Radio receivers of modern radio engineering systems operate in complex electromagnetic environments created by sources of various natures and frequency ranges. The sensitivity of the receiver is calculated and implemented from the condition of the minimum signal power at its input, which, depending on the operating frequency range, can be fractions of femtowatts in the microwave range and microwatts for kilometer and hectometer wavelengths. The most dangerous is the influence of in-channel and out-of-channel concentrated interference at frequencies ωint1 and ωint2 in the preselector without tuning to the received frequency, which creates a component with a signal frequency |kωint1 ± mωint2| ≈ ωs in the output current of the AE where k and m are integers. The most intense interference is intermodulation interference of the second and third orders, the influence of which is estimated by the distance from the points of intersection of IIP2 and IIP3 with the amplitude characteristic of the cascade at the one-dB compression point КР1, provided that their total power is equal to the power of the useful signal at the receiver input. A measure that eliminates intermodulation distortion is the correct frequency planning of the terrestrial radio access network, which excludes intermodulation distortion. The impact of intense out-of-band interference can lead to nonlinear distortion and even blocking of the input stages of the radio path by such interference. The elimination of such distortions can be achieved by expanding the dynamic range of the input amplifying stages of the radio path and by increasing the filtering properties of the input circuit. Such techniques reduce the level of out-of-band interference, which can significantly exceed the level of the useful signal, leading to the appearance of nonlinear distortions in the output stages of the radio path and even blocking. However, the use of such passive methods for dealing with lumped interference is insufficient, especially in digital mobile communication systems. This is especially true for BS receivers when using methods of correlation signal processing, when a large number of signals act at their input, which have very different signal levels coming from closely or far-located ATs, real signal paths, and dynamically changing radio channel parameters. The spread of the levels of the influencing signals can exceed 50–60 dB, which leads to the appearance of mutual modulation or suppression of weak signals. As a measure to combat such phenomena, the method of controlling the power of the AT transmitter is actively used when information on the state of the radio channel obtained on the basis of an estimate of the error probability of the received signal is transmitted from the AT on the opposite channel. This allows, on command from the BS, adjusting the transmitter power of each AT and equalizing the levels of the signals coming to its input. In addition to the described methods for regulating the signal level (interference) acting at the receiver input, traditional methods for automatically controlling the gain of the RF front-end cascade, described in the first section of the chapter, are actively used. When implementing them, it should be taken into account that their use in discrete RF front-end designs cannot always be repeated in an integrated design. The active use of digital methods of carrier (or carrier) modulation in the transmitter of data transmission systems requires the use of correlation processing of received signals based on strict time synchronization of signals from individual sources of information sequences at all stages of their processing. This cannot be achieved without the use of reference oscillators that generate a frequency with a relative instability of 10−7–10−8 and frequency synthesizers covered by a PLL system that allows the formation of a frequency grid for VCOs that generate harmonic voltages synchronous and in-phase with the carrier of the received signal. The second part of the chapter describes the methods of generating the reference signal, the principles of constructing VCOs with smooth and discrete control of the generation frequency, and the principles of frequency synthesis. In the appendix to the chapter in the MicroCAP environment, a fragment of the RF front-end of the IC with gain control by the depth of the NFB is studied.