Tektronix AM-6786U Technical Manual

TM 11-6625-2749-14&P

OPERATOR’S, ORGANIZATIONAL, DIRECT SUPPORT, AND GENERAL SUPPORT

MAINTENANCE MANUAL

(INCLUDING REPAIR PARTS AND SPECIAL TOOLS LISTS)

FOR

DIFFERENTIAL AMPLIFIER AM-6786/U

(TEKTRONIX TYPE 7A22, VERTICAL PLUG-IN UNIT)

(NSN 6625-00-478-0597)

HEADQUARTERS,

DEPARTMENT OF THE ARMY

OCTOBER 1979

CLEANING THE INTERIOR

Use an eye-shield when cleaning with pressurized air. Hardened dirt can be removed with a paint brush, cotton tipped swab or cloth dampened with a water and mild detergent solution. Avoid the use of chemical cleaning agents that might damage the plastic parts.

SPECIFIC COMPONENT REPLACEMENT INFORMATION

Disconnect the instrument from the power source before removing or replacing components.

The pushbutton switches are not repairable and should be replaced if defective. Components which are mounted on the circuit board associated with the pushbutton switch can be replaced using the normal replacement procedure. See the information under Light-Bulb Replacement for bulb replacement.

TM 11-6625-2749-14&P

This manual contains copyright material reproduced by permission of Tektronix, Incorporated.

TECHNICAL Manual

HEADQUARTERS

DEPARTMENT OF THE ARMY

No. 11-6625-2749-14&P

OPERATOR’S, ORGANIZATIONAL, DIRECT

MAINTENANCE

SUPPORT,

MANUAL

Washington, DC,

AND GENERAL SUPPORT

10 October

1979

(INCLUDING REPAIR PARTS AND SPECIAL TOOLS LISTS)

FOR

DIFFERENTIAL AMPLIFIER AM-6786/U

(TEKTRONIX TYPE 7A22, VERTICAL PLUG-IN UNIT)

NSN 6625-00-478-0597)

Current as of 20 March 1979

REPORTING OF ERRORS

You can improve this manual by recommending improvements using DA Form 2028-2 located in the back of the manual. Simply tear out the self-addressed form, fill it out as shown on the sample, fold it where shown, and drop it in the mail.

If there are no blank DA Forms 2028-2 in the back of your manual, use the standard DA Form 2028 (Recommended Changes to Publications and Blank Forms) and forward to the Commander, US Army Communications and Electronics Materiel Readiness Command, ATTN: DRSEL-ME -MQ, Fort Monmouth, NJ 07703.

In either case, a reply will be furnished direct to you.

This manual is an authentication of the manufacturer’s commercial literature which, through usage, has been found to cover the data required to operate and maintain this equipment. Since the manual was not prepared in accordance with military specifications, the format has not been structured to consider levels of maintenance.

i-0.1

TM 11-6625-2749-14&P

TABLE OF

SECTION O

Introduction . . . . . . . . . . . . . . . . . . .

SECTION 1 SPECIFICATION

Introduction . . . . . . . . . . . . . . . . . . . .

Electrical Characteristics . . . . . . . . . . .

Deflection Factor . . . . . . . . . . . . . . .

Gain . . . . . . . . . . . . . . . . . . . . . . . .

Differential Dynamic Range . . . . . .

DC Offset . . . . . . . . . . . . . . . . . . . .

Frequency Response . . . . . . . . . . . .

Bandwidth limit . . . . . . . . . . . . . . .

Recovery Time . . . . . . . . . . . . . . . .

Common-Mode Signal Range . . . .

Common-mode Rejection Ratio . . . .

Maximum input Voltage . . . . . . . .

Input Rand C . . . . . . . . . . . . . . . . .

Maximum Input Gate Current . . . .

Variable Balance . . . . . . . . . . . . . .

Step Attenuator DC Balance . . . . . .

Displayed Noise

(Tangentially Measured) . . . . . . .

DC Drift . . . . . . . . . . . . . . . . . . . . .

Isolation between + and — Inputs

Environmental Characteristics . . . . . . .

Altitude . . . . . . . . . . . . . . . . . . . . .

Transportation . . . . . . . . . . . . . . . .

Physical Characteristics . . . . . . . . . . . .

Finish . . . . . . . . . . . . . . . . . . . . . . .

Dimensions . . . . . . . . . . . . . . . . . . .

Weight . . . . . . . . . . . . . . . . . . . . . .

