SU951650A1 - Readjustable filter - Google Patents

Description

(54) REJECTABLE FILTER

one

This invention relates to radio engineering and can be used in input circuits of radio receivers.

Tunable resonant circuits are known that contain inductive nodes and varicaps whose capacitance is controlled by the voltage U applied to their clamp T In this case, the device’s dependence on the resonance frequency can be expressed as follows

.fp - fpoll K), CO device resonance frequency

po

where wa in the absence of a control voltage (J;

is an exponent depending on the type of varicap transition (j);

and

K-normalized manager

U ((i. Parameter (U is the contact potential difference of the varicap) ... From expression (1), since -j 5 it follows that for a true device, the frequency overlap ratio

. frudx

win

VY11M

very small (usually M 3). In addition, since the fragmentation of the varicap drops sharply with increasing frequency, the passband of the known device significantly varies in the range.

The closest to the technical essence of the invention is a device containing an amplifier with adjustable gain, the output and input of which are connected via an inductor, and a capacitor 2} is connected parallel to the input of a device with an adjustable gain.

The use of variable feedback allows for a smooth change in the equivalent reactance of the circuit formed by the capacitance of the capacitor and equivalent inductance. With an atom of the transmission coefficient K, the unit service, which in this case is the control parmmeter (), the resonance frequency fp depends as follows: f where j is ijl. (1) 1 This shows that the coefficient. shut off this device M -filb22: rL. k) p-hr ;;;) - ,. maybe (for K ,, 99, М 1О is significantly greater than that of the known device ij. But if the relative sensitivity of the frequency ip to the deviations of the parameter K (defined stability (p). at the upper limit of the range (Kyy, 99 reaches S -50, then T04 is difficult to adjust p under such conditions. Thus, the known devices, which have nonlinear laws for controlling the resonance frequency (1) and (2), are not satisfactory either due to the small overlap factor frequency, or due to low tuning accuracy. The invention’s circuit is an tuning and extending the tuning range at constant bandwidth. To achieve the goal, a tunable filter containing an amplifier with a controlled gain, parallel to the input of which is the filter input, is connected to a capacitor, and an inductor is connected between the input and output An additional amplifier with an adjustable gain factor is connected to the output of the JC amplifier with adjustable gain; between the output of which and the filter input an additional coil is connected and inductance, with both amplifiers having a common control body. Fig. 1 shows the structural electrical circuit of the proposed device; in fig. 2 is a graph showing his work. The device comprises cascade-connected amplifiers 1 and 2 with adjustable gain factors, a capacitor 3, inductance coils 4 and 5. The device works as follows. The use of a two-loop variable-frequency feedback link allows a change in the equivalent input reactance (reactance) of the circuit formed by capacitor 3 and equivalent inductance, according to a square law, which in turn allows linearization of the device’s resonance frequency control. The value of the equivalent inductance is adjusted by synchronously changing the coefficients K and K of Amplifiers 1 and 2. Starting from the graph (Fig. 2), the input response of the device is .., K- (2. -ja / li-urb C - capacitor 3 ; B is the inductance of inductors 4 and 5; fU is the loss resistance of coils 4 and 5 (capacitance is neglected). Hence the resonant frequency is determined by the expression. Ff / i-iL-J Y p g), (3) f - - ± - / 9 P ° itV2lc At the same time, the Q-factor of the amppi-fatigue characteristic is equal to QQ Л-J -lllKl-V: 2) oFrom expression (3) and (4), we find the gender Su transmission. ip .ro L Q 2. TCL which (taking into account the assumptions made) does not depend on K, i.e., it remains constant in the entire range of realignment. Linear adjustment law f is guaranteed at LM) 22 As can be seen from (3), the proposed device ensures the fulfillment of the identity (b) with K, --- UK, Ki-K / 2 Ki -2.K, CK | 2), where K - parameter adjustment.

Claims (1)

Claim Tunable filter containing variable gain amplifier 951650 6 amplification, parallel to the input of which a filter is connected, a capacitor is connected, and an inductance coil is connected between the input and output, which is characterized by the fact that, in order to increase the tuning accuracy and expand the tuning range with a constant passband, the amplifier output with adjustable gain / an additional amplifier with an adjustable gain is connected, between the output of which and the filter input an additional inductor is connected, while both amplifiers have common RGANI gain.