US3622919A

US3622919A - Step attenuator of low inductance and high bandwidth

Info

Publication number: US3622919A
Application number: US830071A
Authority: US
Inventors: Darrel I Wilhoit
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1969-06-03
Filing date: 1969-06-03
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23
Also published as: GB1252874A; FR2053910A5; NL7007885A; JPS5021335B1; DE2027286A1

Abstract

A drum attenuator for selectively connecting a plurality of attenuator sections of different value into an electrical circuit is described, in which the selected attenuator section is grounded through at least two adjacent parallel current paths of approximately equal length for each ground point to reduce inductance and to prevent signal distortion due to ''''ringing.'''' The attenuator sections are each in the form of a ceramic strip having a pair of voltage divider resistors and a damping resistor coated thereon and having capacitors formed integrally therewith. The attenuator sections are releasably mounted within slots around the periphery of the attenuator drum by metal clips which also serve as ground planes. The resulting step attenuator apparatus has an extremely wide frequency bandwidth and may be used in a high-frequency cathode-ray oscilloscope to attenuate its vertical input signal by a plurality of predetermined steps.

Description

United States Patent [72] Inventor Darrell. Wilhoit Beaverton, Greg. [2]] Appl. No. 830,071 [22] Filed June 3, 1969 [45] Patented Nov. 23, 1971 [73] Assignee Tektronix, Inc.

Beaverton, Oreg.

[54] STEP A'ITENUATOR 0F LOW INDUCTANCE AND HIGH BANDWIDTH l 1 Claims, 7 Drawing Figs.

[52] 11.5. C1 333/81, 333/81 A, 333/97 [51] Int. Cl 1101p l/22 [50] Fieldof'Search 333/81,81 C, 81 A, 97

[561- References Cited UNITED STATES PATENTS 3,215,959 11/1965 Velsink 333/81 3,219,953 11/1965 Schwartz 333/81 3,299,373 1/1967 Conney v. 333/81 Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Buckhorn, Blore, Klarquist and Sparkman ABSTRACT: A drum attenuator for selectively connecting a plurality of attenuator sections of difierent value into an electrical circuit is described, in which the selected attenuator section is grounded through at least two adjacent parallel current paths of approximately equal length for each ground point to reduce inductance and to prevent signal distortion due to ringing." The attenuator sections are each in the form of a ceramic strip having a pair of voltage divider resistors and a damping resistor coated thereon and having capacitors formed integrally therewith. The attenuator sections are releasably mounted within slots around the periphery of the attenuator drum by metal clips which also serve as ground planes. The resulting step attenuator apparatus has an extremely wide frequency bandwidth and may be used in a highfrequency cathode-ray oscilloscope to attenuate its vertical input signal by a plurality of predetermined steps.

PATENTEDHBV 23 ml 3,622,919

SHEEI 2 [IF 2 FIG. 6

56 58 6O 9 62 64 5 INPUT OUTPUT y II I I DARREL I. WI LHOIT INVENTOR BUCKHORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS STEP ATTENUATOR F LOW INDUCTANCE AND HIGH BANDWIDTH BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to step attenuators for varying the amplitude of an electrical signal in predetermined steps by selectively connecting a plurality of attenuator sections of different value into the electrical circuit through which such signal is transmitted. More particularly, the present invention relates to drum attenuators having a plurality of attenuator sections mounted on a rotatable drum, of the type shown in Us. Pat. No. 3,074,01 l of L. C. Hedrick, which is owned by the assignee of the present invention.

The attenuator sections are releasably mounted in slots on the periphery of the attenuator drum for easier replacement as shown in US. Pat. No. 3,299,373 of M. Conney. However, the present attenuator apparatus differs from such prior apparatus by grounding the selected attenuator section through, at least two closely adjacent parallel current paths of approximately the same length for each ground point to cause the lead inductances to be lowered to a minimum, thereby reducing inductance and preventing signal distortion due to ringing. In addition, attenuator sections of the present invention are in the form of ceramic strips having voltage divider resistors coated thereon and capacitors formed integrally therewith. A damping resistor is provided in each attenuator section and connected between the junction of the voltage divider resistors and the output terminal of a coupling capacitor so that such damping resistor prevents ringing without affecting the DC signal.

While the step attenuator apparatus of the present invention may be employed in any electronic circuit where it is desired to change the amplitude of a signal in predetermined steps, it is especially useful in cathode-ray Oscilloscopes because of its extremely wide frequency bandwidth. This improved frequency response is the result of the low inductance as well as a minimum input and'output capacitance and minimum ground impedance of the present attenuator. The attenuator sections used in such attenuator enable a reduction in size, lower cost and fewer parts to be assembled. The drum attenuator of the invention switches one selected attenuator section into the circuit which isolates and shields the selected attenuator section in use. Such drum attenuator also enables the input signals to be completely shielded from the output signal which greatly improves performance at high attenuation values.

