CA1145422A - Digitally programmable active rc bandpass filter with constant absolute bandwidth - Google Patents
Digitally programmable active rc bandpass filter with constant absolute bandwidthInfo
- Publication number
- CA1145422A CA1145422A CA000333229A CA333229A CA1145422A CA 1145422 A CA1145422 A CA 1145422A CA 000333229 A CA000333229 A CA 000333229A CA 333229 A CA333229 A CA 333229A CA 1145422 A CA1145422 A CA 1145422A
- Authority
- CA
- Canada
- Prior art keywords
- terminal
- resistor
- output
- network
- center frequency
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H19/00—Networks using time-varying elements, e.g. N-path filters
- H03H19/008—Networks using time-varying elements, e.g. N-path filters with variable switch closing time
Abstract
D-20,684 DIGITALLY PROGRAMMABLE ACTIVE RC
BANDPASS FILTER WITH CONSTANT ABSOLUTE BANDWIDTH
by Gideon Willoner ABSTRACT OF THE DISCLOSURE
A narrow-band RC active bandpass filter, in which the center frequency may be changed by changing the resistance values of certain resistive elements, has the certain resistive elements switchably connectable into and out of the RC network so that the ratio of time during which the resistors are con-nected to the network to the time during which resistors are not connected to the network determines the effective resistance thereof. A programmable timing circuit has a plurality of outputs with each output supplying a different number of equal width pulses during a predetermined like time period whereby each output corresponds to a different ratio. The timing circuit actuates and deactuates the switchable connection from a selected output thus deriving the required effective resistance for a prede-termined center frequency. The bandwidth of the passband is set by a different resistive element and remains substantially unchanged as the center frequency is changed by changing the effective resistance of the certain resistive elements.
BANDPASS FILTER WITH CONSTANT ABSOLUTE BANDWIDTH
by Gideon Willoner ABSTRACT OF THE DISCLOSURE
A narrow-band RC active bandpass filter, in which the center frequency may be changed by changing the resistance values of certain resistive elements, has the certain resistive elements switchably connectable into and out of the RC network so that the ratio of time during which the resistors are con-nected to the network to the time during which resistors are not connected to the network determines the effective resistance thereof. A programmable timing circuit has a plurality of outputs with each output supplying a different number of equal width pulses during a predetermined like time period whereby each output corresponds to a different ratio. The timing circuit actuates and deactuates the switchable connection from a selected output thus deriving the required effective resistance for a prede-termined center frequency. The bandwidth of the passband is set by a different resistive element and remains substantially unchanged as the center frequency is changed by changing the effective resistance of the certain resistive elements.
Description
D-20,684 ~5~22 DIGITALLY PROGR~MABLE ACTIVE RC
2 BANDPASS FILTER WITH CONSTANT ABSOLUTE BANDWIDTH
1. Field of the Invention 6 This invention relates to narrow-band RC active bandpass 7 filters and in particular to narrow-band filters in which the 8 absolute bandwidth is substantially constant and is substantially 9 independent of the c'enter frequency.
2. Description of the Prior Art 11 In keeping with the size reduction in components, it 12 has been necessary to devise new techniques to obtain filter 13 networks of comparable reduced size. Thus, inductor simulation 14 has been refined and the use of RC networks with operational amplifiers to obtain bandpass, low-pass, and bandstop ~ilters 16 is well known. A problem that was created by the use of miniature~;
17 and integrated circuits was that of obtaining high resistance 18 values in excess of the values of re Lstance available in such circuits. One way in which this problem was solved was to switch resistors in and out of the circuit. In this way, the effective 21 resistance is increased well beyond the actual values available~
22 and in direct proportion to the ratio of the off-period~to the 23 on-period ~resistor connected in the circuit).
24 Using this technique, filters reduced in size but which ;
approximate the characteristics of the prior conventional large ~6 size filters were derived as well as reduced size filters which 27 permit the scaling of transfer functlons, N-path multiplex filter:
28 and digital filters. Howe~er, none of the reduc:d size filter: -29 can be used as a narrow-bandpa:s filter in which the center ~0 frequency may be changed by digital programming and Ln which the 31 absolute bandwidth remains substantially constant.