SECTION 2 OPERATING INSTRUC-

TIONS

Introduction . . . . . . . . . . . . . . . . . . . . 2-1

Front Panel Controls and Connectors . 2-1

Operating Instructions . . . . . . . . . . . . . 2-2

First Time Operation . . . . . . . . . . . 2-2

General Operating Information . . . . . 2-4

Trace Drift . . . . . . . . . . . . . . . . . . . .

Page

0-1

1-1 1-1 1-1 1-1 1-1 1-1 1-1

1-1 1-1 1-1 1-1

1-3 1-3 1-3 1-3 1-3

1-3

1-3 1-3 1-3 1-3

1-3

2-4

CONTENTS

Input Gate Current . . . . . . . . . . . . .

Voltage Measurement . . . . . . . . . . .

Signal Input Connectors . . . . . . . . .

High Input Impedance . . . . . . . . . . .

Display Polarity . . . . . . . . . . . . . . .

Deflection Factor . . . . . . . . . . . . . .

Bandwidth Selection . . . . . . . . . . . .

Voltage Comparison Measurements 2-10

Differential Operation . . . . . . . . . . 2-10

DC Offset Operation . . . . . . . . . . . . 2-11

Input Overdrive Indicator . . . . . . . . 2-12

Readout . . . . . . . . . . . . . . . . . . ...2-12

Trace Identify . . . . . . . . . . . . . . . 2-13

SECTION 3 CIRCUIT DESCRIPTION

Introduction . . . . . . . . . . . . . . . . . . . . 3-1

Block Diagram Description . . . . . . . . . 3-1

Input Coupling . . Input Attenuators

Preamp . . . . . . . .

L F -3 dB Point . Output Amplifier

H F -3 dB Point

General Information

Feedback Amplifiers . . . . . . . . . . . . 3-2

Differential Configuration . . . . . . . 3-3

Detailed Circuit Description . . . . . . . . 3-3

Input Coupling . . . . . . . . . . . . . . . . 3-3

Input Attenuators . . . . . . . . . . . . . . 3-4

Gate Current Compensation . . . . . . 3-4

Input Amplifier . . . . . . . . . . . . . . . . 3-4

X15 Preamp . . . . . . . . . . . . . . . . . 3-4

DC Balance . . . . . . . . . . . . . . . .

Floating Power Supply . . . . . . . . 3-5

Common-Mode Rejection . . . . . . 3-5

Cross Neutralization . . . . . . . . . . 3-6

. . . . . . . . . . . . . .

. . . . . . . . . . . . . . 3-2

. . . . . . . . . . . . . .

‘age

2-4 2-4 2-4 2-5 2-5

2-5 2-5

3-1 3-1

3-1 3-1 3-2

3-2

3-4

TM 11-6625-2749-14&P

TABLE OF

Input Overdrive Protection . . . . . 3-7

DC Off set . . . . . . . . . . . .

Offset Generator . . . . . .

Input Overdrive Indicator LF

-3 dB Point Selector

Output Amplifier . . . . . . . .

Gain Switching Amplifier Variable Output Stage . . . Signal Output Amplifier . Trigger Output Amplifier .

Indicator Oscilloscope Readout . . . . 3-11

Probe Sensing . . . . . . . . . . . . . . . 3-11

Trace Identify . . . . . . . . . . . . . .

SECTION 4 MAINTENANCE

Introduction . . . . . . . . . . . . . . . . . . . .

Preventive Maintenance . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . .

Cleaning Front Panel . . . . . . . . . . . .

Cleaning Interior . . . . . . . . . . . . . . .

Visual Inspection . . . . . . . . . . . . . .

Transistor Checks . . . . . . . . . . . . . .

Calibration . . . . . . . . . . . . . . . . . . .

Corrective Maintenance . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . .

Obtaining Replacement Parts . . . . .

Special Parts . . . . . . . . . . . . . . . . . .

Soldering Techniques . . . . . . . . . . .

Component Replacement Information

Troubleshooting . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . .

Indicator Oscilloscope . . . . . . . . . . .

Operating Procedure and

Control Settings . . . . . . . . . . . . .

Trouble Symptoms Visual Inspection . Calibration Check

. . . . . . . . . . . . . . 4-4

. . . . . . . . . . . . . .

. . . . . . 3-7

. . . . . . 3-8

. . . . . . 3-9

. . . . . .

. . . . . . 3-9

. . . . . .