It is therefore one object of the present invention to provide an improved step attenuator apparatus of small size, low cost and wide frequency bandwidth.

Another object of the present invention is to provide an improved drum attenuator in which the selected attenuator section is grounded through at least two adjacent parallel current paths for each ground point to reduce inductance and prevent signal distortion due to ringing.

A further object of the invention is to provide an improved drum attenuator having a plurality of attenuator sections releasably mounted in slots on the periphery of the attenuator drum to enable easy replacement of the sections and to electrically isolate or shield such sections.

An additional object of the invention is to provide an improved attenuator section including a ceramic strip upon which resistors are formed as coatings and having capacitors formed integrally therewith, to provide an attenuator section of reduced size, lower cost, a minimum of parts and with better frequency response.

Still another object of the present invention is to provide an improved attenuator section which includes a damping resistor connected between the output terminal of a coupling capacitor and the junction of a pair of voltage divider resistors to provide an improved frequency response by reducing ringing without affecting the DC signal.

LII

' attenuator apparatus beside the drum [0 and BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof and from the attached drawings, of which:

FIG. 1 is an elevation view of one embodiment of the drum attenuator apparatus of the present invention;

FIG. 2 is a side elevation view of the apparatus of FIG. I with parts broken away to show the grounding contacts;

FIG. 3 is a view taken along the line 3-3 of FIG. 2 with parts broken away to show the attenuator sections and the switch contacts;

FIG. 4 is an enlarged view of a portion of FIG. 3 showing the end of one of the attenuator sections as it is engaged by a grounding contact;

FIG. 5 is an elevation view of one of the attenuator sections of the present invention with parts broken away for clarity;

FIG. 6 is a partially schematic view showing the relative positions of the electrical components and the ground contact points on the attenuator section of FIG. 5; and

FIG. 7 is a schematic diagram of the electrical circuit of the attenuator section of FIGS. 5 and 6.

DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIGS. 1, 2 and 3, the step attenuator apparatus of the present invention includes an attenuator drum l0 having a plurality of attenuator sections 12 of different attenuator values mounted in uniformly spaced relationship on the periphery ofsuch drum. The drum [0 is mounted on a shaft 14 which rotates such drum between a plurality of switch positions determined by a detent mechanism 16 of conventional type. For example, l2 attenuator sections may be employed in which case the detent mechanism [6 provides 12 different attenuator positions each spaced apart by 30. The shaft 14 may also be coupled to a pair of conventional rotary switches l8 which may be employed to actuate a switch position readout device that provides a visual display of the attenuator drum position.

A coaxial cable input 20 of the BNC type is secured to the adjacent to a knob 22 attached to the end of the drum actuator shaft 14. The inner conductor ofthe coaxial connector 20 has its output end 24 connected to an electrical circuit (not shown) mounted on an etched circuit board 26. The electrical circuit is a switch circuit used in an oscilloscope for switching its vertical signal input between AC and DC coupling and ground and has one output connected to a switch contact 28 providing the signal input to the attenuator drum. through a flexible leaf spring conductor 30 as shown in FIG. 3. The input switch contact 28 is insulatingly mounted on a leaf spring 32 so that such contact extends through a hole 34 in a grounded support member 36 and one end of the spring is attached to the support member by a bolt 38. Similarly, an output switch contact 40 is mounted on another spring attached to the support member 36 by means of a bolt 42 at a position longitudinally spaced along the drum axis from switch contact 28. The output switch contact 40 is connected by another flexible leaf spring conductor 43 to an output terminal 44 supported on a standoff post 45 of insulating material on the exterior of a shielded housing. The housing includes a first housing portion 46 containing the input signal circuit on the circuit board 26,

and a second housing portion 48 is provided about the attenuator drum 10 so that the output signal of such attenuator drum is completely shielded from its input signal. It should be noted that a pair of feed-

through capacitor terminals

50 and 51 may be provided on housing 46 for connecting to a relay in the switching circuit on the circuit board 26 to an external source of DC operating voltage and for transmitting a readout signal corresponding to the switch position.