32 ~
~ ,.
: : , '':
D-20,684 ~ S422 2 A tunable narrow-band bandpass filter wherein the
1. Field of the Invention 6 This invention relates to narrow-band RC active bandpass 7 filters and in particular to narrow-band filters in which the 8 absolute bandwidth is substantially constant and is substantially 9 independent of the c'enter frequency.
2. Description of the Prior Art 11 In keeping with the size reduction in components, it 12 has been necessary to devise new techniques to obtain filter 13 networks of comparable reduced size. Thus, inductor simulation 14 has been refined and the use of RC networks with operational amplifiers to obtain bandpass, low-pass, and bandstop ~ilters 16 is well known. A problem that was created by the use of miniature~;
17 and integrated circuits was that of obtaining high resistance 18 values in excess of the values of re Lstance available in such circuits. One way in which this problem was solved was to switch resistors in and out of the circuit. In this way, the effective 21 resistance is increased well beyond the actual values available~
22 and in direct proportion to the ratio of the off-period~to the 23 on-period ~resistor connected in the circuit).
24 Using this technique, filters reduced in size but which ;
approximate the characteristics of the prior conventional large ~6 size filters were derived as well as reduced size filters which 27 permit the scaling of transfer functlons, N-path multiplex filter:
28 and digital filters. Howe~er, none of the reduc:d size filter: -29 can be used as a narrow-bandpa:s filter in which the center ~0 frequency may be changed by digital programming and Ln which the 31 absolute bandwidth remains substantially constant.
32 ~
~ ,.
: : , '':
D-20,684 ~ S422 2 A tunable narrow-band bandpass filter wherein the
3 bandwidth of the passband remains substantially constant as the
4 center frequency is changed includes an RC active network having ~ a first resistive element, the resistance value of which sets the 6 bandwidth of the filter. Certain other resistive elements are 7 switchably connectable into and out of the RC network, and a 8 switching device is interposed in said RC networ~ to effect a ~ connection of these certain resistive elements to the RC network when said switching device is actuated and to break this said 11 connection when the device is deactuated. A programmable timing 12 means provides a plurality of timed outputs for different 13 selective connection to said switching device. Each timed output 14 has a liXe fixed period, T, but sets a different fraction of the fixed time period during which the switch~means~is actuated. Thus, 16 the different outputs yield different ratios, i.e., off-period to 17 on-period, to selectively vary the effective value of the certain 18 resistive elements and to vary the center frequency accordlngly.
The single figure presented is a schematic dlagram of 21 a preferred embodiment of applicant's programmably tunab~le 22 narrow-band bandpass filter having a predetermined absolute 23 passband.
24 DETAILED DESCRIPTION OF THE INV~NTION
Referring to Fig. 1, the signal is applied between 26 input 2 and ground 28. As will be shown herein below, the 27 resistance value of resistor RO determines the absolute bandwidth.
28 Resistor P~O has one end connected to input terminal 2 and the 29 other end connected to junction A which is connected along~path 4 to the non-inverting input of operational amplifier 6.
D-20,684 ;~ S92Z
1 The feedback network of operational amplifier 6 consists 2 of resistor R2 having one end thereof connected to the output of 3 amplifier 6 via path 8, path 12 and junction 14. The other end 4 of R2 is connected to one side of switch 30 which has the other ~ side thereof connected via paths 32 and 16 to the inverting input 6 of operational amplifier 6, to one end of capacitor C2, and to the ~ inverting input of operational amplifier 24. Thus, resistor R2 8 is switchably connectable in the feedback network of operational 9 amplifier 6; and, as will be shown herein below, the resistance value of resistor R2, in conjunction with Rl, Cl and C2, is 11 used in setting the center frequency of the bandpass filter~ The 12 output of operational amplifier 26 is connected to one side of 13 switch 34 which is connected to one end of resistor Rl, the other 14 end of resistor Rl being connectea to path 4 and the non-inverting input to operational amplifier 6. Thus, resistor Rl is 16 switchably connectable in the feedback network of operational 17 amplifier 6 and operates in conjunction with R2 and capacitors 18 Cl and C2 to set the center frequency of the network as the 19 effective resistance of each of these two resistors is changed.