. . . . . . 3-11

Page

3-9

3-11 3-11

3-11

4-1 4-1 4-1 4-1 4-1 4-1 4-1 4-1 4-1 4-1 4-2 4-2 4-2 4-3 4-3 4-3 4-4

4-4

CONTENTS

Detailed Troubleshooting . . . . . . . . . . 4-4

General . . . . . . . . . . . . . . . . . 4-4

Test Equipment Recommended . . . . 4-4

DC Balance Check . . . . . . . . . . . . .

Troubleshooting by Direct

Replacement . . . . . . . . . . . . . . . . 4-5

Component Checks . . . . . . . . . . . . . 4-6

Transistors . . . . . . . . . . . . . . . . .

Diodes . . . . . . . . . . . . . . . . . . . . 4-6

Resistors . . . . . . . . . . . . . . . . . . . 4-6

Capacitors . . . . . . . . . . . . . . . . . 4-7

SECTION 5 PERFORMANCE CHECK/

CALIBRATION PROCEDURE

Introduction . . . . . . . . . . . . . . . . .

Test Equipment Required . . . . . . . .

General . . . . . . . . . . . . . . . . . .

Short Form Procedure . . . . . . . . . .

Performance Check/Calibration

Procedure . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . .

Preliminary . . . . . . . . . . . . . . . .

Check or Adjust . . . . . . . . . . . .

AC Atten Balance . . . . . . . . .

Variable Balance . . . . . . . . .

Coarse DC Balance. . . . . . .

+ Gate Current Zero . . . . .

- Gate Current Zero . . . .

GAIN . . . . . . . . . . . . . . . . . .

VARIABLE Control Ratio . . VOLTS/DIV Gain Switching .

Isolation between + and -

Inputs . . . . . . . . . . . . . . . .

Total DC Offset Range . . . . .

C241 + Cross Neutralization C141 - Cross Neutralization Cl 15 Xl +Input Atten Time

Constant . . . . . . . . . . . . . .

. . . 5-1

. . . 5-1 . . .

. . . 5-2

,.. . . . 5-3

. . . 5-3 . . . 5-4 . . . 5-4 . . . 5-4 . . . 5-4 . . . . . . 5-5 . . . 5-5 . . . 5-5 . . . 5-5

. . . 5-7 . . . 5-7 . . . 5-7 . . . 5-8

. . . 5-8

Page

4-4

4-6

5-1

5-3

5-5

TABLE OF CONTENTS

TM 11-6625-2749-14&P

Page

C215 x1

Constant . . . . . . . . . . . . . . . . . 5-8

Input Atten Accuracy . . . . . . . . . 5-9

Input Atten Differential Balance . 5-9

+ Input Atten

- Input Atten Time

Series

Compensation . . . . . . . . . . . . . 5-10

- Input

-input

Atten Compensation . . . 5-10

Atten Shunt

Compensation . . . . . . . . . . . . . 5-11

HF -3dB Point . . . . . . . . . . . . 5-12

-3dB Point . . . . . . . . . . . . . . . . . . . . . . .5- 12

CMRR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .5-13

Input Overdrive Indicator . . . . . . 5-13

Differential Dynamic Range . . . . 5-14

Overall Noise Level

(Measured Tangentially) . . . . . 5-15

Overdrive Recovery Time . . . . . . 5-15

SECTION 10 CALIBRATION TEST EQUIPMENT REPLACEMENT

APPENDIX A. REFERENCES

SECTION 6 ELECTRICAL PARTS LIST

Abbreviations and Symbols

Parts Ordering Information Index of Electrical Parts List Electrical Parts List

SECTION 7 DIAGRAMS AND ME-

CHANICAL PARTS ILLUSTRATIONS

Diagrams

Mechanical Parts Illustrations

SECTION 8 MECHANICAL PARTS LIST

Mechanical Parts list Information

Index of Mechanical Parts Illustrations

Mechanical Parts List

SECTION 9 PART NUMBER-NATIONAL STOCK NUMBER CROSS REFERENCE

INDEX.

A - 1

COMPONENTS

B. C. ADDITIONAL AUTHORIZATION LIST (Not Applicable) D. MAINTENANCE ALLOCATION

Section I. General

Maintenance Allocation Chart for Differential Amplifier

II.

AM-6786/U

III.

Tool and Test Equipment Requirements for Differential

Amplifier AM-6786/U

APPENDIX E.

