CN212811646U - Pre-and post-filters for elliptic filters - Google Patents
Pre-and post-filters for elliptic filters Download PDFInfo
- Publication number
- CN212811646U CN212811646U CN202022325403.7U CN202022325403U CN212811646U CN 212811646 U CN212811646 U CN 212811646U CN 202022325403 U CN202022325403 U CN 202022325403U CN 212811646 U CN212811646 U CN 212811646U
- Authority
- CN
- China
- Prior art keywords
- filter
- operational amplifier
- pin
- resistor
- filters
- 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.)
- Active
Links
Images
Landscapes
- Networks Using Active Elements (AREA)
Abstract
The utility model discloses an elliptical filter's leading and rearmounted wave filter. To a filter and more particularly to improvements to pre-and post-filters for elliptic filters. The front-end and rear-end filters of the elliptic filter with a simple structure comprise an operational amplifier N1, a resistor R1-resistor R5 and a capacitor C1-capacitor C3, wherein a pin III of an operational amplifier N1 is connected with an input end XS1 through a resistor R2 and a resistor R1 in sequence, a pin III of an operational amplifier N1 is connected with a pin I of the operational amplifier N1 through a resistor R2 and a capacitor C2 in sequence, and a pin III of an operational amplifier N1 is grounded through a capacitor C1; the gain of the pre-filter and the gain of the post-filter are only 1.056, the pre-filter and the post-filter are three-order Butterworth low-pass filters, the pass band is flat to the maximum, the required components are few, the circuit board area is small, the structure is simple, meanwhile, the attenuation of the stop band is high, and the requirements of the elliptic filter module on the pre-filter and the post-filter are met.
Description
Technical Field
The present invention relates to a filter, and more particularly, to an improvement of a front filter and a rear filter of an elliptic filter.
Background
At present, the anti-aliasing low-pass filter has high requirement on the attenuation rate of the stop band (> -100dB/oct), and the general low-pass filter can not reach the index, so the special chip MAX293 of the elliptic filter is adopted. However, since the chip MAX293 dedicated to the elliptic filter employs a switched capacitor circuit, it is required to add a pre-low pass filter at the front end of the MAX293 to eliminate the aliasing effect of the MAX293, and add a post-low pass filter at the back end of the MAX293 to serve as a smoothing filter of the MAX 293. The pre-filter and the post-filter select a second-order BUTTERWORTH low-pass filter. The cut-off frequency of the anti-aliasing low-pass filter is generally divided into 11 grades, and the grades are divided from 10Hz, 20Hz, 50 Hz-10 KHz and 20KHz according to the rule of 1, 2 and 5. The front filter and the rear filter are also divided into 11 grades, and the circuit is relatively complicated.
SUMMERY OF THE UTILITY MODEL
In order to overcome the defects of the prior art, the utility model provides a simple structure's leading and trailing wave filter of elliptic filter
The utility model discloses a following technical scheme realizes: the front-end filter and the rear-end filter of the elliptic filter comprise an operational amplifier N1, a resistor R1-a resistor R5 and a capacitor C1-a capacitor C3, wherein a pin III of the operational amplifier N1 is connected with an input end XS1 through a resistor R2 and a resistor R1 in sequence, a pin III of the operational amplifier N1 is connected with a pin I of the operational amplifier N1 through a resistor R2 and a capacitor C2 in sequence, and a pin III of the operational amplifier N1 is grounded through a capacitor C1;
the pin I of the operational amplifier N1 is connected with the pin II of the operational amplifier N1 after being divided by a resistor R4 and a resistor R3, the pin I of the operational amplifier N1 is connected with the pin V of the operational amplifier N1 through a resistor R5, the pin I of the operational amplifier N1 is connected with a capacitor C3 through a resistor R5 and then grounded, and the pin VI of the operational amplifier N1 is connected with the pin seven of the operational amplifier N1 and then connected with an output end XS 2.
The first pin of the input end XS1 is grounded, and the second pin of the input end XS1 is connected with the third pin of the operational amplifier N1 through a resistor R1 and a resistor R2 in sequence.
Pin two of the output terminal XS2 is grounded, and pin one of the output terminal XS2 is connected to pin seven of the op-amp N1.
And a pin four of the operational amplifier N1 and a pin eight of the operational amplifier N1 are vacant.
The operational amplifier N1 is an LF353 operational amplifier chip.