As shown in FIGS. 1 and 3, the attenuator sections 12 are mounted within longitudinal slots 52 separated by ridges or fingers 54 and uniformly spaced about the periphery of the attenuator drum 10. Such attenuator drum may be made of a resilient plastic material, such as nylon, so that the fingers 54 between such slots are resilient to enable easy insertion and removal of the attenuator sections and to hold such sections within such slots. A metal clip member 56 provided on each of the attenuator sections 12 has a pair of side flanges which engage notches in the finger 54 on opposite sides of the slot 52 to hold such sections within such slot. In addition. the metal clips 56 also function as ground planes as hereafter discussed with reference to FIGS. 4, and 6. The attenuator drum [0 of insulating material electrically isolates the attenuator sections from each other so that such sections are not grounded until they are moved into the selected position where their contacts engage the input contact 28 and the output contact 40.

The selected attenuator section is grounded by a plurality of leaf spring ground contacts 58 which are formed as part of a single sheet 59 of metal attached to the grounded support member 36 bolts 60 as shown in FIGS. 2 and 4. Thus one end of each spring contact 58 is fixed to the support member by such bolts. The intermediate portion of ground contact 58 en gages a ground contact point on the grounding plane portion of the clip 56 of the selected attenuator section. while the free end of the contact engages the surface of the support member 36 and slides across such surface during the flexing of such contact. As a result each ground contact portion or point on the selected attenuator section is connected to ground through two adjacent parallel

current paths

62 and 64 of approximately the same length formed by the opposite ends of the ground contact 58. The current which flows in opposite directions through contact 58 along such two paths as indicated by

arrows

62 and 64 to the electrical ground point 66, creates two magnetic fields of equal strength and opposite polarity which tend to cancel each other and lower the lead inductance in such ground contact. This reduces inductance in the attenuator sections and prevents signal distortion due to ringing, which is the oscillation of a signal at the resonant frequency of the circuit inductance.

As shown in FIGS. 5, 6 and 7, the attenuator sections l2 each include a ceramic strip 68 which is soldered or otherwise bonded to the metal clip member 56. A pair of

voltage divider resistors

70 and 72 connected in series between an input contact 74 and the grounded clip 56. are formed as coatings of resistive material on the bottom of the ceramic strip. The common terminal of such voltage divider resistors is connected to an output contact 76. Both of

contacts

74 and 76 are insulatingly mounted on the metal clip member by a sleeve 78 and extend through an aperture in the ceramic strip 68 so that such contacts can be connected by solder at their lower ends to the voltage divider resistors. A variable input capacitor 80 is formed integral with the ceramic strip so that a metallized portion of the ceramic material provides part of such capacitor and a partially metallized ceramic disc the other part, such disc being rotatable by a slotted adjustment member to vary the capacitance. The input capacitor is connected between the input contact 74 and the grounded clip 56. A variable coupling capacitor 82 is also formed integral with the ceramic strip and is connected between the input contact 74 and one terminal of a damping resistor 84. The damping resistor 84 is provided as a resistive coating on the top of the ceramic strip 68 and has its other terminal connected to the output contact 76 at the common junction of the voltage divider resistors. The damping resistor prevents ringing by damping the signal oscillations without attenuating the DC signal transmitted through the voltage divider resistors. Such damping resistor a value on the order of about 75 ohms when the values of the

voltage divider resistors

70 and 72 are, respectively, 600 kilohms and 675 kilohms. Of course, the values of the voltage divider resistors change for each different attenuation value, as does the value of the

capacitors

80 and 82. A shunt capacitor 86 may be employed in the attenuator section, and is connected between ground and the common terminal of coupling capacitor 82 and damping resistor 84. Two pairs of grounding fingers 85 extending from the sides of the ground plane 56 connect the capacitor 86 to ground. Thus

capacitors

82 and 86 provide an AC voltage divider while

resistors

70 and 72 provide a DC voltage divider for the signal being attenuated. Input capacitor 80 is adjusted to provide each attenuator section with the same input capacitance and compensates for changes in the values of

capacitors

82 and 86, as well as other capacitance.

As shown in FIGS. 5 and 6, four ground contacts 58 engage the ground clip 56 including one contact between the input terminal 74 and a hole 88 provided through such clipto enable adjustment of the input capacitor 80. In addition. two other ground contacts engage the clip on opposite sides of a second hole 90 provided for adjustment of the coupling capacitor 82. The fourth ground contact is placed immediately above the fixed capacitor 86 so that is is connected to ground through the low inductance current paths of such contact. The input and

output contacts

74 and 76 are separated by these last three ground contacts. As a result the attenuator sections of the present invention have extremely low inductance and a wide frequency bandwidth.