EXPENDABLE SUPPLIES AND MATERIALS LIST (Not Applicable)

OF END ITEM LIST (Not Applicable)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D-1

D-4

D-5

iii

TM 11-6625-2749-14&P

LIST OF ILLUSTRATIONS

Figure

1-1 1-2

2-1

2-2 2-3

2-4A

2-4B

2-5

Page

Type 7A22 Differential Amplifier. . . . . . . . . . . . . . . . .

CMRR vs Frequency for signal not exceeding Common Mode

Signal Range . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1-2

Signals applied to the + INPUT Connector Produce an

Upright Display, while signals applied to the - INPUT

are inverted . . . . . . . . . . . . . . . . . . . . . . . . . .

Location of AC ATTEN BAL Control . . . . . . . . . . . . . . . .

2-1 2-3

Location of wire strap between input line and internal

gate . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2-5

7A22 CMRR at 10 uV/Div to 10 mV/Div. Inputs DC Coupled.

Worst-case degradation due to source resistance . . . . . . . .

2-6

7A22 Bandwidth and VOLTS/DIV Error as functions of Source

Resistance . . . . . . . . . . . . . . . . . . . . . . . . . .

2-7

7A22 FREQUENCY RESPONSE as a function of LF -3 dB POINT

SETTING (HF-3 dB POINT at l MHz) . . . . . . . . . . . . .

2-8

2-6

2-7 2-8

2-9 3-1

3-2 3-3 3-4 3-5 3-6 3-7

7A22 FREQUENCY RESPONSE as a function of HF -3 dB POINT

settings (LF -3 dB POINT of DC).

. . . . . . . . . . . . . . . . . . . . . . .

Improving signal-to-noise ratio by setting bandwidth . . . . .

2-9 2-10

Waveforms showing differential rejection of a common-mode

signal

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Connecting a differential amplifier across a circuit . . . . .

2-11 2-12

Generalized feedback system showing the relation between

input and output . . . . . . . . . . . . . . . . . . . . . .

Feedback amplifier (Single-ended version).

. . . . . . . . . . . . . . .

Feedback amplifier (Differential configuration). . . . . . . . . .

Gate Current Compensation . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

Common-mode Rejection using floating power supply . . . . . . . .

Input cross neutralization

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Effects of high impedance to ground in the - INPUT . . . . . . .

3-2

3-3

3-4 3-5 3-6

3-7

TM 11-6625-2749-14&P

Figure

3-8

3-9

3-10 3-11

4-1 4-2 4-3 4-4 4-5 4-6

Page

(A) Overdrive Protection System showing current paths.

(B, C) Sequence of events leading to excessive

overdrive

Offset system showing (A) Conditions with zero offset, (B)

0.25V DC offset Functional arrangement of the offset generator Partial Low Frequency -3 dB Point selector for + Input

amplifier

Removing or replacing component on circuit board

Transistor junction-voltage measurements Transistor base pin and socket arrangement

Diode polarity and color code Color code for resistors and ceramic capacitors

Plain circuit board (components)

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . .

. . . . . . . . . . .

. . . . . . . . .

. . . . . . . . . .

3-8

3-9 3-10

3-11 4-2 4-6 4-7 4-8 4-8 4-10

4-7 4-8

5-1 5-2 5-3

5-4

5-5

5-6

5-7 5-8

Main circuit board (wire color code)

Bandwidth circuit board (components and wire color code) . Left side of the 7A22 showing adjustment locations Equipment required for steps 6 through 19

Typical waveform showing (A) Cross neutralization properly

adjusted, (B) incorrectly adjusted

Typical waveform showing (A) Correct adjustment of Input

Attenuator Time Constant. (B) and (C) incorrect adjustment

Typical display obtained when Input Attenuators are

adjusted for optimum differential balance

Typical display obtained with - Input attenuator adjusted

for optimum Common-Mode signal rejection. (A) C208C

adjusted properly (B) C209C adjusted properly Equipment required for steps 20 through 24 Right side of Type 7A22 showing location of adjustments . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . .

. . . . . . . . . . . . . . .

4-11 4-12 5-4 5-6

5-7

5-8

5-9

5-10 5-11 5-12

TM 11-6625-2749-14&P

Figure

5-9 5-10

5-11 5-12

Page

Equipment required for step 25

. . . . . . . . . . . . . . . . . . . . . . . . . . .

5-14

Typical display showing (A) two noise bands and (B) merging

noise bands . . . . . .