Compared with the prior art, the utility model discloses leading filter and post filter's gain is only 1.056, is three-order butterworth low pass filter, and the biggest is flat in the passband, and required components and parts are few, and the circuit board area is little, simple structure, and the stopband attenuation is high simultaneously, has satisfied the requirement of elliptical filter module to leading filter and post filter.
Drawings
Fig. 1 is a schematic diagram of the filter circuit of the present invention;
Detailed Description
The present invention will be further described with reference to the accompanying drawings and the detailed description, and it should be noted that the embodiments or technical features described below can be arbitrarily combined to form a new embodiment without conflict.
As shown in fig. 1, the front and rear filters of the elliptic filter include an operational amplifier N1, a resistor R1-a resistor R5 and a capacitor C1-a capacitor C3, a pin iii of the operational amplifier N1 is connected to an input terminal XS1 through a resistor R2 and a resistor R1 in sequence, a pin iii of the operational amplifier N1 is connected to a pin i of the operational amplifier N1 through a resistor R2 and a capacitor C2 in sequence, and a pin iii of the operational amplifier N1 is grounded through a capacitor C1; the pin I of the operational amplifier N1 is connected with the pin II of the operational amplifier N1 after being divided by a resistor R4 and a resistor R3, the pin I of the operational amplifier N1 is connected with the pin V of the operational amplifier N1 through a resistor R5, the pin I of the operational amplifier N1 is connected with a capacitor C3 through a resistor R5 and then grounded, and the pin VI of the operational amplifier N1 is connected with the pin seven of the operational amplifier N1 and then connected with an output end XS 2. The first pin of the input end XS1 is grounded, and the second pin of the input end XS1 is connected with the third pin of the operational amplifier N1 through a resistor R1 and a resistor R2 in sequence. Pin two of the output terminal XS2 is grounded, and pin one of the output terminal XS2 is connected to pin seven of the op-amp N1. And a pin four of the operational amplifier N1 and a pin eight of the operational amplifier N1 are vacant.
The front low-pass filter and the rear filter are formed by adopting a three-order Butterworth filter, preferably an LF353 operational amplifier chip as a core device, and the gain of the three-order Butterworth filter is slightly larger than 1. As shown in FIG. 1, for the MAX293 prefilter, since it is a third order Butterworth low pass filter, the transfer function is
Where fc is the cut-off frequency of the third order low pass filter. The third-order low-pass filter is formed by connecting a second-order low-pass filter and a first-order low-pass filter in series, and the transfer function of the second-order low-pass filter is H(S)＝1/(S2+ S +1), the transfer function of the first order low pass filter is H (S)1/(S + 1). The cut-off frequency of the anti-aliasing low-pass filter is divided into 11 grades, and the cut-off frequency of the 11 grades of filters is divided into 4 groups from 10Hz, 20Hz, 50 Hz-10 KHz and 20KHz, wherein the first group is as follows: 10Hz, 20 Hz; second group: 50Hz, 100Hz, 200 Hz; third group: 500Hz, 1KHz and 2 KHz; and a fourth group: 5KHz, 10KHz, 20 Hz. The anti-aliasing low-pass filter has a set of cut-off frequencies corresponding to one cut-off frequency of the third-order Butterworth low-pass filter, so that the third-order Butterworth low-pass filter has 4 cut-off frequencies.
If the third set of cut-off frequencies of the anti-aliasing low-pass filter is selected, the corresponding cut-off frequency of the third order butterworth low-pass filter is 5.9 KHz. As the third group of cut-off frequencies are respectively 500Hz, 1KHz and 2KHz, namely the cut-off frequencies of the anti-aliasing low-pass filter are respectively 500Hz, 1KHz and 2KHz, the MAX293 clock frequencies are respectively 50KHz, 100KHz and 200 KHz. The frequency of the sine wave input signal is also 500Hz, and according to Shannon's law, the sine wave signal with the frequency of 500Hz is converted into a signal containing various frequency components after passing through MAX293, wherein the frequency of the sine wave signal is 500Hz, 49.5KHz (100-1), 50.5KHz (100+1), 99KHz (200+1), 101KHz (200+1) and the like. Except for the 500Hz signal, the signals of other frequencies are redundant signals and are filtered by a low-pass filter. If the cut-off frequency fc of the third-order low-pass filter is 59KHz, the signal attenuation of 49.5KHz is-55.4 dB. The frequencies of 50.5KHz, 99KHz and 101KHz, … … are higher than 49.5KHz, and the stop band attenuation is far greater than-55.4 dB, so that the frequency is not considered.
For the rest two-gear cut-off frequencies of 1KHz and 2KHz, the redundant signal frequency is far greater than 49.5KHz, so that the redundant signal frequency is not needed to be considered.
