US3369173A

US3369173A - Attenuator apparatus

Info

Publication number: US3369173A
Application number: US508784A
Authority: US
Inventors: Roland E Andrews
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1965-11-19
Filing date: 1965-11-19
Publication date: 1968-02-13
Anticipated expiration: 1985-02-13
Also published as: DE1541874A1; GB1116730A; NL6614872A; FR1501221A

Description

Feb. 13, 1968 Filed Nov. 19, 1965 R. E. ANDREWS 3,369,173

ATT ENUATOR APPARATUS 2 Sheets-Sheet 1 PRIOR ART o o --o X o -o 0- 0 KI V V V V ROLAND E. ANDREWS "INVENTOR BUCKHORN, BL ORE, KLAROU/ST a SPAR/(MAN v ATTORNEYS United States Patent i ABSTRACT OF THE DISCLOSURE A plurality of multiposition switches are ganged and are operable together for selecting multiple attenuation values. Plural attenuating sections are included between first and second switches, and other attenuating sections are included between third and fourth switches, while the second and third switches are normally coupled together. However, in at least one switching position wherein attenuation is not desired a shunt connection is provided from the first switch to a simultaneously selected contact of the fourth switch, for bypassing the various attenuator sections as well as the second and third switches so that circuit input capacitance is reduced.

This invention relates to attenuator apparatus and particularly to attenuator apparatus having low input capacitance and adaptable for accurately adjusting an input signal presented to an oscilloscope.

In oscilloscopes the basic deflection factor of the device determines the amplitude of the input signal as displayed on the viewing screen of the oscilloscopes cathode ray I tube. Through the use of amplifiers, an oscilloscope is usually quite sensitive and may therefore display signals of extremely small amplitude. However, in order to render the instrument useful in also displaying and measuring a range of larger amplitude signals, suitable attenuator apparatus is employed.

An aten'uator apparatus found extremely useful heretofore involves the cascading of selected attenuator sections in varying combinations by switching means having a plurality of switch sections and a plurality of switch positions. For example, an attenuator section designed for reducing an input signal by a factor of 2.5 can be employed in series with other attenuator sections having a selectable attenuation ratio of and 100 thereby resulting in an overall reduction selectable between 2.5, 25 and 250. A larger range of attenuation is possible with this type of cascaded arrangement than in a circuit employing a separate attenuator for each attenuation required.

An attenuator apparatus also desirably employs one or more switch positions wherein no attenuation is inserted in the circuit. In these switch positions the basic deflection factor of an oscilloscope will hold true. It has been the usual practice'with attenuator apparatus of the eascaded type to provide one or more switch positions which include no attenuator sections and involve direct connections between switch sections at the points where series attenuator sections would normally be located.

Although reduction of component parts and a wider choice of attenuation values is achieved with a cascaded attenuator, this type of arrangement contributes input shunt capacitance to the circuit. However reduction of such capacitance is an important factor in improving the response of an oscilloscope or the like. The worst case of attenuator input capacitance has been the instance wherein attenuator sections are excluded from the circuit, that is wherein straight-through connections or other substantially non-attenuating coupling means are substituted for the individual attenuator sections. This is because the at tenuator sections have the effect of reducing the effective 3,369,173 Patented Feb. 13, 1968 'ice capacitance as seen at the input terminal of the apparatus. Therefore the non-attenuating switch position is the worst offender. In order to maintain a constant input impedance for all attenuator switch positions, all other switch positions are adjusted to have the same relatively high capacitance as the worst case.

It is therefore an object of the present invention to provide an improved attenuator apparatus having lower input capacitance in the selectable positions thereof.

It is another object of the present invention to provide an improved attenuator apparatus achieving a wide range of attenuation values with economy of component parts, while holding the shunt input capacitance of the apparatus down to a minimum level in the switch position of the apparatus having minimum attenuation.