Equipment required for step 26

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . .

Typical waveform showing overdrive recovery time . . . . .

5-15 5-16 5-16

TM 11-6625-2749-14&P

Type 7A22

Fig. 1-1. Type 7A22 Differential Amplifier.

SECTION 0

INTRODUCTION

TM 11-6625-2749-14&P

0-1.

6786/U and provides instructions for operation and maintenance. Throughout this manual, the AM-6786/U is referred to as Tektronix Type 7A22 Differential Amplifier.

included is a repair parts list.

0-2.

whether there are new editions, changes, or additional publications pertaining to the equipment.

modification work orders (MWO’S) pertaining to the equipment. 0-3.

forms, records, and reports which are to be used by maintenance personnel at all maintenance levels are listed in and prescribed by TM 38-750.

DD Form 6 (Packaging Improvement Report) as prescribed in AR 700-58/NAVSUPINST

4030.29/AFR 71-13/MCO P4030.29A and DLAR 4145.8.

SCOPE

This manual describes Vertical Plug-In Unit, Differential Amplifier AM-

The maintenance allocation chart appears in appendix D. Also,

INDEXES OF PUBLICATIONS

DA Pam 310-4.

DA Pam 310-7.

FORMS AND RECORDS

Reports of Maintenance and Unsatisfactory Equipment. Maintenance

Report of Packaging and Handling Deficiencies. Fill out and forward

Refer to the latest issue of DA Pam 310-4 to determine

Refer to DA Pam 310-7 to determine whether there are

Discrepancy in Shipment Report (DISREP) (SF 361). Fill out and for-

ward Discrepancy in Shipment Report (DISREP) (SF 361) as prescribed in AR

55-38/NAVSUPINST 4610.33B/AFR 75-18/MCO P461O.19C and DLAR 4500.15. 0-4.

tions for preparing EIR’s are provided in TM 38-750, The Army Maintenance Management System. EIR’s should be mailed direct to Commander, US Army Communications and Electronics Materiel Readiness Command; ATTN: DRSEL-ME-MQ, Fort Monmouth, NJ 07703.

0-5.

shall be in accordance with TM 750-244-2. 0-6.

in accordance with TM 750-244-2.

REPORTING EQUIPMENT IMPROVEMENT RECOMMENDATIONS (EIR)

EIR’s will be prepared using DA Form 2407, Maintenance Request. Instruc-

A reply will be furnished directly to you.

ADMINISTRATIVE STORAGE

Administrative storage of equipment issued to and used by Army activities

DESTRUCTION OF ARMY ELECTRONICS MATERIEL

Destruction of Army electronics materiel to prevent enemy use shall be

0-1

TM 11-6625-2749-14&P

SECTION

SPECIFICATION

Introduction

h Type 7A22 Vertical Plug-in is

enticd

amplifier with excellent common-mode reiection char-

acteristics and high gain for low level applications.

The DC Offset capability of the Type 7A22 allows the

display of the very small low-frequency signals containing

large DC component, at deflection factors not possible with AC coupling. The vertical deflection factor range of the Type 7A22 is from 10 pV to 10 V. The high and low frequency —3 dB points to set the bandwidth of the instrument. Thus, for low fre-

quency applications the signal-to-noise ratio can be improved

by restricting the bandwidth of the Type 7A22. The bandwidths selection and excellent drift-with-time provide measurement capabilities in the biomedical, trans-

ducer, and other areas which require stable, low deflection

factor, low noise measurements.

The Type 7A22 is designed for use in Tektronix 7000 Series

oscilloscopes.

The electrical characteristics described in this section are

volid over the stated environmental range for instruments ccdibrated at an ambient temperature of +20”C to +30” and after

a 5

can be

minute warmup unless otherwise noted.

ELECTRICAL CHARACTERISTICS

Characteristic

Deflection Factor ~VOLTS/DIV]

Gain Ratio Accuracy

VAR (CAL IN) Range

GAIN

Differential Signal Range [DC OFFSET not used)

10 pV/Div to

mV/Div

20 mV/Div to

0.1 V/Div

0.2 V/Div to 1 V/Div

2 V/Div to

10 V/Div

Performance

Within 2% with GAIN ad@sted at 1 mV/div

Continuously variable; extends deflection factor to at least 25 V/div

Permits ad@stment of deflection

factor at 1 mV/DIV for all mainframes

At least AI V

At least *1O V

At least A1OO V

At least A1OOO V

a DC

selected at the front panel,

coupled differ-

characteristics

Requirements

DC OFFSET

COARSE Range from Electrical Zero

10 pV/Div to 10mV/Div

20 mV to 0.1 V/Div

0.2 V to 1 V/Div 2V to 10 V/Div

Frequency Response (8

div Reference)