Operational amplifier input 24 has its non-inverting input 21 connected with path 22 to the junction of resistors R3 and R4. ;
22 The value of resistors R3 and R4 are not critical but in the 23 normal application, they are e~ual and are selected to provide a gain of 2 for amplifier 24.
~5 Switches 30 and 34 are preferably semiconductor 26 switches or gating circuits which are operated by pulses ~rom 27 a binary pulse train, and are connected via path 40 to pulse 28 rate converter 38. ~hile a specific connection to output 41 of 29 pulse rate converter 38 is shown, it should be understood that .
lead 40 may be connected to any other output lead 42-45, or to any 31 combination thereof, depending upon the ratio of off-period to 32 on-period desired. As is well known, the effective resistance , .
D-20,684 ~ ~5~Z2 1 varies in direct relation to this ratio. Thus, the minimum value of resistance is obtained when the resistor is permanently 3 connected in the circuit and the effective value increases as the on-period is decreased.
From practical considerations such as the effect of 6 transients and the accuracy of the center frequency~ the clock ~ frequency of clock 36 should be chosen to be of two orders of 8 magnitudes (100 times) greater than the highest center frequency 9 contemplated. For example, if the highest center frequency is 10,000 Hz, the clock frequency should be fc = 100 x 10,000 = 1 mHz.
11 The clock frequency is divided in pulse rate converter 38 so as 12 to obtain ~arious ratios of on-period to off-period.
13 One method of doing this is to use a binary rate 14 multiplier. A first step here is to determine the minimum on-period, and it is preferable if this value is compatible with 16 binary techniques. For example, the minumum on time could be 17 taken as 1/32. A duty cycle would then be the time period ~or 18 32 pulses. The binary rate multiplier could then include well 19 known logic circuits to provide 16, 8, 4, 2 and 1 output pulses during the duty cycle. By proper connection of the outputs of 21 the logic circuits, the equivalent of 31 pair of resistances 22 could be obtained which in turn would tune the filter to 31 23 different center frequencies.
24 It is apparent from the foregoing discussion that such an arrangement could most efficlently be used in a multichannel 26 data system employing relatively low baud rates. A major 27 advantage of such an arrangement is that only one filter design 28 is required for a plurality of filters. This facilitates 29 interchangeability and considerably simplifies manufacturing of 31 the filters.
, ~, :
_ 4 - ~ -D-20,684 ~5~2 1 A filter uslng the configuration shown in the drawing was built in which the relative bandwidth, ~, is related to R0 3 as follows:
4 -V~-3-R0 - _ (1) 6 2~
7 and the center frequency was determined by 9 Wo = (2) 11 The filter characteristics and element values were:
12 center frequency selected = 500 Hz 13 Cl = C2 = C lO~F
14 Rl = R2 = R = 31.8 kQ~
bandwidth = 100 Hz - , 17 ~ = = 0.2 19 R0 = 134 kQ*
~*standard values were used, i.e.,~
21 R = 31.6 k8 and R0 = 133 kQ.
22 Further, changing the value of resistors Rl and R2 changed the 23 center frequency but the bandwidth remained constant.
While the in~ention has been particularly shown and 26 described with reference to a preferred embodiment thereof, it 27 will be understood by those skilled in the art that change in 28 form and detail may be made therein without departing from the ~ ;
29 spirit and scope of the invention. ~
~0 . : ~ ~
_ 5 _ -,: ' ;: `
The single figure presented is a schematic dlagram of 21 a preferred embodiment of applicant's programmably tunab~le 22 narrow-band bandpass filter having a predetermined absolute 23 passband.
24 DETAILED DESCRIPTION OF THE INV~NTION
Referring to Fig. 1, the signal is applied between 26 input 2 and ground 28. As will be shown herein below, the 27 resistance value of resistor RO determines the absolute bandwidth.
28 Resistor P~O has one end connected to input terminal 2 and the 29 other end connected to junction A which is connected along~path 4 to the non-inverting input of operational amplifier 6.
