Receiver input circuit with electronic tuning

The tuning of the frequency contour of the input circuit and subsequent circuits tuned upon reception of signals by different carrier frequencies can be electronically performed using a varicap or ferrovarometer.

Adjustment by ferrovariometers.  

Electrical circuit diagram of the input circuit when setting up a ferrovarometer has the following form:

Figure 8.9.

        - capacitor carrying out external capacitance with the antenna;

        - contour parameters;

        - coupling capacitor with the first cascade;

        - control coil of the ferrovarometer;

        - the source controlling the ferrovarometer;

        - divider that provides the specified control current.

The setting of ferrovarometers has significant drawbacks:

- a relatively large power required to change the inductance;

- large temperature instability of the magnetic permeability of ferrite;

- ambiguity settings due to hysteresis;

- inertia in the process of establishing inductance;

- low quality factor.

Therefore, the varicap setting is used more often.

Varicapes setting.  

The schematic diagram of a single-circuit input circuit with electronic rearrangement has the following form:

Figure 8.10.

        - external capacitance with the antenna;

        - additional capacity, together with the nominal capacity of the varicap providing the permissible maximum capacity of the circuit;

        - loop inductance;

        - blocking capacitor, eliminating the influence of the control source circuit parameters on the quality factor of the circuit;

        - the source controlling the change in the capacity of varicap.

        - coupling coil, providing transformer coupling with the input of the first cascade.

Figure 8.11.

Advantages of varicapal adjustment;

- high speed adjustment;

- lack of mechanical contacts in the circuit;

- high resistance to climatic and mechanical stress;

- ease of implementation of remote control settings;

- low cost element adjustment;

- ease of implementation of the necessary law of changing the frequency settings when moving the knob.

Disadvantages:

- non-linear effects accompanying signal reception, since addiction nonlinear;

- the instability of the characteristics of the input circuit, due to the regeneration of the signal.