Block diagram of the input circuit of the receiver

An input circuit is a device that connects an antenna or antenna feeder to the input of the first amplifying or transducer cascade of a receiver.

The purpose of the input circuit is the most complete transfer of energy from the antenna to the input of the first stage and the frequency selectivity to the input of the first nonlinear element.

The input device does not contain in its composition sources of energy, does not have, as a rule, non-linear elements and, as a result, it can be represented as a passive linear quadrupole.

Figure 5.1.

The antenna directly or through the antenna feeder is connected to the input circuit at points 1-1 ', and the first stage of the receiver at points 4-4'.

For frequency selection, the input circuit contains an electoral system consisting of one, two or more resonant circuits or special filters. Voltage transmission and selective properties of the electoral system are estimated by the transfer coefficient.

Communication electoral system with the antenna or feeder can be transformer, autotransformer, capacitive or combined. The corresponding transforming system in the block diagram is represented by block Tr ₁ .

The electoral system with the input cascade of the receiver can be connected not only directly, but also with the help of transformer, autotransformer connection or communication through a divider. The diagram shows the corresponding transforming system in the form of block Tr ₂ .

To assess the transformation of voltages or currents at the input and output of the electoral system, depending on the type of connection, the concepts of transformation ratio, degree of coupling, inclusion coefficient, etc. are used.

In the general case, the inclusion coefficient is a complex value. However, under certain conditions, the transforming systems Tr ₁ and Tr ₂ can be represented as ideal transformers, and their switching factors (transformations) are real values.

Accordingly, for transforming systems Tr ₁ and Tr _2, the inclusion coefficients are expressed

The representation of real transforming systems in the form of an ideal transformer is approximate, but the use of this concept greatly simplifies the analysis and makes it possible to obtain the ratios necessary for engineering calculations.