D-20,684 ;~ S92Z
1 The feedback network of operational amplifier 6 consists 2 of resistor R2 having one end thereof connected to the output of 3 amplifier 6 via path 8, path 12 and junction 14. The other end 4 of R2 is connected to one side of switch 30 which has the other ~ side thereof connected via paths 32 and 16 to the inverting input 6 of operational amplifier 6, to one end of capacitor C2, and to the ~ inverting input of operational amplifier 24. Thus, resistor R2 8 is switchably connectable in the feedback network of operational 9 amplifier 6; and, as will be shown herein below, the resistance value of resistor R2, in conjunction with Rl, Cl and C2, is 11 used in setting the center frequency of the bandpass filter~ The 12 output of operational amplifier 26 is connected to one side of 13 switch 34 which is connected to one end of resistor Rl, the other 14 end of resistor Rl being connectea to path 4 and the non-inverting input to operational amplifier 6. Thus, resistor Rl is 16 switchably connectable in the feedback network of operational 17 amplifier 6 and operates in conjunction with R2 and capacitors 18 Cl and C2 to set the center frequency of the network as the 19 effective resistance of each of these two resistors is changed.
Operational amplifier input 24 has its non-inverting input 21 connected with path 22 to the junction of resistors R3 and R4. ;
22 The value of resistors R3 and R4 are not critical but in the 23 normal application, they are e~ual and are selected to provide a gain of 2 for amplifier 24.
~5 Switches 30 and 34 are preferably semiconductor 26 switches or gating circuits which are operated by pulses ~rom 27 a binary pulse train, and are connected via path 40 to pulse 28 rate converter 38. ~hile a specific connection to output 41 of 29 pulse rate converter 38 is shown, it should be understood that .
lead 40 may be connected to any other output lead 42-45, or to any 31 combination thereof, depending upon the ratio of off-period to 32 on-period desired. As is well known, the effective resistance , .
D-20,684 ~ ~5~Z2 1 varies in direct relation to this ratio. Thus, the minimum value of resistance is obtained when the resistor is permanently 3 connected in the circuit and the effective value increases as the on-period is decreased.
From practical considerations such as the effect of 6 transients and the accuracy of the center frequency~ the clock ~ frequency of clock 36 should be chosen to be of two orders of 8 magnitudes (100 times) greater than the highest center frequency 9 contemplated. For example, if the highest center frequency is 10,000 Hz, the clock frequency should be fc = 100 x 10,000 = 1 mHz.
11 The clock frequency is divided in pulse rate converter 38 so as 12 to obtain ~arious ratios of on-period to off-period.
13 One method of doing this is to use a binary rate 14 multiplier. A first step here is to determine the minimum on-period, and it is preferable if this value is compatible with 16 binary techniques. For example, the minumum on time could be 17 taken as 1/32. A duty cycle would then be the time period ~or 18 32 pulses. The binary rate multiplier could then include well 19 known logic circuits to provide 16, 8, 4, 2 and 1 output pulses during the duty cycle. By proper connection of the outputs of 21 the logic circuits, the equivalent of 31 pair of resistances 22 could be obtained which in turn would tune the filter to 31 23 different center frequencies.
24 It is apparent from the foregoing discussion that such an arrangement could most efficlently be used in a multichannel 26 data system employing relatively low baud rates. A major 27 advantage of such an arrangement is that only one filter design 28 is required for a plurality of filters. This facilitates 29 interchangeability and considerably simplifies manufacturing of 31 the filters.
, ~, :
_ 4 - ~ -D-20,684 ~5~2 1 A filter uslng the configuration shown in the drawing was built in which the relative bandwidth, ~, is related to R0 3 as follows:
4 -V~-3-R0 - _ (1) 6 2~
7 and the center frequency was determined by 9 Wo = (2) 11 The filter characteristics and element values were:
12 center frequency selected = 500 Hz 13 Cl = C2 = C lO~F
14 Rl = R2 = R = 31.8 kQ~
bandwidth = 100 Hz - , 17 ~ = = 0.2 19 R0 = 134 kQ*
~*standard values were used, i.e.,~
21 R = 31.6 k8 and R0 = 133 kQ.
22 Further, changing the value of resistors Rl and R2 changed the 23 center frequency but the bandwidth remained constant.
