Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS3603898 A
Publication typeGrant
Publication dateSep 7, 1971
Filing dateFeb 2, 1970
Priority dateMar 14, 1969
Also published asDE2017285A1, DE2017285B2, DE2017285C3
Publication numberUS 3603898 A, US 3603898A, US-A-3603898, US3603898 A, US3603898A
InventorsDawson John Howard, Fripp David George Francis
Original AssigneeMarconi Co Ltd
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
N-parallel-path capacitive switched filter in which only incomplete spaced portions of input signal are sampled
US 3603898 A
Abstract  available in
Images(8)
Previous page
Next page
Claims  available in
Description  (OCR text may contain errors)

nit @ttea tet l l NH ll llL'lllEllt llhl Wllllll Cllll ONLY WEQMIPLETE SPACEID llt'lllflNS Oll llltlll lll'll STEIN/ML AlltlE SAMPLED l2 Claims, 11) Ill/trawling ll iga.

US. Ell. 333/70 A,

llnt. lCl lllllllh 7AM Wield oil Search 333/70, 70

[56] References Eited UNITED STATES PATENTS 2,584,986 2/1952 Clark 333/70 AU OTHER REFERENCES LePage et al. Analysis of a Comb Lifter Using synchronously Conimutated Capacitors, AIEE, March I953 333-7OA Primary Examiner llerman Karl Saalbach Assistant Examiner-Paul L. Gensler AttorneylBaldwin, Wight & Brown 'll'lltlliC'l: The invention provides a band-stop filter including a plurality n of switchably selectable capacitive parallel paths. Switch means are provided for sequentially and selectively selecting the paths, the switch means being controlled to select each path for a period. of time which is less than l/nth (usually 1/411) of the period of a complete switching sequence of all of the paths.

M ii LOW 77 S 7 3 /T R PATENTEUSEP 7197i 3,603,898

SHEET 2 OF 8 FIG. 31

FIG 4 INVENTORS 061, 9% A fmm/ 7% BY 5154 062) WW U41 a: M62225 PATENTEUSEP 7:971 $603,898

SHEET b 0F 8 INVENTORS JMW W BY MM WW K Um/acznwu ATTORNEY PATENTEUSEP Hem 3,603,898

SHEET s or 8 LOW PASS F/LTER v) 8 5 I\ E R i PASS BAND STOP FREQUENCY BAND LOW 77 PASS FILTER m INVE TORS B) aWU/MM ZZ A Afifi ATTORNEYS PATENTEB SEP "/1911 3593 9 sum 5 BF 8 VOLTAGE ACROSS INV NTORS PATENTED SEP Hen 3.603; 898 SHEET a nr 8 INVENTORS BY MW WWW MANOR 13g:

N-WilliALlLlEIL-FAIII CAFA CITIVlE SWII'IICIIIIIEID FIllL'IlEIIi IN WllIlltCIll UhllL'I! llhltJtIh/IIPLIE'IIIE SF/tClED IPQIIITIIINS OF Il IlPlU'll I BIGNAIL Mill IiAilt/IIPILIEID This invention relates to filter networks and more particularly to N-path band-stop filter networks.

The invention is explained by reference to the accompany ing drawings in which:

FIG. I is a schematic diagram of a typical known N-path band-stop filter network; and

FIGS. 2 to 6 are explanatory diagrams relating thereto;

FIGS. 7 to 112 are explanatory diagrams relating to N-path band stop filters in accordance with the present invention;

FIG. I3 is a schematic diagram of a modification of an N- path band stop filter in accordance with the present invention; and

FIGS. I l, 15 and I6 are explanatory diagrams relating thereto;

FIG. I7 illustrates a further modification;

FIG. lid is a schematic diagram of another modification of an N-path band stop filter in accordance with the present invention; and

FIG. I9 is an explanatory diagram relating thereto.

