Tuning receiver in the frequency range

   Methods of tuning: Modern receivers usually provide reception of signals in a wide range of operating frequencies. Rebuilding receivers from receiving signals from one station to receiving another can be done as follows:

1) smoothly - by smoothly changing the capacitances or inductances of the circuits of tunable electoral systems, as a result of which the receiver tunes in resonance to any of the frequencies within the operating range from

before ;

2) discretely - by abrupt switching of elements of all tunable electoral systems or by smoothly changing elements of electoral systems of the received frequency path and using a grid of discrete frequencies in the heterodyne, resulting in receiving signals at discrete fixed frequencies

, , ..., ;

3) in a combined way - by jumps when moving from one part of the operating frequency range to another and smoothly within each section. Discrete parts of the general range are usually called subranges, and if the transition from one subrange to another is performed by abrupt switching of inductances, then a smooth rearrangement within the subband is a smooth change in capacitance and vice versa.

Smooth adjustment.

It is possible to smoothly adjust the circuit in a given frequency range by changing the inductance or capacitance, or both.

Consider the configuration of the circuit by changing the inductance at a constant capacitance circuit .

Express the parameters of the circuit through its capacity:

- wave resistance ;

- contour attenuation .

Due to the surface effect in the coil wires and the dielectric loss, the resistance r increases approximately in proportion to the tuning frequency.

Then attenuation called constructive will be proportional to the square of the frequency

Quality factor also dependent on frequency. The bandwidth is 0.707.

will be proportional to the cube of the tuning frequency.

Thus, when you adjust the circuit inductance, its performance in the range changes dramatically. This leads to changes in the performance of the receiver as a whole, which is undesirable.

Now consider the receiver tuning capacity at a constant inductance .

The parameters of the circuit expressed through its inductance are:

- wave resistance .

- contour attenuation .

- contour attenuation .

- contour quality factor .

Still, considering r proportional to frequency , we see that the attenuation of the circuit and its quality factor are practically independent of frequency.

Bandwidth

will be proportional to the setting frequency.

Thus, the tuning capacity is accompanied by less drastic changes in the parameters of the contours. Therefore, adjust the contours in a relatively wide frequency range, usually by changing the capacitance. Tuning by inductance is used for small overlaps of the range, as well as in cases where tuning by capacitance may be inconvenient for constructive reasons. For example, in receivers working in conditions of strong shaking.

Smooth restructuring of electoral systems can be carried out using mechanical or electronic devices. With mechanical adjustment methods, the parameters of the electoral system, i.e. capacitance and inductance are varied by mechanically moving capacitor plates or variometer coils. In the case of electronic adjustment methods, the capacitance or inductance of a noise system varies with the effect of variable electrical quantities — voltage or current. For smooth electronic tuning, varicaps, varicades, ferrovarometers and varactors can be used.

For reference.  

        A varicap is a semiconductor diode whose capacitance ( pn junction capacitance ) non-linearly depends on the applied voltage. Semiconductor diodes with reverse bias are used as varicaps, when only the pn junction barrier capacitance is manifested . Made on the basis of Ge, Si, GaAs .

Key parameters: nominal (initial) capacitance C , quality factor Q , coefficient of overlap in capacitance

        .

        Varaktor - varicap used in the HF and MF bands.

        Variconde is a ferroelectric ceramic capacitor with a pronounced nonlinear dependence of the capacitance on the electrical voltage or temperature.

With mechanical methods of receiver tuning, the time for tuning the receiver to a given frequency is relatively large compared with the tuning time when using electronic methods. Mechanical methods of restructuring are structurally more complex, the dimensions and weight of the corresponding tunable systems are usually large. However, when using currently known elements for electronic rearrangement, instability of their parameters should be considered when exposed to various factors. In addition, the parameters of the radio receiver are deteriorating, in particular its selectivity. This is due to the nonlinearity of the adjustment elements, i.e. the dependence of their parameters on the amplitude, and their relatively low stability under the influence of various destabilizing factors.