A sine wave signal with a frequency of 2KHz (corresponding to an anti-aliasing low-pass filter with a cut-off frequency of 2 KHz) is attenuated to-0.0066 dB after passing through a third-order pre-low-pass filter (with a cut-off frequency of 5.9KHz), while a sine wave signal with a frequency of 500Hz (corresponding to an anti-aliasing low-pass filter with a cut-off frequency of 500 Hz) and a sine wave signal with a frequency of 1KHz (corresponding to an anti-aliasing low-pass filter with a cut-off frequency of 1 KHz) are attenuated to a far lower level than-0.0066 dB after passing through the third-order pre-low-pass filter (with a cut-off frequency of 5.9 KHz).
After passing through MAX293, the 500Hz sine signal becomes a signal modulated by 50KHz frequency, is a step wave, needs a low-pass filter for smooth filtering, reproduces the original 500Hz sine signal, and is attenuated by-0.0066 dB after passing through a three-order post-low-pass filter. From the above analysis, the cut-off frequencies of the pre-filter and the post-filter are only 4 steps, namely 59Hz, 590Hz, 5.9KHz and 59KHz, and the circuit is simple and easy to implement.
The third-order butterworth low-pass filter adopts a VCVS filter, the circuit diagram of which is as shown in fig. 1, and for the convenience of realizing the circuit function, the resistor R1 and the resistor R2 are set to be the same, i.e., R1 ═ R2 ═ R, and the relationship between the voltages of the nodes of the circuit is as follows:
V3＝KV2…………………………………(3)
wherein, K is 1+ R4/R3, which is the gain of N1 operational amplifier
Formula (4) is obtained by substituting formula (2) with formula (3) V2V 3/K, wherein V1 is V2(1+ SRC 1):
when the formulas (3) and (4) are substituted into the formula (1), the following are provided:
through calculation, the following formula is obtained:
thereby obtaining
Where Q is the quality factor of the second order filter.
In contrast to the transfer function of a third-order filter, Q is 1, i.e.
When K is 1, C2 is 4C 1. 2 1NF were connected in series to give C1 ═ 0.5 NF; 2NF were connected in parallel to give C2 ═ 2 NF. The disadvantage of this solution is that 4 capacitors are needed, which occupy the circuit board and are not used.
If C1 is 1NF and C2 is 3.3NF, K is 1.056, which is slightly larger than 1, and the total gain can be adjusted to 1 in the subsequent circuits.
Cut-off frequency of second order filter
Let R1 ═ R2 ═ R ═ 1.5K Ω, C1 ═ 1NF, and C2 ═ 3.3NF, obtain the cutoff frequency of the second order filter of 58.9 KHz.
Cut-off frequency of first order filter
Taking R5 ═ 2.7K Ω and C3 ═ 1NF, the cut-off frequency of the first order filter was 58.9 KHz.
The resistance values of the resistors R1, R2 and R5 are changed, the cut-off frequency of the three-order pre-filter and the cut-off frequency of the three-order post-filter are divided into 4 grades, the cut-off frequency is respectively 59Hz, 590Hz, 5.9KHz and 59KHz, and the gains are all 1.056. The relationship between the resistance values R1, R2, R5 and the third order filter is shown in the following table:
| cutoff frequency of third order filter | R1(KΩ) | R2(KΩ) | R5(KΩ) |
| 59Hz | 1500 | 1500 | 2700 |
| 590Hz | 150 | 150 | 270 |
| 5.9KHz | 15 | 15 | 27 |
| 59KHz | 1.5 | 1.5 | 2.7 |
The transfer function of the second order filter is H(S1)＝1/(S2+ S +1), the transfer function of the first order filter being H(S2)When the second-order filter and the first-order filter are connected in series, the transfer function H of the third-order filter is obtained(S)＝H(S1)*H(S2)＝＝1/(S2+S+1) × 1/(S +1), where S is normalized j ω. Substituting S to j ω then H(ω) 2＝1/((1-ω2)2+ω2)*
1/(1+ω2)＝1/(1-ω2+ω4)*1/(1+ω2)＝1/(1+ω6) A third order butterworth low pass filter. The gain of the pre-filter and the gain of the post-filter are only 1.056, the pre-filter and the post-filter are three-order Butterworth low-pass filters, the pass band is flat to the maximum, the required components are few, the circuit board area is small, the structure is simple, meanwhile, the attenuation of the stop band is high, and the requirements of the elliptic filter module on the pre-filter and the post-filter are met.