Referring to FIG. II, the known N-path band-stop filter network therein represented consists of a number n of capacitors ll, 2, 3, 4i each in series with a switch 5, s, 7 or d to form n switchably selectable capacitive paths connected in parallel in one arm of the network between input terminals 9, l and output terminals 11, 12. Each capacitor 1, 2, 3 or t may be associated, by closing the respective switch 5, s, 7 or 8 with a common shunt resistor I3 to form a CR filter. Between the CR filter so formed and the output terminal III is connected a low pass filter M having no appreciable loading effect on the previous circuit.

The switches S, 6, 7 and 8 are closed sequentially and in turn by means not shown, the switch 6 closing as the switch 5 opens, the switch 7 closing as the switch 6 opens and so on. In other words each switch is closed for a period equal to l/nth of the period of a complete switching sequence, where n is the number of switchable capacitive paths. This is illustrated in FIG. 2 in which the excursions from the base line S5 represent times during which switch 5 of FIG. I is closed; the excursions from the base line S6 represent time during which switch s of FIG. I is closed; the excursions from the base line S7 represent times during which switch '7 of FIG. I is closed; and the excur sions from the base line S8 represent times during which switch II of FIG. I is closed. Ifa sinusoidal input as represented in FIG. SI having a period IIT, where dT (see FIG. 2) is the period of a complete switch sequence of the switches 5, a, 7 and ii, is applied to the input terminals 9, III) of FIG. ll, switch 5 will sample the portions of the sinusoidal waveform shown shaded in FIG. 3 and of duration T; switch a will sample the following waveform portions of duration T; and so on. It will be seen that no charge accumulates in the capacitors and the whole of the sinusoidal wave appears as output across resistor I3.

If, as shown in FIG. I, a sinusoidal input having a period tlT is applied to the input each capacitor has applied to it a voltage similar in magnitude and sign at each sampling and thus charges up to the average of the sinusoidal input during the time its switch is closed. The output across resistor I3 is thus reduced. FIG. 5 shows the output across resistor 113 under these conditions when switch 5 is closed. The overall response is a series of notches, as shown in FIG. 6, occurring at zero frequency and harmonics of frequency 1/41. The shape of the CR filter provided by each of the I i-paths is reproduced on either side of the center rejection frequencies. The effect of the low pass filter M is shown in dashed line. The notch bandwidth at 3 db. attenuation is l/ NrrrCR.

Except at zero frequency, where the notch depth is infinite, the notch depth is limited since there is still a considerable output at the synchronous frequency 1/41.

The object of the present invention is to provide improved N-path band stop filter arrangements of increased notch depth.

According to this invention a band-stop filter including a plurality n of switchably selectable capacitive parallel paths and means for sequentially and repeatedly selecting said paths comprises means for selecting each path for a period of time which is less than l/nth of the period of a complete switching sequence of all of said paths.

The important result achieved by the invention is that there are during the switching sequence period, periods during which none of the paths are selected to be in circuit.

In a preferred embodiment of the invention, each path is selected for a period of time which is equal to l/4n of the period ofa complete switching sequence of all of said paths.

Preferably the band-stop filter is provided with sampling and holding means connected and arranged to hold the output signal of a selected path substantially level during the period which follows the switching off of that path and during which none of the paths are selected.

Preferably the sampling and holding means is included between a common shunt resistor at the output ends of said paths and a low pass further filter through which output from the band-stop filter is taken.

Preferably the sampling and holding means consists of a series connected switch which, when closed, applies output signals from said paths to charge a shunt connected capacitor, said series connected switch being so controlled as to be closed during periods in which any of the paths is selected and open during periods in which none of said paths is selected.

Preferably again the control of the series connected switch is such that it is closed after a short delay following the selection of each path. This short delay may conveniently be one quarter ofthe time period during which the path is selected.

Preferably again the control of the series connected switch is such that it is opened at approximately the middle of the time period during which a path is selected. Preferably means are provided for adjusting the time of opening of the said series connected switch.

