SU1764142A1 - Subcarrier band filter - Google Patents

Abstract

The invention relates to radio engineering and can be used in frequency selective radio electronic devices. The purpose of the invention is to reduce the output resistance and increase the accuracy of the formation of the transfer function. The band-pass SC filter contains the operational amplifier 1, the first capacitor 2, the first switch 3, the second capacitor 4, the second switch 5, the third capacitor 6. the third switch 7. The operation of the band-pass SC filter is based on the periodic closing of switches 7.5, 3, the result is a transfer function of a second-order bandpass filter. 2 Il.

Description

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The invention relates to radio engineering and can be used in selective electronic devices, mainly in microelectronic performance.

Known bandpass filter (Handbook of microelectronic pulse technology / VN Yakovlev, VV Voskresensky, SV Miroshnichenko and others - Kiev:, 1983, Table. 13.5, link number 1), containing the first and second capacitors The third, fourth and fifth switched capacitors, the first output of the fifth switched capacitor is the filter input, the second is connected to the first output of the second capacitor and the first output of the fourth switched capacitor, the second output of which is connected to the output of the amplifier, and the first capacitor and third to mutated capacitor connected between the input of the amplifier and the common bus, wherein the output filter is the output of the amplifier.

The disadvantage of a bandpass filter is that it contains a large number of capacitors and switches and cannot control the pole frequency with a single element.

Also known is the Wien bridge, tunable by one element (Patent No. 126343, CL. H 03 V 5/26, published 15.12.85), containing two capacitors, a variable resistor, six resistors, the first capacitor being connected to the signal source by the first output and the second output is connected to the common bus through serially connected from first to fourth resistors, the second capacitor is connected to the first output between the second and third resistors, and the second output connects to the common bus, the variable resistor is connected between the connection node of the first and second resistors and between the third and fourth resistor junction, the fifth and sixth resistors form a resistive voltage divider and are connected between the signal source and the common bus, the output of the device is the output of the resistive divider and the junction of the second, third resistors and the second capacitor.

The disadvantage of this device is: low quality, complexity (the device contains seven resistors and two capacitors), therefore it is difficult to realize in microelectronic design, it has a narrow tuning range due to the limited ratio of the resistance of the tunable resistor to the sum of the resistances of the first to fourth resistors.

Closest to the invention is a band-pass filter with switched capacitors (avt.se. USSR № 1510072), which contains four capacitors, three switches, two resistors, an operational amplifier, the first capacitor being connected to the amplifier output by the first and the second terminal to the output of the fourth capacitor and the first output of the first

key, the second capacitor is the first output connected to the common bus, and the second to the converting input of the operational amplifier and the first output of the second key, the third (switching) capacitor and the third key are connected between the common bus and the second output of the first and second keys, the first output of the fourth capacitor is connected to the input of the device, the first resistor of the voltage divider by the first output

connected to the common bus and the second to the inverting input of the operational amplifier; the second resistor of the voltage divider is connected between the output and the inverting input of the amplifier, the device output

is the output of the operational amplifier or its inverting or non-inverting input.

The disadvantage of the device is a large output resistance, high sensitivity to the parameters of the containers, a large number of elements.

The purpose of the invention is to reduce the output resistance and increase the accuracy of the formation of the transfer function.

In the proposed SC bandpass filter containing the first, second and third capacitors, the first, second and third switches, the operational amplifier, the first output of the first capacitor is connected to the output of the operational amplifier, and the second output of the first capacitor is connected to the input of the first switch, the first the output of the second capacitor is connected to the common bus, the second output of the second capacitor

connected to the input of the second key; to achieve the goal, the output of the operational amplifier is connected to the input of the third key, the output of which is connected to the second output of the second capacitor, the second output of the first capacitor is connected to the inverting input of the operational amplifier, the non-inverting input of which is connected to the common bus, the second output of the third the capacitor is the input of the bandpass SC-filter; in the care of the first switch is connected to the second output of the second capacitor, the output of the band-pass SC filter is the output operationally on the amplifier. The ability to achieve the objectives of the invention is due to the following:

the bandpass SC filter is simplified (compared to the prototype device, it contains one capacitor and two less resistors); has a lower output impedance (the output is the output of the op amp).

