SU1322420A1 - Active rc-filter - Google Patents

Abstract

The invention relates to radio engineering. The purpose of the invention is to reduce the error in changing the quality factor when changing the cutoff frequency. The active RC-fststr contains a passive two-tier ladder RC circuit 1, amplifier 2, a gain control unit consisting of two multiplying DACs 3 and 4, a differential amplifier 5 and an adder 6. The goal is achieved by introducing a differential amplifier 5 and an adder 6, s With the help of KOTopbLX, the feedback circuit under the influence of the control digital code generates feedback coefficients that determine the value of the active RC filter cut-off frequency corresponding to this code, while maintaining the constant of its quality factor. 1 il. (LL) / X5 Upro6lyyush, ii qi (written under

Description

The invention relates to radio engineering and can be used in radio engineering devices for various purposes.

The purpose of the invention is to reduce the error of changing the quality factor with a change in the cutoff frequency.

The drawing shows an electrical functional diagram of an active RC filter.

The active RC filter contains a passive two-tier ladder RC circuit 1, amplifier 2, a gain control unit containing the first multiplying digital-to-analog converter (UCAP) 3 ,, second UCAP 4, differential amplifier 5, adder 6.

UBWX

one

Bh

1, (1 .-.,) | 1. (l) (5ll5l, .L)

The necessary type of transfer functions for a high-pass or low-pass filter provides an appropriate selection of elements of the passive two-link ladder RC-circuit 1.

For the implementation of the transfer function of the high-pass filter as elements of z and z are used

2 -, 1-ci2 (-04

K (p) --R-R r r GT-51

p RiMlCl + p, + R, (C, + C,) M

By the condition of RjC ,, R (), then the drain (2) is converted into expansion (3)

one

C, C,

,

to (p)

,

P - R; K; C; C (3)

R, R,

As can be seen from the extrude for w,, 40, the cutoff frequency of the filter can be switched by changing the feedback coefficient i at constant values of R, R, C, C, however with Imagine the transfer function, this changes the quality factor of the high pass filter in canonical 45 q . In order to ensure the consistency of Q Q at frequency shifts

2 cuts choose the form of representation

K (p) is the feedback coefficient a l such

p2 + + (l1

50 way to Q "

/ ZD-CONSTANT RATE

1-0 / 1 „l RiCi V

where u} n V 2-Z-a 7 dionality.

 1., Aif,

From equality ---: - - UQ- filter cutoff frequency; ,,, - з

. about - the quality factor of the filter, From the condition RiC ,,) is - Ri Cr

DIM --- t; R

one

g + g

l l

from where

L j

find with 2 - d

i

Complicated nonlinear znS from C12 impedes n

Active RC-filter works as follows.

The input signal passes through a series-connected passive two-stage RC circuit 1 and amplifier 2 is fed to the output of an active RC filter,

The output signal is fed through the feedback circuit to the first and second taps of the passive two-tier ladder RC circuit 1 with transmission coefficients s and d, respectively.

We take z, 2., Zj and Z. - resistances of the elements of a passive two-tier ladder RC circuit 1. Using the superposition principle, we find the transfer function of the active RC filter

.L

capacitors, as elements of Zg and 24 - resistors. In this case

l jlj

- -rs:

psg

 R.

z Rj.

Substituting the indicated values of z ,, Z |, zеiz4B expression} xeni (1), we obtain

one

C, C,

,

(3)

which

50 way to Q "

1 b1 2

/ ZD - CONSTANT coefficient proportional / 5s

From equality ---: - - -Ы, - з

L j j

we find with 2 - dv-.

i PO

The complex nonlinear dependence of ujg НЫ, on С12 makes it difficult to practical

31322420

implementation of active RC-filter with digital control. To eliminate this difficulty, we present the coefficient in

feedback loop d-, in the form of

-

Where

auxiliary

coefficient. Substituting oi expressions for Id and d we obtain the calculated ratios for the high-pass filter

o

; Q

 one

(BUT

i R..C RjC, R DS. + Cf;

., -.

As can be seen from the calculated ratios, the cutoff frequency of the filter ui is directly proportional while maintaining the Q quality constant. The change in the coefficient o is influenced by the control digital code supplied to the digital inputs of UCAP 3 and 4. The possibility of changing the coefficient o, over a wide range in small steps, it allows to obtain a wide set of filter cut-off frequencies with unchanged parameters of the passive two-stage RC circuit.

Formation of feedback coefficients and in accordance with the calculated ratios is carried out as follows.