Briefly, in accordance with the present invention, attenuator apparatus comprises first and second groups of attenuating means wherein different attenuating means establish different degrees of attenuation. Switching means couple selected attenuating means from the first group with selected attenuating means of the second group, such switching means including a coupling section intermediate the first and second groups. The switching means has, however, at least one switching position for bypassing the groups of attenuating means and also bypassing the switching means coupling section therebetween to provide a low capacitance, low attenuation circuit.

In accordance with an embodiment of the present in vention, an attenuator apparatus includes first, second, third and fourth multiple position switching means which are preferably ganged together to form a multi-position attenuator switch. Various attenuator sections are included between first and second switching means and other attenuating sections are included between the third and fourth switching means, the second and third switching means being coupled together. The switching means are connected to place an attenuator section from a first group between first and second switching means in a cascaded relationship with a selected attenuator section from a second group between the third and fourth switching means. However, instead of providing straight-through connections in place of various attenuator sections in the instance when attenuation is not desired, a shunt connection is provided in at least one position of said switching means from the first switching means to corresponding or simultaneously selected contact of the fourth switching means, in this manner bypassing the second and third switching means.

In bypassing the second and third switching means the capacitance to ground thereof is avoided and the input capacitance of the attenuator apparatus, in what would otherwise be a high capacitance position, is substantially reduced. Moreover, not only is capacitance reduced in this position, but the input capacitance of other switch positions may be reduced while still maintaining constant input impedance for all switch positions.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further advantages and objects thereof, may be best understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a schematic diagram of attenuator apparatus in accordance with the prior art;

FIG. 2 is a schematic diagram of an improved attenuator apparatus in accordance with the present invention;

FIG. 3 is a schematic diagram of an attenuator section employed in the circuit of FIG. 2;

FIG. 4 is a schematic diagram of a second attenuator section employed in the circuit of FIG. 2;

FIG. 5 is a schematic diagram of a third section employed in the circuit of FIG. 2; and

FIG. 6 is a schematic diagram of a fourth section employed in the circuit of FIG. 2.

Referring to FIG. 1, illustrating a conventional prior art attenuation circuit, the circuit includes

attenuator sections

3, 4, 5 and 6 and also includes

input terminals

10 and 12, the latter terminal being grounded and the former terminal being connected to movable contact 14 of a multiple position switch section 16. Switch section 16 is ganged to form a control switch with

similar switch sections

18, 20 and 22, as well as a switch (not shown) within amplifier 24. The switch sections are operable together, for example by means of a single control. Switch section 18 has a movable contact 26 coupled via small resistor 28 to movable contact 30 of switch section 20. Movable contact 32 of the switch section 22 is connected attenuator attenuator to input resistor 34 of amplifier 24 with the lower end of the resistor returned to ground. Between resistor 34 and amplifier 24,

series resistors

36 and 38 are interposed with the former having a capacitor 40 shunted thereacross. A variable capacitor 42 is located between the interconnection of

resistor

36 and 38 and ground.

In the first three positions designated by Roman numerals I to III of the ganged switching means including

switch sections

16, 18, 20 and 22, the input contacts of switch section 16 are connected to the first three output contacts of switch section 18. Likewise the first three input contacts of switch section 20 are connected to the first three output contacts of switch section 22. Also, the fourth and fifth input contacts of switch section 16 are connected to the fourth and fifth output contacts of switch sections 18. Between the sixth, seventh and eighth input contacts of switch section 16 and the sixth, seventh and eighth output contacts of switch section 18 is interposed the attenuator section 5 having the property of attenuating a signal presented thereto by a factor of ten. Hence, attenuator section 5 is labeled with the designation +10. Between input contacts nine, ten and eleven of switch section 16 and the ninth, tenth, and eleventh output contacts of switch section 18, there is interposed the attenuator section 6 for dividing the signal presented thereto by one hundred.