In another way of carrying out the invention the band stop filter is provided with sampling and holding means which precede the plurality of switching selectable capacitive parallel paths, and are adapted and arranged to sample an input waveform prior to a path being selected and to hold the samples value thereof substantially level during the period in which said path is selected.

Preferably the said last mentioned sampling and holding means is arranged to sample the input waveform during each period in which none of the paths are selected and to hold the samples value substantially level between consecutive samples.

Preferably also said last mentioned sampling and holding means comprises a series connected switch which, when closed, applies the input waveform to charge a shunt connected capacitor which is connected to apply voltage thereacross to said paths.

Referring to FIGS. 7 to I7 illustrating the present invention, theory shows that the operation of an Npath band'stop filter is only as described with reference to FIGS. I to t for input frequencies of up to l/2T. For ease of explanation of the embodiments of the invention now to be described, a 4i path band-stop filter network will be assumed and the explanation will be concerned only with the notches at zero frequency and 1 /4T. As mentioned hereinbefore the notch at zero frequency is infinitely deep. The purpose of embodiments of the present invention now to be described is, in all cases, to increase the depth of the notch at I MT.

In accordance with this invention the switches 5, s, 7 and II of a band stop filter which, regarded purely as a circuit, is like that of FIG. I, are each closed (i.e. rendered conductive) for a period which is less than l/nth of the total switching sequence period IT in FIG. 2) where n is the number of paths, in this case I. In other words the switches 5, s, 7 and h of FIG. I are now so controlled by means (not shown) that, in contradistinction to a known band-stop filter as above-described with reference to FIGS. 1 to 6, all the switches are open (i.e. nonconductive) together for parts of the period following the closing of one of the switches. In the specific example being described, each switch is closed for a period equal to l/l6th (i.e. l/4n) of the total switching sequence period. This is illustrated in FIG. 7 which, like FIG. 2, shows base lines S5, S6, S7 and S8, the excursions from which represent times during which the switches 5, 6, 7 and 8 are closed. The corresponding sampling of an input sinusoidal wave of period 4T by all the switches is shown in FIG. 8, whilst the output due to the wave of period 4T developed across resistor 13 is shown in FIG. 9. This corresponds to a stop band. The corresponding sampling of an input sinusoidal wave of period 8T by all the switches is shown in FIG. 10, whilst the output which is due to a wave of period ST and is developed across resistor 13 is shown in FIG. 11. This corresponds to the pass band. FIG. 12 shows the overall response. Again the dashed line represents the effect of the low pass filter 14.

It is obviously unnecessary to illustrate or describe in detail the means for controlling the switches since such means may take any of a number of suitable forms which will suggest themselves to those skilled in the art.

The output (at the input of the low pass filter) of the band stop filter above described with reference to FIGS. 7 to 12 will be a series of narrow pulses within the pass band. These narrow pulses give, when filtered, an attenuated value of the input signal.

In the further embodiment of the invention illustrated in FIG. 13 a larger proportion of the value of the input signal may be obtained at the output. The arrangement shown in FIG. 13 is generally similar to that above described with reference to FIGS. 7 to 12 but with the addition of a sampling and level holding circuit which precedes the input to the low pass filter 14 and which consists of a switch 15, a capacitor 16, and a buffer stage 17. This sampling and holding circuit is operated to hold substantially constant the level of a pulse applied to resistor 13 whilst the switches 5, 6, 7 and 8 are open. FIG. 14 illustrates the operation of 15. As with FIG. 7, excursions from the base lines S5, S6, S7, S8 and S15 of FIG. 14 represent the periods of closing of the switches 5, 6, 7, 8 and 15 respectively.

When switch 15 is closed capacitor 16 quickly charges up to the level of the signal pulse, and this level is held during the period in which switch 15 is open.