The output signal of the active RC filter (supplied to the analog input of the first multiplying digital-to-analog converter 3, to the digital input of which a control digital code is applied that determines the value of the transfer coefficient of the converter df,. At the output of the first UCAP 3, the voltage dg is output. Ugb ,,;, which is fed to the analog input of the second UCAP 4. The same control digital code is applied to the digital input of UCAP 4, as a result of which the output of the second UCAP 4 is applied

ten

° -out output The feedback coefficient oi is formed by a differential amplifier 5, the non-inverting input of which is fed to the output signal of the active RC filter Ugj ,, y, and the inverting input is given by the voltage dl Uj, x from the second UCAP 4. On the output of the differential amplifier 5 is a voltage .1, -dl and „,,, (1-oi2);

20

15

25 by 2

 0 84

Psch, which enters the second branch of the passive two-link RC circuit 1.

The feedback coefficient 0 is generated by the adder 6, the inputs of which are fed to the output signal of the active RC filter of the first UCAP 3 and the voltage - oto U g, from the output of the second UCAP 4. The transmission coefficients for the inputs of the adder 6

equal to K 1

one

and

Cs

one,

to „- Eo

as a result, a feedback voltage is acting on its code

and. ,, Л1 + dl - 1), U6 ,,,,

Ws comes on

which

the first branch of the passive two-link RC circuit. Thus, the feedback circuit under the influence of the control digital code forms feedback coefficients of and с / 2, which determine the value corresponding to this cut-off code of the active RC-filter If) while maintaining the constant Q of Q.

To implement the transfer function of the low-pass filter as elements of the passive two-tier ladder RC circuit z and z. resistors are used as well as HZ elements.

- capacitors .. r ;; z R

AT

Z, ---- 1 pc i

by this

one

pc;

Substitute indicated

1 we get

z and z in cuff (p)

one

p4l-o 2) RiR2C Cj + p (1-ciT), + (1 - «fp (Ri + RT) CJ +1

Comply with R., C () Cj, k (p),

then expression (5) is converted to „g +

expression.Q

one

p, .- .. L (f,) - where

 p2 (1-ai) R, R, C ,, C ((2) +1

Represent the transfer function of the low-pass filter in canonical form.

one

Wn1

-ciT), + (1- «fp (Ri + RT) CJ +1

(five)

Q

WnIM

5P22420

From condition (found CL. RI

C, R, + R,

F

on well in re no o tse

,,eleven.

R.

Vboi2

.

According to the considerations outlined in analyzing the high-pass filter, the feedback coefficients are about (, and c / it is advisable to form by the same algorithm, then we obtain the calculated ratios for the low-pass filter

one

0, R..Cj

RI R R

2

R, C,

.

(R, + R2) C ;; 0/2

.1 S -;.

The 2Q transmission center, the outputs of the adder (7) and the differential amplifier are connected respectively to the first and second taps of the passive two-link

ladder KS circuit, the output of the amplifier 25 is connected to the non-inverting input

As follows from the ana-differential amplifier and the first leasing, the functional diagrams and the input of the adder, the second input of which feedback circuits are connected to the inverting input of the differential filter in the high and low frequencies of the ana- nential amplifier and the output are logical, and the filters differ in the second multiplying digital-analogue one only in the structure of the passive two-converter unit, the input of which is a sub-ladder KS circuit. It is connected to the third input of the adder.

Or N. Lazarenko 2875/52

Compiled by S. Muzyuk Tehred I. Popovich

Cor Pod

Circulation 901 VNIIPI USSR State Committee

for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., 4/5

Production and printing company, Uzhgorod, st. Project, 4

6

Formula ieobreten and

An active RC filter containing a series-connected passive two-tier ladder RC circuit, whose input is the input of an active RC filter, an amplifier and a gain control unit, characterized in that, in order to reduce the error of quality or Q, the differential is introduced when the cut-off frequency is changed. the amplifier and the adder, and the gain control unit is complete in the form of series-connected

t5 of the first and second multiplying digital-to-analog converters, with the input of the first multiplying digital-analog converter, being the input of the transmission ratio factor adjusting unit 2, the outputs of the adder 7) and the differential amplifier are connected to the first and second taps of the passive two-stage, respectively

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Claims (1)

FIELD OF INVENTION An active RC filter comprising a passively connected two-tier ladder RC circuit, the input of which is an input of an active RC filter, an amplifier and a transmission coefficient adjustment unit, characterized in that, in order to reduce errors in the Q-factor when changing the cutoff frequency are introduced by a differential amplifier and adder, and the transmission coefficient adjustment unit is made in the form of series-connected first and second multiplying digital-to-analog converters, while input q of the first multiplying digital-to-analog converter, is the input of the gear coefficient adjustment block, the outputs of the adder and differential amplifier are connected respectively to the first and second taps of a passive two-link ladder RC circuit, the amplifier output is connected to a non-inverting input of the differential amplifier and the first input of the adder, the second input of which is connected to the inverting input of the differential amplifier and the output of the second multiplying digital-to-analog converter, the input of which is connected to the third input of the adder.