Overall Frequency Response DC (Direct) Coupled Input

AC (Capacitive) Cou pled Input Lower Bandwidth Frequencj

Bandwidth Limit

Accuracy

HF –3dB POINT

100Hz to 1 MHz

Accuracy

LF –3 dB POINT

0.1 Hz to 10 kHz Accuracy

Recovery Time

Common Mode Signal

10 pV/Div to

10 mV/Div

20 mV/Div to

0.1 V/Div

0.2 V/Div to 10 V/Div

Common-Mode Reiection Ratio

DC (Direct) Coupled

AC (Capacitive)

Coupled

At least +1 V to –1 V

At least +1OV to –1OV

At least +100 V to –1OOV At least +1000 V to –1OOOV

DC to within 10~o of 1 MHz at –3 dB

2 Hz or less

9 steps in

Within 10~o of selected frequency 6 steps in

Within 12% of selected frecluency 10 ps or less to recover to within

0.5% of zero level after the removal of

plied for 1 s. Test signal not to exceed Specified aberration (0.5%) based on test signal amplitude

At least +1OV and –1OV

At least +1OOV and –1OOV

At least +500 V and –500 V

See Verification Points on graph, Fig. 1-2.

1-3 sequence

1-10 sequence

a + or —

Differential Signal Range.

test input ap-

1-1

1-2

Fig. 1-2. CMRR vs. Frequency for signals not exceeding Common Mode Signal Range.

TM 11-6625-2749-14&P

ELECTRICAL CHARACTERISTICS (cont)

Characteristic

Maximum Input Voltage krch insxst}

Input R and C

Maximum Input Gate

Current

Varicrkde Balance

Displayed Noise (Tangentially Measured)

,-.

DC (Direct) Coupled, EK + Peak AC, AC Component 1 Mt-lz or less

10pV/Div

mV/Div

20 mV/Div to 0.1 V/Div

.2 V/Div to 10 V/ Div

AC (Capcrcitive) Coupled Input DC Voltage

AC (Capacitive] Coupled

Input DC Reiection

Resistcmce Capacitance F! & C Product

10 ~V/Div to 10 mV/ Div

20 mV/Div to IOV Div

Display Shift at 10 pV jDiv (AC Coupled]

Performance

M least 4 X

lMa & l~o Approximate

Within 3=1 Y. between all deflection factors

+25°

+-20 pA *1OO pA each input +40 pA *200 pA Differentially

&lOpA

&20pA

A4 div &20 div

).2 div or less shift with VARIABLE :ontrol turned from fully cw to UIIY ccw position

6 pV or 0.1 Div (whichever is

~reater), 1 MHz HF –3 dB POINT,

ource resistance

Requirements

05:1

47.0 pF

25 Q or

-4-50°c

less

TM 11-6625-2749-14&P

DC Drift

Drift with Time (Ambient Temperature and Line Voltage Constant)

Short Term

Long Term

Drift with Ambient

Temperature (Line

Voltage Constant)

Isolation between + and

– Inputs (+ INPUT to an Open — INPUT, — INPUT to on Open INPUT)

5 pV (P-P) or 0.1 div (whichever is greater) in any minute after 1 hour warmup

10 pV (P-P) or 0.1 div (whichever is greater) in any hour after 1 hour warmup

50 pV/ “C or less

-f-

At least 200:1, DC to 1 MHz

ENVIRONMENTAL

PLUG-IN TESTED OUT OF INDICATOR

ITEM

Altitude

Non-operating

Transportation

CHARACTERISTIC

To 50,000 feet and —55° C Qualified under National Safe

Tronsit Committee test procedure

1A, Category

PHYSICAL

ITEM CHARACTERISTIC

Finish Dimensions

Weight

Front panel is anodized aluminum s

145/8

478 inch H

25/8

Ibs

inch L X

OSCILLOSCOPE

2S/8

inch W X

1-3

TM 11-6625-2749-14&P

SECTION 2

OPERATING INSTRUCTIONS

Introduction

This section opens with a brief functional description of

the front-panel controls, input overdrive lamp, and input

connectors. Following the front-panel description is a familiarization procedure and finally a general discussion of the operation of the Type 7A22.