FIG. 15 shows a typical waveform for a signal in the pass band for one period T following the closing of switch 15. Period T is the period during which the switch 15 is closed and capacitor 16 is charging up. Period T2 is the period during which switch 15 is open and the signal level is held by capacitor 16.

FIG. 16 shows a typical waveform for a signal in the stop band for one period T following the closing of switch 15. Again period T is the period during which switch 15 is closed and period T is the period during which switch 15 is open. T For a sample having a width T much less than T, the line AB in FIG. 16 tends towards the linear and P, the point of zero output, becomes the center point. If, as illustrated in FIG. 17, switch 15 is controlled to be closed for a short period at or about the point P the stop band attenuation is improved whilst the pass band characteristic is maintained substantially unchanged.

The use of the sampling and holding circuit 15, 16 as described also simplifies the task of the low pass filter 14 of smoothing the output since, as will be appreciated, the output thus provided is much closer to the finally desired output than is an output consisting of a series of relatively short pulses.

Again the means (not shown for controlling the switching of switch 15 may take any number of suitable forms, known per se.

The means controlling the opening and closing of the switch 15 may be such that the point P, i.e. the time at which switch 15 opens, is adjustable so as to permit the maximum attenuation at the notch center, or at a nearby frequency, to be obtained.

Providing an adjustable delay before switch 15 is closed and providing means for adjusting the duration of the signal which controls switch 15 are two ways of achieving this. Again the practical implementation of this feature of the present invention is within the competence of the skilled man and detailed description or illustration is unnecessary.

The filter arrangement in FIG. 18 generally similar to that described with reference to FIGS. 7 to 12 but with the addition in front of the band-stop filter of a sampling and level holding circuit consisting ofa switch 18 and a capacitor 19. As will be seen a buffer stage 20 is interposed between the sampling and holding circuit 18, I9 and the N-path band stop filter. The operation of the arrangement of FIG. 18 is explained with reference to FIG. 19 in which:

a. shows a sinusoidal input wave of frequcncyf=( l/4T) b. shows the times of closing of switch 18 as excursions from a base line S18;

0. shows the charge upon capacitor 19 as a result of sampling the input wave (a) by switch 18; and

d. shows the times of closing of switches 5, 6, 7 and 8 as excursions from base lines S5, S6, S7 and S8 respectively.

As will be seen switch 18 samples the input wave for short periods and the voltage present is held by capacitor 19, resulting in a substantially flat topped voltage waveform being present for sampling by the switches 5, 6, 7 and 8. The switches 5, 6, 7 and 8 are synchronized to sample in the periods when switch 18 is open, i.e. on flat portions of the voltage waveform (c). Thus there can be no appreciable change of input voltage whilst any of the switches 5, 6, 7 or 8 are sampling. At or near the frequencyf=( l/4T) the average value of the input during sampling by switches 5, 6, 7 and 8 is removed and the portion of the input wave e the low pass filter 14 is reduced, thus tending further increasing the notch depth at l/4T.

We claim:

1. A band-stop filter including a plurality n of switchably selectable capacitive parallel paths, means for sequentially and repeatedly selecting said paths wherein each path is selected for a period of time which is less than l/nth of the period of a complete switching sequence of all of said paths, a common shunt impedance at the output end of said paths, and a low pass filter through which output from the band-stop filter is taken.

2. A filter as claimed in claim 1 wherein each path is selected for a period of time which is equal to l/4n of the period of a complete switching sequence of all of said paths.

3. A band-stop filter as claimed in claim 2 wherein there is provided sampling and holding means connected and arranged to hold the output signal of a selected path substantially level during the period which follows the switching off of that path and during which none of the paths are selected.

4. A band-stop filter as claimed in claim 3 wherein the sampling and holding means is included between said common shunt impedance at the output ends of said paths and said low pass filter through which output from the band-stop filter is taken, said shunt impedance being a resistor.