The attenuator section 3 has a division factor of 2.5 and is connected between the fourth, seventh and tenth input contacts of switch section 20 and the fourth, seventh, and tenth output contacts of switch section 22. Furthermore, the attenuator section 4 having a division factor of 5 is connected between the fifth, eighth and eleventh input contacts of switch section 24] and corresponding output contacts of switch section 22. A direct or divide by 1 connection is present between the sixth and ninth input contacts of switch section 20 and the sixth and nineth output contacts of switch section 22.

The circuit of FIG. 1 illustrates an attenuator apparatus employing a switching arrangement of the general type as heretofore utilized. This apparatus, of the cascaded attenuator type, accomplishes the purpose of selectively reducing or dividing the input signal presented across

terminals

10 and 12 by selected predetermined amounts. A principal use of this type of circuit is in the input circuit of an oscilloscope. An input signal may have a wide variety of values but in each case the waveform portrayed on the screen of an oscilloscope cathode ray tube should occupy a reasonable portion of such screen and moreover the size of the waveform on the cathode ray tube screen may be used as an accurate measure of the waveform amplitude. In one position of the switching means, position III in the device of FIG. 1, the amplitude of the input signal can be determined from the basic deflection factor of the oscilloscope apparatus. However, in all other positions of the switching means, the input signals amplitude is altered by a predetermined factor.

In positions I and II of the switching means, amplification in addition to that providing such basic deflection factor is inserted in the circuit within amplifier 24 by switching means which is not shown. However, from position IV to position XI, a selectable amount of attenuation is inserted between the input terminal 10 and the amplifier 24.

Very little attenuation is attributable to

resistances

28, 36 and 38 since these resistances are small and their primary function is one of high frequency damping. In the position IV of the switching means, the input signal is divided by 2.5 inasmuch as the signal passes through series attenuator section 3 having that factor of attenuation. In position V of the switching means, the input signal is similarly divided by five through attenuator section 4. In position VI, atenuator section 5 causes a reduction of ten in the amplitude of the signal and the

switch sections

20 and 22 provide straightthrough connections. However, in position VII of the switching means, both attenuator section 5 and attenuator section 3 are cascaded for a total of attenuation of 25. In successive positions VIII to XI of the switching means, successive combinations of

attenuator sections

3, 4, 5, and 6 are cascaded. The cascaded attenuator arrangement reduces the number of attenuator sections and component parts required for a given range of attenuation values. A great deal more attenuation circuitry would be required, for example, if a separate attenuator section had to be inserted in series with the input and output for each desired value of attenuation.

However useful, the foregoing circuitry is not without disadvantages inasmuch as the attenuator apparatus presents an input capacitance tending to increase the rise time and reduce the high frequency response of signals presented at the input terminal. Some switch positions of the attenuator apparatus contribute a greater effective shunt capacitance to ground than certain other switch positions of the attenuator apparatus, or the shunt capacitance at all switch positions is made equal to the switch position which would otherwise provide the greatest shunt capacitance.

For a fuller appreciation of the problems involved, reference is made to FIGS. 3 through 6, more fully illustrating attenuator sections 3 through 6 respectively, and in which similar elements are referred to with the same reference numerals. Each of the attenuator sections of FIGS. 3 through 6 includes an input terminal 44, an output terminal 46 and a voltage

divider comprising resistors

48 and 50 connected between terminal 44 and ground.

Output terminal 46 is coupled to the midpoint of this divided via damping resistor 52. This voltage divider is an effective means for reducing the amplitudes of DC or low frequency signals substantially in the proportion the resistance of resistor 50 bears to the sum of the resistance of

resistors

48 and 50. However, as the frequency increases, AC compensation is required and hence a capacity voltage

divider including capacitors

54 and 56 is included. Capacitor 54 is shunted across resistor 48 and capacitor 56 is shunted across resistor 50. In the circuit of FIG. 3, the capacitor 56 is shown dotted since the capacitance thereof is contributed by portions of the circuit including any circuit to which the output terminal 46 is connected, as hereinafter described. However, an acual capacitor 56 is also included in the other three attenuator sections illustrated. At higher frequencies, the voltage divider becomes elfectively a capacitance divider. In order to avoid problems of phase shift and the like, the RC time constant of each resistor-capacitor combination, including the capacitance of the circuitry across output terminal 46, is made equal to the RC time constant of the remaining parallel pair. Capacitor 58 is a standardizing element for balancing the different attenuators whereby to provide substantially equal input capacitance for each.