FIG. 1 given the electrical circuit of the proposed band-pass SC-filter; in fig. 2 is a timing diagram explaining his work.

The band-pass SC filter contains an operational amplifier 1, a first capacitor 2, a first switch 3, a second capacitor 4, a second switch 5, a third capacitor 6, a third switch 7. The first output of the first capacitor 2 is connected to the output of the operation amplifier 1, and the second output of the first capacitor The 2nd input of the first key 3. The first output of the second capacitor 4 is connected to the common bus, and the second output of the second capacitor1 4 is connected to the input of the second switch 5. The first output of the third capacitor 6 is connected to the output of the second switch 5. The output of the operational amplifier 1 is connected to stroke third switch 7 whose output is connected to a second terminal of the second capacitor 4. A second terminal of the first condenser 2 is connected to the inverting input of the operational amplifier 1, the non-inverting input connected to the common bus. The second terminal of the third capacitor 6 is the input of the SC bandpass filter. The output of the first switch 3 is connected to the second terminal of the second capacitor 4. The output of the bandpass SC filter is the output of the operational amplifier 1.

Band-pass SC-filter works as follows.

After the supply of control pulses in accordance with the timing diagram shown in FIG. 2, periodically start the third, second and first keys 7, 5, 3, respectively

As a result of such a sequence of key operation, a bandpass SC filter has a bandwidth transfer function

F (z)

)

&. C1

 + &) -N + - & + NW

C1

where Ci, C2, Сз are the capacities of the first, second and third capacitors 2, 4, b, respectively;

0

z is the discrete Laplace transform operator,

In the general form, the transfer function of the second-order chain of the strip type in the R-plane (Handbook on the calculation and design of ARC-schemes / SA Bukashin, V. Vlasov, V. F. Zmiy, et al. / Ed. By .A. Lanae. - M .: Radio and Communications, 1984) can be represented in the form

F (p) M

pdpop

p2 + pdp 0) p + o $

(2)

where M is the scale transfer coefficient at the pole frequency;

dp, Sch - attenuation and frequency of the pole; p is the operator of the continuous Laplace transform

In order to represent expression (1) in the form of (2), it is necessary to change the variables z p, which is valid when the switching frequency fs is much higher than the frequency of the signal being processed fc:

fs where Ts is the key switching period

Is

3, 5, 7 (FIG. 2), for example, using the inverse difference method (Allen F., Sanchez-Sinensio E. Electronic Circuits with Switched Capacitors / Trans., From English - M: Radio and Communication, 1989, c . 82):

p (1-z 1) / Ts,

40

(3)

Compared in expressions (1) and (3) the coefficients for the same powers of p, ne echo 45 d t the main parameters of the device:

Sz Ci

Shr

one

C2

TS vcicT

50

utg

Sz

55

From the analysis of the last expressions it follows: the attenuation of the pole is determined by the ratio of the capacities of the first 2 and third 6 capacitors; the scaling factor and the attenuation of the pole do not depend on the capacitance of the second capacitor 4, therefore the frequency of the pole can be controlled by changing the value of this element.

Claims (1)

Invention Formula Band SC-fidr containing the first, second and third capacitors, operational amplifier, the first, second and third keys, the first output of the first capacitor is connected to the output of the operational amplifier, and the second output of the first capacitor is connected to the input of the first Key, the first output of the second capacitor is connected with a common bus, and the second output of the second capacitor is connected to the input of the second switch, the first output of the third capacitor is connected to the output of the second switch, which is echoed by the fact that five reducing the output impedance and improving the accuracy of the transfer function; the output of the operational amplifier is connected to the input of the third key, the output of which is connected to the second output of the second capacitor, the second output of the first capacitor is connected to the inverting input of the operational amplifier, the non-inverting input of which is connected to the common bus, the second output the third capacitor is the input of the bandpass SC filter; the output of the first switch is connected to the second output of the second capacitor; the output of the bandpass SC filter is .operatsionnogo output amplifier.