5. A band-stop filter as claimed in claim 3 wherein the sampling and holding means consists of a series connected switch which, when closed, applies output signals from said paths to charge a shunt connected capacitor, said series connected switch being so controlled as to be closed during periods in which any of the paths is selected and open during periods in which none of said paths is selected.

6. A band-stop filter as claimed in claim 5 wherein the control of the series connected switch is such that it is closed after a short delay following the selection of each path.

7. A band-stop filter as claimed in claim 6 wherein the delay is substantially one quarter of the time period during which the path is selected.

ll. A band-stop filter as claimed in claim 5 wherein the control of the series connected switch is such that it is opened at approximately the middle of the time period during which a path is selected.

9. A band-stop filter as claimed in claim 5 wherein means are provided for adjusting the time of opening of the said series connected switch.

110. A band-stop filter as claimed in claim ll wherein the band-stop filter is provided with sampling and holding means which precede the plurality of switehably selectable capacitive parallel paths, and are adapted and arranged to sample an input waveform prior to a path being selected and to hold the sampled value thereof substantially level during the period in

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2584986 *Apr 24, 1946Feb 12, 1952Fed Telephone & Radio CorpSelective wave filter
Non-Patent Citations
Reference
1 *LePage et al. Analysis of a Comb Lifter Using Synchronously Commutated Capacitors, AIEE, March 1953 333-70 70 A
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3761816 *Sep 8, 1972Sep 25, 1973Bell Telephone Labor IncData set employing a commutating capacitor, tracking, notch filter
US4039979 *Jun 18, 1975Aug 2, 1977Bell Telephone Laboratories, IncorporatedReduction of aliasing distortion in sampled signals
US4151474 *Aug 18, 1977Apr 24, 1979Raytheon CompanyVariable bandwidth pass-band filter
US4218665 *Mar 17, 1978Aug 19, 1980Nippon Electric Company, Ltd.Band-pass filter
US4392068 *Jul 17, 1981Jul 5, 1983General Electric CompanyCapacitive commutating filter
US5466976 *Dec 28, 1992Nov 14, 1995Nec CorporationTime constant detecting circuit and time constant adjusting circuit
US5473526 *Apr 22, 1994Dec 5, 1995University Of Southern CaliforniaSystem and method for power-efficient charging and discharging of a capacitive load from a single source
US5574633 *Feb 23, 1994Nov 12, 1996At&T Global Information Solubions CompanyMulti-phase charge sharing method and apparatus
US6016018 *Nov 21, 1997Jan 18, 2000Alps Electric Co., Ltd.Electronic apparatus incorporating battery
US6985142Sep 3, 1999Jan 10, 2006University Of Southern CaliforniaPower-efficient, pulsed driving of capacitive loads to controllable voltage levels
US7502603 *Jun 3, 2005Mar 10, 2009Maxim Integrated Products, Inc.DC cancellation in zero-IF receivers
US7663618Nov 18, 2005Feb 16, 2010University Of Southern CaliforniaPower-efficient, pulsed driving of capacitive loads to controllable voltage levels
US20130335163 *Mar 28, 2013Dec 19, 2013Nokia CorporationMethods and apparatuses for implementing variable bandwidth rf tracking filters for reconfigurable multi-standard radios
USRE37552 *Dec 5, 1997Feb 19, 2002University Of Southern CaliforniaSystem and method for power-efficient charging and discharging of a capacitive load from a single source
USRE38918 *Jan 10, 2001Dec 13, 2005University Of Southern CaliforniaSystem and method for power-efficient charging and discharging of a capacitive load from a single source
USRE42066Jan 21, 2005Jan 25, 2011University Of Southern CaliforniaSystem and method for power-efficient charging and discharging of a capacitive load from a single source
Classifications
U.S. Classification333/173, 333/172, 327/556
International ClassificationH03H19/00
Cooperative ClassificationH03H19/002
European ClassificationH03H19/00A