CONTROLS AND CONNECTORS

INPUT Input overdrive indicator lamp turns on OVERDRIVE

VOLTS/DIV Volts per displayed division. Nineteen

VARIABLE (CAL IN]

GAIN

POSITION IDENTIFY

HIGH

FREQUENCY

-3 dB POINT

to indicate excessive differential drive to the input amplifier stage. Lights when the differential dynamic range between input connectors is exceeded.

position switch used to select the calibrated deflection factors.

Two-position switch activated by the VARIABLE knob to select calibrated or uncalibrated deflection factors. At the IN position, the VARIABLE control is inoperative and the deflection factor is calibrated. When pressed and released, the knob moves outward to activate the VARIABLE control for uncalibrated de-

flection factors. The uncalibrated posi-

tion provides continuously variable uncalibrated attenuation between the calibrated deflection factors and extends the deflection factor to at Ieost 25 VOLTS/DIV.

Screwdriver adjust control to set the CRT

display scale factor to agree with the

VOLTS/DIV switch indication. Adjusted

for proper deflection with the VOLTS/

DIV switch set to the 1 mV position. The control that vertically positions the

trace or display. Momentary contact, push-button switch,

concentric with POSITION. Will cause the trace, representing the output of the Type 7A22, to move a small amount when pressed. Aids in identifying the Type 7A22 trace when multiple traces are displayed.

Nine position switch to select the approximate high frequency -3 dB point. The switch positions are: 100 Hz, 300 Hz,

1 kHz, 3 kHz, 10 kHz, 30 kHz, 100 kHz,

300 kHz, and 1 MHz.

LOW

FREQUENCY

-3 dB POINT

+INPUT

Fig. 2-1. Signals applied to the + INPUT connector produces an upright display, while signals applied to the — INPUT are inverted.

+ AC-GND-DC

Eight position switch to select DC coupling or the approximate low frequency

-3 dB points. The switch positions are: DC OFFSET, DC, .1 Hz, 1 Hz, 10 Hz, 100 Hz, 1 kHz, and 10 kHz.

Signal input connector. Positive input produces deflection upward (see Fig. 2-l).

Signal input connector. Positive input produces deflection downward (see Fig.

Fig. 2-1

A miniature illuminated push-button type switch. The buttons are interlocked so that only one button may be depressed at one time. When the ‘AC’ button is depressed the signal is coupled through

0.1 µF to the Input Amplifier and only the varying component of the input sig-

nal is amplified.

When the ‘DC’ button is depressed the signal is coupled directly to the Input

Amplifier and the entire input signal, both AC and DC, is amplified. When the ‘GND’ button (or none of the buttons) is depressed, the signal is coupled

Mfl

through 0.1 µF and through 1

ground. The Amplifier Input is grounded

in this condition.

2-1

TM 11-6625-2749-14&P

aEoch push-button is illuminated from behind when the button is depressed.

– AC-GND-DC

The following when the LOW is set to DC

STEP ATTEN

DC BAL (DC MODE the amplifier input stage. With

ONLY)

DC OFFSET

(DC OFFSET

mode only)

RELEASE LATCH Gray rectangular knob near the bottom

Same function as the + AC-GND-DC switch but applied to the — INPUT.

IMPORTANT

two controls ore operative only

FREQUENCY

DC OFFSET.

Front panel cantrol for DC balancing

nals applied ta the input connectors, the control is adlusted for shift as the VOLTS/DIV switch is moved from the 10 mV pasition to the 10 position.

Coarse and fine controls to provide internal offset bias while maintaining

the differential

range of the offset bias depends upon the settings of the VOLTS/DIV and is indicated by shaded gray bands (OFFSET RANGE) around the VOLTS/ DIV switch.

left of the front panel. Pull out to remove Plug-In fram Plug-In compartment.

—3 dB

POINT switch

capability. Available

switch,

sig-

trace

~LV

OPERATING INSTRUCTIONS

First-Time Operation

Steps

to help place the trace the unit for immediate use. Steps 6 through 8 are used to check remaining are intended to demonstrate some of the basic functions of the Type 7A22.

ment.

clockwise and turn the oscilloscope Power ON. Preset the

time-base and triggering controls and automatic triggering.

oscilloscope to warm up.