It will be obvious to those having ordinary skill in the art that many changes can be made in the above-described preferred embodiment of the present invention without departing from the spirit of the invention. For example, the attenuator drum 10 can be made of metal rather than plastic, and each ground contact 58 can be divided into two contacts whose free ends engage the ground clip 56. Therefore the scope of the present invention should only be determined by the following claims.

I claim:

1. A step attenuator apparatus comprising:

input means for applying input signals to the attenuator apparatus including a fixed input switch contact;

output means for transmitting an attenuated output signal from said apparatus including a fixed output switch contact;

a plurality of attenuator sections of different attenuation values;

carrier means for movably supporting the attenuator sections to selectively connect said sections between the input switch contact and the output switch contact of the attenuator apparatus; and

ground means including switch contact means for selectively connecting the selected attenuator section to an electrical ground by engagement with at least one ground contact portion of said selected section, said switch contact means providing at least two parallel current paths extending in opposite directions for each ground contact portion to reduce inductance and prevent signal distortion.

2. An attenuator apparatus in accordance with claim 1 in which the two current paths are provided by the opposite ends of a single spring contact having an intermediate portion forming one end of each of said current paths to provide a common terminal, said current paths being closely adjacent each other and of approximately the same length.

3. An attenuator apparatus in accordance with claim 2 in which the spring contact is mounted so that one end is fixed to a support member, its intermediate contact portion releasably engages said one ground contact portion of the selected attenuator section, and its other end slidably engages said support member.

4. An attenuator apparatus in accordance with claim 3 which includes a plurality of other spring contacts whose intermediate contact portions engage a plurality of other ground contact portions on said selected attenuator section.

5. An attenuator apparatus in accordance with claim 1 in which the carrier means includes a rotary drum having longitudinal slots provided in its outer surface and mounting means for releasably mounting the attenuator sections in said slots.

6. An attenuator apparatus in accordance with claim 5 in which the drum is made of resilient plastic material which insulates the attenuator sections from each other so that only the selected section is grounded.

7. An attenuator apparatus in accordance with claim 6 in which the attenuator sections each include a ceramic strip on which resistors and capacitors are provided to form the attenuator section and a metal clip member issecured to said ceramic strip to mount such section in the drum slots and to provide a ground plane for said section on which the ground contact portion is located.

8. An attenuator apparatus in accordance with claim 4 in which the input and output switch contacts are resiliently mounted on the support member in holes positioned between said spring contacts.

9. An attenuator section comprising:

voltage divider means including a pair of resistors connected in series between an input terminal and ground, and an output terminal connected to the common connection of said pair of resistors; an input capacitor connected between said input terminal and ground;

a coupling capacitor having one terminal connected to said input terminal; and

a damping resistor connected between the other terminal of said coupling capacitor and said common connection a strip of ceramic material on which all of said resistors are provided as coating of resistive material and which also forms part of said input and coupling capacitors, and a metal ground plane member to which said strip is secured.

10. An attenuator section in accordance with claim 9 which also includes a shunt capacitor of fixed value connected between the other terminal of the coupling capacitor and ground.

11. An attenuator section in accordance with claim 9 in which the input and coupling capacitors are variable and the ground plane member is provided with apertures therethrough in alignment with such capacitors to enable adjustment thereof.

Claims

1. A step attenuator apparatus comprising: input means for applying input signals to the attenuator apparatus including a fixed input switch contact; output means for transmitting an attenuated output signal from said apparatus including a fixed output switch contact; a plurality of attenuator sections of different attenuation values; carrier means for movably supporting the attenuator sections to selectively connect said sections betweeN the input switch contact and the output switch contact of the attenuator apparatus; and ground means including switch contact means for selectively connecting the selected attenuator section to an electrical ground by engagement with at least one ground contact portion of said selected section, said switch contact means providing at least two parallel current paths extending in opposite directions for each ground contact portion to reduce inductance and prevent signal distortion.

7. An attenuator apparatus in accordance with claim 6 in which the attenuator sections each include a ceramic strip on which resistors and capacitors are provided to form the attenuator section and a metal clip member is secured to said ceramic strip to mount such section in the drum slots and to provide a ground plane for said section on which the ground contact portion is located.

9. An attenuator section comprising: voltage divider means including a pair of resistors connected in series between an input terminal and ground, and an output terminal connected to the common connection of said pair of resistors; an input capacitor connected between said input terminal and ground; a coupling capacitor having one terminal connected to said input terminal; and a damping resistor connected between the other terminal of said coupling capacitor and said common connection, a strip of ceramic material on which all of said resistors are provided as coating of resistive material and which also forms part of said input and coupling capacitors, and a metal ground plane member to which said strip is secured.