As will be appreciated by those skilled in the art, the input capacitance of one of these attenuator sections, not

considering the standardizing capacitor, is .substantially equal to capacitance 54 in series with capacitance 56 including circuit capacitance across the latter. Also stray capacitance appears between input terminal 44 and ground attributable to switch elements and the like.

In an instance wherein an attenuator section is switched into circuit of FIG. 1, the input capacitance seen at the input terminal of the attentuator can be less than the input capacitance of the amplifier to which the attenuator section connects. Since series capacitance 54 is placed in series with capacitance 56, including the input capacitance of the amplifier stage to which terminal 46 is coupled, the input capacitance seen at terminal 44 is reduced. For example, the input capacitance at terminal 44 is equal to The capacitance in the attenuator section thus has the effect of reducing the input capacitance of the apparatus as seen at terminal 44 and approximately by the attenuator ratio.

However, in some positions of the attenuator apparatus switching means, no attenuator sections are included, but rather straight connections are substituted for cascaded attenuator sections. For example, in the first three switching positions in the apparatus of FIG. 1, input terminal is effectively connected to the input of amplifier 24 via the switching sections and there is no reduction of shunt capacitance by attenuator sections. These positions I to III, i.e., the straight-through positions of the switching means, represent the worst input capacitance case for the apparatus. Less capacitance can be tolerated in the switching means in these three positions since the amplifier input capacitance is fully present at terminal 10 and is not isolated by an attenuator. Capacitance can be reduced somewhat through good design practices and the use of high quality switching components. However, high input capacitance has heretofore been a limiting factor in the design of oscilloscopes and the like wherein an accurate response to high frequency signals is desired.

In accordance with the present invention, the problem of high input capacitance is alleviated by employing apparatus of the type illustrated in FIG. 2. The apparatus of FIG. 2, like FIG. 1, includes ganged switching means for cascading attenuating

sections

3, 4, 5, and 6 in various combinations to provide a wide range of attenuation values with a minimum of attenuator structure. The principal difference between the circuit of FIG. 1 and that of FIG. 2 is that in the circuit of FIG. 2 a shunt connection 60 is provided betwen switch section 16 and switch section 22 in the positions I to III of the ganged switching means, that is, for those positions involving no attenuation. As can be seen from FIGS. 1 and 2, the circuit of FIG. 2 differs from the prior apparatus in that

intermediate switch sections

18 and 20 are bypassed in straight-through positions of the switching means and the function of damping resistor 28 in FIG. 1 is provided by means of

resistors

62, 64 and 66 in the FIG. 2 apparatus.

Referring further to the FIG. 2 apparatus, a connection 60 extends directly between the first 3 contacts 68, 70 and 72 of the switch section 16 (those for the .005, the .01, and the .02 volt positions in this particular attenuator apparatus) and

corresponding contacts

74, 76 and 78 of switch section 22. In this manner, the input capacitance to the attenuator apparatus is reduced fifty percent, with a consequent fifty percent improvement in rise time. Reduction in capacitance is principally attributable to reduction in the switch capacitance present in the positions which insert no attenuating sections into the circuit.