While the in~ention has been particularly shown and 26 described with reference to a preferred embodiment thereof, it 27 will be understood by those skilled in the art that change in 28 form and detail may be made therein without departing from the ~ ;
29 spirit and scope of the invention. ~
~0 . : ~ ~
_ 5 _ -,: ' ;: `
Claims (2)
1. A tunable narrow-band bandpass filter in which the absolute band-width and the center frequency can be controlled independently comprises:
a 3-terminal RC active network in which a ground terminal provides one connection to both the input and the output ports, a first terminal provides the ungrounded connection to the input port, and a second terminal pro-vides the ungrounded connection to the output port, said RC active network comprising:
a first resistor having one terminal end thereof connected to said first terminal of the input port, said first resistor establishing the band-width in accordance with the relation , where .delta. is the relative bandwidth, and RO is the value of the first resistor;
a first operational amplifier having the non-in-verting input terminal connected to the other terminal end of said first resistor, having an inverting input terminal and having an output terminal connected to the second ter-minal of said active network;
a first capacitor having one terminal end connected to the other terminal end of said first resistor and having the other terminal end connected to said ground terminal;
a feedback network including resistive elements the resistance value of which set the center frequency of the filter in accordance with the relationship Wo = 1/RC, where Wo = 2.pi.fo, and fo is the center frequency, R is the value of the resistance of the resistor and C is the value of the capacitor in the network; said feedback network having an input terminal connected to the output terminal of said first operational amplifier, having a first output terminal connected to the inverting input terminal of said first operational amplifier and having a second output terminal connected to the non-inverting input of said first opera-tional amplifier; and switch means interposed in said feedback network to effect a connection between said resistive elements and said first and second output terminals of the feedback network when said switch means is actuated and opening said connection otherwise; and programmable timing means providing a plurality of timed outputs for selective connection to said switch means, each timed output setting a pre-determined time period during which said switch means is actuated and there-by selectively varying the effective value of the resistive elements so that the center frequency of the filter may be programmably changed.
D-20,684
a 3-terminal RC active network in which a ground terminal provides one connection to both the input and the output ports, a first terminal provides the ungrounded connection to the input port, and a second terminal pro-vides the ungrounded connection to the output port, said RC active network comprising:
a first resistor having one terminal end thereof connected to said first terminal of the input port, said first resistor establishing the band-width in accordance with the relation , where .delta. is the relative bandwidth, and RO is the value of the first resistor;
a first operational amplifier having the non-in-verting input terminal connected to the other terminal end of said first resistor, having an inverting input terminal and having an output terminal connected to the second ter-minal of said active network;
a first capacitor having one terminal end connected to the other terminal end of said first resistor and having the other terminal end connected to said ground terminal;
a feedback network including resistive elements the resistance value of which set the center frequency of the filter in accordance with the relationship Wo = 1/RC, where Wo = 2.pi.fo, and fo is the center frequency, R is the value of the resistance of the resistor and C is the value of the capacitor in the network; said feedback network having an input terminal connected to the output terminal of said first operational amplifier, having a first output terminal connected to the inverting input terminal of said first operational amplifier and having a second output terminal connected to the non-inverting input of said first opera-tional amplifier; and switch means interposed in said feedback network to effect a connection between said resistive elements and said first and second output terminals of the feedback network when said switch means is actuated and opening said connection otherwise; and programmable timing means providing a plurality of timed outputs for selective connection to said switch means, each timed output setting a pre-determined time period during which said switch means is actuated and there-by selectively varying the effective value of the resistive elements so that the center frequency of the filter may be programmably changed.
