RU2333594C1 - Tuneable pass-band filter - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention refers to tuneable pass-band filters (PBF) of receivers and transmitters. In input and output PBF contains, two serial communications, i.e. inductive (1) and transformer (4), and between PBF resonant circuits transformer coupling communications (10) designed as structural component with windings on one frame. Note that communication windings of non-adjacent resonant circuits are located on both sides relative to winding of centre resonant circuit.

EFFECT: maintenance of constant relative pass-band within whole operating frequency range.

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Description

The invention relates to radio engineering and can be used in short-wave receivers and transmitters.

Signal transmission in the receiver or transmitter should be accompanied by suppression of interference, the frequencies of which are tuned to the operating frequency by 5-10%. For this purpose, tunable band-pass filters (PPPF) are used, in which the ratio of the minimum obstacle band (Δf _S ) to the operating frequency (f ₀ ) must remain constant:

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To ensure optimal attenuation values in the Δf _S and passband Δf bandwidths, the band ratio during filter tuning in the operating frequency range should remain constant:

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Frequency tuning of filters is usually accomplished by smooth or discrete changes in the capacitance of capacitors of parallel resonant circuits. Filters with parallel circuits are characterized as quasi-polynomial. An analysis of the frequency properties of such circuits given in [1, p. 222] shows that for the constancy of Ω when the filter is tuned in frequency, the coupling coefficient between the resonant circuits, determined by the ratio of the circuit inductances (L _K , L _{K + 1} ) and the coupling inductance ( L _K , L _{K + 1} ), must remain constant. For a U-shaped circuit of coupled resonant circuits L _K , _{K + 1} >> L _K , and for a T-shaped circuit, L _K , _{K + 1} << L _K.

According to the analysis, the value of the equivalent resistance of the extreme resonant circuits at the lower frequency of the operating range should be determined as:

,

,

where R _H , R _B are the equivalent resistance of the input (output) resonant circuit at the lower f _H and upper f _B frequencies of the operating range.

None of the schemes given in [1, p. 222] satisfies this requirement.

Closest to the technical solution is the circuit of a quasi-polynomial filter with coupling inductances at the input, output and between circuits, given in [1, p. 222, Fig. D.1.2, circuit “e”].

This circuit is selected as a prototype.

The disadvantages of this prototype scheme include:

- the impossibility of ensuring the required regularity of change in the equivalent resistance of the input (output) resonant circuit (3);

- the unacceptability of the constructive use of coupling inductances L _K , _{K + 1} U-shaped circuit due to the large values (L _K , _{K + 1} >> L _K ).

Using a T-shaped circuit of coupled resonant circuits is also not feasible, because to perform a design with a low value of the coupling inductance (L _K , _{K + 1} << L _K ) with high precision implementation is not always possible.

The objective of the invention is to find circuit and structural solutions that enable the implementation of a tunable bandpass filter with a constant relative passband at all frequencies of the operating range.

This problem is solved by using two sequential types of coupling in the input and output PFFP: inductive and transformer, and between resonant filter circuits — transformer coupling made in the form of a structural element with windings located on one frame, and the coupling windings of non-adjacent resonant circuits are located both sides relative to the winding of the middle resonant circuit. Transformer coupling at the input (output) allows to reduce the optimal equivalent resistance of the input (output) resonant circuit to a value at which the inductive coupling matches the transformed resistance of the resonant circuit with the nominal resistance of the generator (load) in the operating frequency range.

The inductance value of the transformed resonant circuit operating in the frequency range is determined from the expression:

,

,

where R is the load resistance of the filter,

α ₁ is the normalized value of the first element of the low-pass filter prototype,

r is the normalizing factor.

_Using this value L _r1T according to the formulas [1], the input inductance L ₀₁ , the transformed value of the capacitance of the resonant circuit C _r1T, and the transformed value of the equivalent resistance of the first circuit R _1T are calculated. The value of the optimal equivalent resistance of the circuit R1 determines the transformation coefficient of the resistances in the input (output) circuit:

.