Referring further to the circuit of FIG. 2 in the IV and V positions of the switching means, input contacts of switch section 16 are connected to output contacts of switch section 18. In the switch position IV of the apparatus, attenuator section 3 is interposed between the input contact of switch section 20 and the output contact of switch section 22. Attenuator section 3 has been illustrated in FIG. 3 and explained in connection therewith. In the position V, attenuator section 4, illustrated in FIG. 4, is connected between the input contact of switch section 20 and the output contact of switch section 22. The eighth and eleventh switch positions numbered VIII and XI. Attenuator section 5, illustrated in FIG. 5, is connected between input contacts of switch section 16 and output contacts of switch section 18 in positions VI, VII and VIII. In the switch position VI, the input contact of switch section 20 is coupled to the output contact of switch section 22 via a damping resistance 66. A capaci-' tance shunted across resistance 66 aids the high frequency response of the circuit. This combination is similarly interposed between the input contact of the switch section 20 in switch position IX thereof, and the output contact of switch section 22. Attenuator section 6, illustrated in FIG. 6, connects the input contacts of switch section 16 to the output contacts of switch section 18 for switch positions IX, X and XI of the apparatus.

Resistors

36, 62 and 64 are damping resistances, 36 and 62 being bypassed with

capacitors

40 and 41 to aid the high frequency response of the circuit.

As hereinbefore noted, one of the straight-through positions of the switching apparatus, for example, position III or the 0.2 volt position, represents the standard straight-through or non-attenuating position for the apparatus. In this position,-the basic deflection factor for an oscilloscope apparatus employing the present circuit is selected. In positions I and II of the switching apparatus, additional amplification is provided by the amplifier 24 and hence amplifier 24 is shown ganged to the switching apparatus.

A variable input capacitor 82 is connected across input resistor 34 of amplifier 24 in order to adjust or standardize the input capacitance of the amplifier. This capacitor is adjusted in the non-attenuating position of the switching apparatus, for example, in position III, or the .02 volt position. Resistor 84 is another high frequency damping resistor. For the positions involving attenuator section 3, the capacitance of capacitor 82 is chosen to be of a value to provide a part of the voltage divider, specifically part of capacitor 56 in FIG. 3. In FIGS. 4, 5 and 6, the capacitance 56 is selected to provide the proper voltage division and time constant for the attenuating sections taking the input capacitance of the amplifier including the capacitance of capacitor 82 into consideration.

Variable capacitor 58 in FIGS. 3, 4, 5, and 6 is employed as a standardizing capacitor and is adjusted to provide the same apparatus input capacitance in the various switch positions. The capacitors 58 for

attenuator sections

5 and 6 are adjusted in switch positions VI and IX respectively, wherein the output terminals of these attenuator sections are connected to the input circuit of amplifier 24, However, in switch positions VII, VIII, X and XI wherein these attenuating sections are placed in cascaded relation with either attenuator sections 3 or 4, an additional shunt capacitance is needed for further standardizing purposes, because of the isolating effect of attenuator sections 3 and 4. Hence an additional capacitor 86 is placed in shunt in switch positions VII, VIII, X and XI via an additional switch section 88 having a ganged contact 90 connected with

switch contacts

26 and 30. The overall result is a cascaded attenuator apparatus having a much lower input capacitance than that of the prior art.

While I have shown and described a specific embodiment of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim:

1. Attenuator apparatus having an input terminal and an output terminal, said apparatus being adapted for the cascading of attenuating means selected from a first group with attenuating means selectedfrom a second group and comprising first switching means having input and output sections for selecting and placing therebetween one of a first group of attenuating means in circuit between said input and output terminals, second switching means having input and output sections for selecting and placing therebetween one of a second group of attenuating means in cascade relation with said one of first group of attenuating means,

means for conjointly operatingboth said first and second switching means to select predetermined combinations of attenuating means from said groups,

and switching means also conjointly operated with said first and second switching means bypassing an output section of said first switching means and an input section of said second switching means in a predetermined switching position for providing a selectable low capacity path between said input and output terminals.