D-20,684
2. A filter as set forth in claim 1 wherein said feedback network fur-ther comprises:
a second operational amplifier having non-inverting and inverting input terminals and an output terminal;
a second resistor having one terminal end thereof connected to the in-put terminal of said feedback network;
a first switching device having one terminal thereof connected to the other terminal end of said second resistor, having a second terminal thereof connected to the non-inverting input of said second operational amplifier and to said first output terminal of said feedback network, and having a third terminal connected to the output of said programmable timing means, whereby the effective value of the resistance of the second resistor may be varied;
a second capacitor having one terminal end thereof connected to the in-verting input and the other terminal end connected to the output of said second operational amplifier;
a third resistor having one terminal end connected to the second output terminal of said feedback network;
a second switching device having one terminal connected to the output of said second operational amplifier, having a second terminal connected to the other terminal end of said third resistor, and having a third terminal connected to the output of said programmable timing means, whereby the effective resistance of the third resistor may be varied in the same manner as the second resistor is varied.
a second operational amplifier having non-inverting and inverting input terminals and an output terminal;
a second resistor having one terminal end thereof connected to the in-put terminal of said feedback network;
a first switching device having one terminal thereof connected to the other terminal end of said second resistor, having a second terminal thereof connected to the non-inverting input of said second operational amplifier and to said first output terminal of said feedback network, and having a third terminal connected to the output of said programmable timing means, whereby the effective value of the resistance of the second resistor may be varied;
a second capacitor having one terminal end thereof connected to the in-verting input and the other terminal end connected to the output of said second operational amplifier;
a third resistor having one terminal end connected to the second output terminal of said feedback network;
a second switching device having one terminal connected to the output of said second operational amplifier, having a second terminal connected to the other terminal end of said third resistor, and having a third terminal connected to the output of said programmable timing means, whereby the effective resistance of the third resistor may be varied in the same manner as the second resistor is varied.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/936,999 US4232269A (en) | 1978-08-25 | 1978-08-25 | Digitally programmable active RC bandpass filter with constant absolute bandwidth |
| US936,999 | 1986-12-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1145422A true CA1145422A (en) | 1983-04-26 |
Family
ID=25469331
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000333229A Expired CA1145422A (en) | 1978-08-25 | 1979-08-03 | Digitally programmable active rc bandpass filter with constant absolute bandwidth |
Country Status (2)
| Country | Link |
|---|---|
| US (1) | US4232269A (en) |
| CA (1) | CA1145422A (en) |
Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4955058A (en) * | 1987-01-29 | 1990-09-04 | Eugene Rimkeit | Apparatus and method for equalizing a soundfield |
| US4785475A (en) * | 1987-01-29 | 1988-11-15 | Eugene Rimkeit | Apparatus and method for equalizing a soundfield |
| US5197334A (en) * | 1991-06-04 | 1993-03-30 | Schlumberger Industries, Inc. | Programmable compensation of bridge circuit thermal response |
| SE511393C2 (en) * | 1997-02-10 | 1999-09-20 | Ericsson Telefon Ab L M | Device and method for programmable analogue bandpass filtering |
| US7143014B2 (en) * | 2002-04-25 | 2006-11-28 | International Business Machines Corporation | System and method of analyzing distributed RC networks using non-uniform sampling of transfer functions |
| US8116862B2 (en) * | 2006-06-08 | 2012-02-14 | Greatbatch Ltd. | Tank filters placed in series with the lead wires or circuits of active medical devices to enhance MRI compatibility |
| WO2010051265A1 (en) * | 2008-10-30 | 2010-05-06 | Greatbatch Ltd. | Capacitor and inductor elements physically disposed in series whose lumped parameters are electrically connected in parallel to form a bandstop filter |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3747007A (en) * | 1972-07-07 | 1973-07-17 | Us Army | Variable compensation for feedback control systems |
| SU438095A1 (en) * | 1972-10-20 | 1974-07-30 | Предприятие П/Я Р-6292 | Band-pass active c-filter |
| SU562913A1 (en) * | 1975-09-01 | 1977-06-25 | Предприятие П/Я А-3724 | Active c-filter |
| US4009400A (en) * | 1975-11-28 | 1977-02-22 | Lockheed Missiles & Space Company, Inc. | Digitally controlled variable conductance |
-
1978
- 1978-08-25 US US05/936,999 patent/US4232269A/en not_active Expired - Lifetime
-
1979
- 1979-08-03 CA CA000333229A patent/CA1145422A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| US4232269A (en) | 1980-11-04 |
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