The above embodiments are only preferred embodiments of the present invention, and the protection scope of the present invention cannot be limited thereby, and any insubstantial changes and substitutions made by those skilled in the art based on the present invention are all within the protection scope of the present invention.
Claims (5)
1. The front and rear filters of the elliptic filter comprise an operational amplifier N1, a resistor R1-a resistor R5 and a capacitor C1-a capacitor C3, and are characterized in that: a pin III of the operational amplifier N1 is connected with an input end XS1 through a resistor R2 and a resistor R1 in sequence, a pin III of the operational amplifier N1 is connected with a pin I of the operational amplifier N1 through a resistor R2 and a capacitor C2 in sequence, and a pin III of the operational amplifier N1 is grounded through a capacitor C1;
the pin I of the operational amplifier N1 is connected with the pin II of the operational amplifier N1 after being divided by a resistor R4 and a resistor R3, the pin I of the operational amplifier N1 is connected with the pin V of the operational amplifier N1 through a resistor R5, the pin I of the operational amplifier N1 is connected with a capacitor C3 through a resistor R5 and then grounded, and the pin VI of the operational amplifier N1 is connected with the pin seven of the operational amplifier N1 and then connected with an output end XS 2.
2. The pre-and post-filters of an elliptic filter of claim 1, characterized in that: the first pin of the input end XS1 is grounded, and the second pin of the input end XS1 is connected with the third pin of the operational amplifier N1 through a resistor R1 and a resistor R2 in sequence.
3. The pre-and post-filters of an elliptic filter of claim 1, characterized in that: pin two of the output terminal XS2 is grounded, and pin one of the output terminal XS2 is connected to pin seven of the op-amp N1.
4. The pre-and post-filters of an elliptic filter of claim 1, characterized in that: and a pin four of the operational amplifier N1 and a pin eight of the operational amplifier N1 are vacant.
5. The pre-and post-filters of an elliptic filter of claim 1, characterized in that: the operational amplifier N1 is an LF353 operational amplifier chip.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202022325403.7U CN212811646U (en) | 2020-10-19 | 2020-10-19 | Pre-and post-filters for elliptic filters |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202022325403.7U CN212811646U (en) | 2020-10-19 | 2020-10-19 | Pre-and post-filters for elliptic filters |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CN212811646U true CN212811646U (en) | 2021-03-26 |
Family
ID=75090753
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN202022325403.7U Active CN212811646U (en) | 2020-10-19 | 2020-10-19 | Pre-and post-filters for elliptic filters |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN212811646U (en) |
-
2020
- 2020-10-19 CN CN202022325403.7U patent/CN212811646U/en active Active
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| JP3181588B2 (en) | Universal filter | |
| WO1995027333A1 (en) | Bootstrapped, ac-coupled differential amplifier | |
| CN105048989A (en) | Low pass filter with common-mode noise reduction | |
| CN111865344B (en) | Analog baseband circuit with variable gain and bandwidth | |
| GB2072445A (en) | Tone control of the operational type | |
| CN212811646U (en) | Pre-and post-filters for elliptic filters | |
| JP2008503964A (en) | Simplified salen key low-pass filter circuit | |
| CN210274516U (en) | Audio amplifying circuit and playback apparatus | |
| CN212259135U (en) | Computer image acquisition device | |
| CN210016446U (en) | Front-end filter circuit of long-wave receiver | |
| US3636466A (en) | Building block for active rc filters | |
| US4303889A (en) | Filter circuit | |
| CN214895863U (en) | Signal processing circuit for metal detector | |
| CN106374871B (en) | A kind of active band-pass filter applied to long-wave time service frequency range | |
| CN215268203U (en) | Filtering and amplifying circuit for small signals | |
| CN216930302U (en) | Circuit for eliminating loudspeaker noise | |
| CN219066131U (en) | Switching circuit and switching wire for USB-C digital audio interface | |
| CN203180863U (en) | An anti-aliasing low-pass filter of an airplane-mounted data collector | |
| CN208001269U (en) | Program control filtering system | |
| JPS6117369B2 (en) | ||
| EP1176711A2 (en) | Apparatus and method for electrical signal amplification | |
| CN203537349U (en) | A low order band-pass filter circuit | |
| US4085380A (en) | Biquad RC active filter | |
| JP2671278B2 (en) | Low-pass filter with delay equalization | |
| CN209218050U (en) | Stereo emulator based on RC double-T network |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| GR01 | Patent grant | ||
| GR01 | Patent grant |