2. Attenuator apparatus having multiple selectable switching positions for cascading a selected one of a first group of attenuating means having input and output terminals with a selected one of a second group of attenuating means having input and output terminals comprising multiple position ganged switching means including a switch section for connecting to an input terminal of a selected attenuating means of a first group, a second switch section for simultaneously connecting to the output terminal of the same attenuating means of said first group, a third switch section for conmeeting to the input terminal of a selected cascaded attenuating means of said second group, a fourth switch section for simultaneously connecting to the output terminal of the same cascaded attenuating means of said second group,

coupling means interposed between said second switching section and said third switching section operable over a plurality of positions of said ganged switching means for completing serial coupling between said selected one of said first group of attenuating means and said selected cascaded attenuating means of said second group,

means for operating said first, second and third and fourth switch sections together for simultaneous selection of attenuating means from said first and second groups,

and means coupling together said first switching section and said fourth switching section in at least one position of said ganged switching means to provide an uncascaded switching position of low attenuation and low capacitance.

3. Attenuator apparatus comprising a first group of attenuating means each having respective input and output terminals, different of said attenuating means establishing a different degree of attenuation between said input and output terminals,

a second group of attenuating means each having input and output terminals with different attenuating means of said second group establishing a different degree of attenuation between their input and output terminals,

switching means for coupling a selected attenuating means from said first group in cascaded relation with a selected attenuating means of said second group, said switching means having a coupling section intermediate said first group and said second group to complete a cascaded circuit relation,

said switching means having at least one switching position connected for bypassing said first group and said second group and also bypassing said section between said first and said second group of attenuating means to provide a low capacitance, low attenuation circuit.

4. The apparatus according to claim 3 wherein each of said attenuating means comprises a resistance voltage divider including a pair of resistors in series connected between input and output terminals thereof, and a capacitor coupled across each resistor for also providing a capacitance voltage divider.

5. The apparatus according to claim 3 wherein said switching means is Wired for coupling one attenuating means from said first group to selected different attenuating means of said second group in diiferent switching positions of said switching means, and wherein further positions of said switching means connect further attenuating means from said first group to the said selected dilferent attenuating means of said second group.

6. Attenuator apparatus having multiple selectable switch positions for cascading selected ones of a first group of attenuating means with a selected one of a second group of attenuating means comprising an apparatus input terminal,

a first multiple position switch for coupling said apparatus input terminal to a selected input of one of a first group of attenuating means,

an apparatus output terminal,

a second multiple position switch for connecting said apparatus output terminal to an output of one of said second group of attenuating means,

coupling means intermediate said first group of attenuating means and said second group of attenuating mens,

a third multiple position switch for coupling an output of one of said first group of attenuating means to said intermediate coupling means,

a fourth multiple position switch for connecting said intermediate coupling means to the input of a selected one of said second group of attenuating means,

means ganging said first, second, third and fourth switches together wherein said third switch in a selected switching position connects to the output of the same attenuating means, the input of which is connected to said first switch, and wherein said fourth switch in the selected position connects to the input terminal of an attenuating means of said second group, the output of which is connected to said second switch,

said switches being interconnected with said attenuating means such that in various positions of the said switches differing possible combinations of attenuating means from said first and second groups are cascaded,

said switches having at least one low attenuation switching position for coupling the apparatus input terminal and the apparatus output terminal via said'first switch and said second switch bypassing said third and fourth switches in order to reduce the shunt capacitance associated with such switchingposition.

7. The apparatus according to claim 6 including means directly connecting a selected contact of said first switch in said low attenuation switching position to a selected contact ofsaid second switch in the said low attenuation switching position.

8. The apparatus according to claim 6 including high frequency damping resistance in series with said apparatus input and output terminals on the remote side of said third and fourth switches from the said coupling means intermediate said first group of attenuating means and said second group of attenuating means.

References Cited UNITED STATES PATENTS 2,999,202 9/1961 Ule 32 -74 3,014,187 12/1961 Sher et al. 333-81 3,252,080 5/1966 Newbold et al. a 32374 JOHN F. COUCH, Primary Examiner.

WARREN E. RAY, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,369,173 February 13, 1968 Roland E. Andrews It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 50, for "divided" read divider column 5, line 50, for "betwen" read between column 6, line 5, after "The" insert same attenuator section is similarly connected in the Signed and sealed this 22nd day of April 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents