulation will be sufficient, and the signal still may not be distinguishable from the noise floor of the receiver. But this situation is not always possible. If the signal is barely audible in the background noise of the receiver, the inclusion of even a small attenuation at the entrance immediately lowers it below these noises, and the reception becomes impossible.
The same situation occurs in the case of very large noise. To significantly weaken the intermodulation requires a strong weakening nor the entrance. But even quite decent signals that are 3 to 5 times greater than the intrinsic noise of the receiver, after switching on the attenuator will be lost in the noise. So, no attenuator by 1 dB does not improve the dynamic range of the receiver. He only negotiates the capabilities of the receiver with the reality in the air. And, not to lose sensitivity, this coordination should be carried out smoothly. But in the receivers we see pretty often rough with step attenuators 20 dB! In professional receivers it right. There is no one pulls VK and ZL from under the neighbor and the attenuator is used for matching speakers for accepted, not by interfering with the signal.
But as soon as professional receiver gets, and the hands of the Amateur, the attenuator must be redone at the degree not more than 6 dB. You can use adjustable UHF from a single resistor. Be adjusted from ASU UHF no and no. His maximum possible gain is entirely determined by the value of the total interference to the PA range, which can create intermodulation levels and not being currently received signal. When you turn on the transceiver for operation on HF bands, where the biggest danger of overload, the controller UHF put in the position of minimum gain. Near the desired frequency is the most clear area and some weak station. Then the gain UHF smoothly increases. It is a weak signal station gradually increases and gets better reception. But at some threshold begin to appear extraneous signals, which was not - started overloading the RF path. The gain is slightly retracted back to the disappearance of the overload and more handle do not touch.
Sometimes the preferred part of the band suddenly there is an obstacle, which previously was not. In order to determine whether or not it works at the range prevent you from the station or is it the fault of the receiver - turn on the attenuator. If the disturbance occurs at this frequency, it will weaken exactly on line with the attenuation of the attenuator. If the disturbance will weaken much stronger than the attenuation of the attenuator or disappear altogether, then this disturbance is the result of intermodulation.
Author: G. Gonchar (UC2LB); Publication: N. Bolshakov, rf.atnn.ru