SU1584085A1 - Switchable filter - Google Patents

Abstract

The invention relates to radio engineering and can be used in devices for frequency selection of radio electronic systems. The aim of the invention is to improve the stability of the amplitude-frequency characteristic (AFC). Switched filter contains an operational amplifier (OA) 1, capacitors 2 and 3, field-effect transistors 4 and 5, the driver 6 pulses with a variable duty cycle. Shaper 6 contains a clock pulse generator 7, a control voltage source 8, an op amp 9, a capacitor 10, field effect transistors 11 and 12. The goal is achieved by performing shaper 6 and connecting the shaper elements 6 with switching filter elements. The frequency and quality factor of the switched filter depend on the ratios of the frequency supply capacitors and the resistances of the field-effect transistors, so the effect of destabilizing factors on the circuit elements will not change the resonant frequency Fp and the quality factor Q. control voltage, while Q remains unchanged. 1 il.

Description

The invention relates to radio engineering N | Ike and can be used in devices for frequency selection of radio electronic devices.

The purpose of the invention is to increase the stability of the amplitude-frequency characteristic.

The drawing shows a schematic electrical circuit of switching an o-g filter.

The switching filter includes an op-amp 1, the first 2 and second 3 capacitors, the first 4 and second 5 field-effect transistors, a variable-duty pulse shaper 6 contains 7 clock pulses, a control voltage source 8, a third amplifier capacitor 10, third 11 and fourth 12 field effect transistors.

1 Switching filter works as follows.

The input signal is fed to the input of the switched filter through the first capacitor 2, and the resistance of the channels of the first 4 and second 5 field-effect transistors, which form the structure of the second capacitor, is equivalent to the Ribbon T-shaped RC circuit, the resonant frequency of which is 1

,

-R

2ff / C "C, RM $ Rnce

(one)

where r0, r

ot us

I. m V and ..

the resistance of the channel of the first 4 and second 5 field-effect transistors, respectively, when a release voltage equal to the amplitude of the control pulses is applied to their gates; the duty cycle of switching pulses with a period of following T and a duration of I, with T 1 1 / f p;

the capacity of the first 2 and second 3 capacitors, respectively.

The duration of the switching pulses is determined by the expression

e t /

. (Ev „+ KvL.S ,.SH (Ny1 | L N1SH.

and

(2)

- resistance of the channel of the third 11 and four

five

0

 U

In addition, 12 field effect transistors, respectively;

- capacitor capacitance 10;

- saturation voltage of the signal comparator (modulo);

jy is the control voltage at the ICD output. Substituting the expression (2) in (1), get a new expression for the resonant frequency of the filter:

 Hundred Ron + Roii 2ff

 "04 K05

Jfo

V6GH

U,

Have

(3)

(SGS3

where is the switching frequency:

.yy + nyy. .-- R-7-).

uw cop + koi

The expression for good quality is

0 lRЈЈ§-l2i / §lC3 VR iCi2 / RЈ1 C3

1 + Ro5e / R0 6

1 + Rft-r / R

US

 04

(four)

0

five

40

45

50

five

From expressions (3) and (4), it follows that the frequency and quality of a switched filter depend on the ratios of the frequency-setting capacitances and resistances of the field-effect transistor channels, therefore the effect of destabilizing factors (temperature, humidity, etc.) on the circuit elements does not change fp and Q.

The control of the tuning frequency f P can be carried out both by changing the switching frequency and also by changing the control voltage, while the value of the quality factor remains unchanged.

Claims (1)

Invention Formula A switching filter comprising a first operational amplifier whose output is a switching filter output, first and second capacitors, first and second keys and a variable duty cycle pulse generator, characterized in that, in order to increase the amplitude-frequency characteristic, the first and second keys made in the form of field-effect transistors, and the pulse shaper with a variable duty cycle is made on the second operational amplifier, which inverts 51 the input of which is connected to the output of the clock pulse generator, and the output of the second operational amplifier through the third capacitor is connected to the source of the third field effect transistor, the drain of which is connected to the non-inverting input of the second operational amplifier and the source of the fourth field effect transistor the gates of the third and fourth field-effect transistors are connected to a negative bias source, and the output of a pulse shaper with a variable duty cycle Is the output of the second operation1 840856 A primary amplifier connected to the gates of the first and second field-effect transistors, the QTOK of the first field-effect transistor is connected to the source of the second, field-effect transistor and the first terminal of the first capacitor, the second terminal of which is connected to the first filter and the first Q terminal the second capacitor and with the output of the first operational amplifier, the non-inverting input of which is connected to the common bus, and the inverter (5 the tiring input is connected to the drain of the second field-effect transistor and the second output home of a second capacitor.