.

_{Using the} values of L _r1T and N, the inductance and capacitance of the first resonant circuit are calculated.

The inter-circuit coupling inductance L _K , _{K + 1 is} calculated for a T-shaped circuit, which is subsequently replaced by a transformer circuit, for which the coupling coefficient between the windings is calculated.

To implement the calculated filter attenuation values, the coupling windings of non-adjacent (first and third) resonant circuits must be located on the structural element on both sides relative to the coupling winding of the middle resonant circuit. In this case, the electromagnetic coupling between non-adjacent resonant circuits is weakened and does not significantly affect the amplitude-frequency characteristic of the filter. The coupling coefficient between adjacent resonant circuits is regulated by the extension of their coupling windings.

A comparative analysis of the considered circuit with the prototype shows that the claimed device is different in structure and in connections, therefore, it meets the criterion of "novelty." The use of inductive couplings or the transformation of resistances in filters is well known, however, the use of their serial connection in the input (output) circuit and the selection of a structural element with transformer couplings allows you to implement a tunable bandpass filter with a constant value of the relative passband (Δf / f ₀ = const, Δf _S / Δf = const), i.e. a new significant filter parameter, from which it follows that the technical solution meets the criterion of "significant differences".

Figure 1 shows the circuit of the analyzed PPSF, consisting of input and output coupling inductances 1, input and output coupling windings 2, inductors of non-adjacent resonant circuits 3 included in the transformers 4, inductors 5 of the middle resonant circuit, permanently connected circuit capacitors 6 , discretely switched capacitors 7, keys 8, transformer links between circuits 9 and structural element 10 with transformer links between resonant circuits, input 11 and output 12 k ntaktov.

Figure 2 shows a structural element with transformer coupling 9.

The device operates as follows.

The input signal comes from input 11 to the coupling inductance 1, and then through the coupling winding 2 of the input transformer 4 is transmitted to the first resonant circuit formed by the inductor 3, which is one of the transformer 4 windings, and capacitors 6 and 7 connected in parallel with the winding; for tuning to the operating frequency, capacitors 7 are used, the terminals of which are closed through the contacts of the relay 8 to the housing; the second end of the inductor 3 is closed to the housing through the winding of the transformer 9, through which the signal is transmitted from the first resonant circuit to the second, consisting of the secondary winding of the transformer 9, inductance 5, capacitors 6 and capacitors 7, the terminals of which are connected to the housing through the contacts of relay 8; the coupling winding of the second resonant circuit has a transformer coupling 9 with the coupling winding of the third resonant circuit; windings of transformer links 9 of three resonant circuits are located on one frame and represent a structural element 10. Through a transformer link 9 between the coupling windings of the second and third resonant circuits, the signal is transmitted to the third resonant circuit (3, 6, 7, 8) and then through the coupling winding 2 transformer 4 and coupling inductance 1 to output 12.

The coupling windings 2 of the input (output) transformer 4 together with the winding of the resonant circuit 3 of the input (output) resonant circuit are wound on one core.

The transformer coupling 9 of the structural element 10 between adjacent resonant circuits is much larger than the stray transformer coupling between non-adjacent resonant circuits due to the structural diversity of their coupling windings on the structural element 10.

In the implemented RFPP, when tuning in the long-range frequency range, the passband changes proportionally to the operating frequency, the standing wave coefficient is within the range of SWR = 1.25-1.35, the attenuation in the passband and obstacle practically does not change.

Information sources

1. G. Hanzel. Reference for the calculation of filters. M .: Soviet Radio, 1974.

Claims (1)

A tunable band-pass filter containing coupled parallel resonant circuits, frequency tunable by changing the capacitance of the circuit capacitors, characterized in that it contains two sequential types of coupling at the input and output: inductive and transformer, and between the filter circuits transformer coupling, made in the form of constructive element with windings located on one frame, and the coupling windings of non-adjacent resonant circuits are located on both sides relative to the middle winding resonant circuit.

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