1 through 5 in the following procedure are intended

the GAIN adjustment. These steps slang with those

Insert the unit inta the ascilloscape plug-in compart-

2. Set the Type 7A22 front-panel controls as follaws:

VOLTS/DIV VARIABLE POSITION

HIGH FREQUENCY

–3dB POINT

LOW FREQ –3 dB AC-GND-DC AC-GND-DC STEP ATTEN DC BAL

3. Turn the oscilloscope Intensity control fully counter-

4. Wait about five minutes for the Type 7A22 and the

(–

INPUT)

the screen quickly and prepare

1 mV IN (CAL)

Midrange

1 MHz

POINT

DC GND GND Midrange

for a .5

ms sweep rate

NOTE

About five minutes is sufficient time for warmup when using the Type 7A22 for short-term DC measurements. For long-term DC measurements the lower deflection factors, allow at least one hour.

5, Adlust the Intensity control for normal viewing of

the trace. The trace shauld appear near the graticule center.

6. Using the POSITION control, position the trace 2 divi-

sions below graticule center.

CAUTION

If the maximum input voltage rating in the 10 /IV

to 10 mV range of the VOLTS/DIV ceeded, the inputs are diode-clamped voltages of approximately

—16.5 volts and damage to the signal source is

possible. If the signal source can supply more than

1/1 6 A of current, the input protective fuse (s)

will open. An apen input fuse is indicated by the

lighting of the INPUT OVERDRIVE indicator with the

input coupling switches set to GND.

7. Apply

coaxial cable to the + INPUT connector

7A22.

8. For DC coupled, single-ended operation, set the +

INPUT AC-GND-DC

should

bottom of the disploy at the reference established in step 6.

9. For AC coupled, single-ended operation, reposition the

display with the Type 7A22 POSITION control to place the

bottom of the display at the graticule center line.

10. Set the + INPUT AC-GND-DC coupling switch to AC

and note that the display shifts downward about 2 divisions

to its average Ieve!,

11. Disconnect the coaxial cable from nector. Connect and — INPUT connectors. Connect the coaxial cable from

the Calibrator

12. For AC coupled differential —INPUT AC-GND-DC brator signal is now coupled to both inputs as

mode signal. A straight line display should

since the common-mode is being relected.

Operational Adjustments

Most of the following checks and adjustments may be made after to the DC drift of the amplifier during

warmup,

adjusted for each check or adjustment, warmup of at least one hour should be allowed before

in conjunction with the Type 7A22 is correctly cali-

brated (refer to the oscilloscope manual) and that

the calibrated output voltage is correct.

a 4 mV

square waves 4 divisions in amplitude with the

the

BAL is attempted. Insure that the oscilloscope used

peak-to-peak calibrator signal through

coupling switch to DC. The display

dual input connector

the dual input connector.

a 5

STEP ATTEN DC

final adjustment of the STEP ATTEN DC

coupling switch

NOTE

minute warmup; however due

16.5 volts and/or

the +

operation, set the

BAL shauld be re-

using

switch is ex-

fixed

the Type

INPUT con-

the + INPUT

AC. The cali-

common-

observed,

subsequent

and

2-2

TM 11-6625-2749-14&P

1. AC ATTEN BAL

LOW FREQ —3 dB POINT switch is used to limit the low frequency response of the Type 7A22, the unit employs AC stabilization. This means that when the switch is set to

crny

position except DC or DC OFFSET, the STEP ATTEN DC BAL and DC OFFSET controls become inoperative. VOLTS/DIV

BALI ;; inkm-ml cdiushent

Fig.

When trcsnsferring the Type 7A22 from ane oscilloscope to another, it may ad~ustment of this control, due to normal power supply varicstions between oscilloscopes.

Ixslance is then controlled with the AC ATTEN

2-2. location

of AC ATTEN BAL

(internal adiustmerst) . When the

(see

Fig. 2-2].

control.

necessary to perform

minor re-

NOTE

An unbalance of up to 30 pV is normal and cannot be reduced by adiusting the AC ATTEN BAL control.

With the Type 7A22 inserted into the oscilloscope, re-

move the left side panel of the oscilloscope and set the

Type 7A22 controls as follows:

VOLTS/DIV AC-GND-DC (+ INPUT) AC-GND-DC (–INPUT) LOW FREQ —3 dB POINT 10 Hz HIGH FREQ –3 dB POINT 100 Hz

Using the POSITION

graticule center.

c. Set the VOLTS/D[V

Adiust the AC ATTEN BAL control, R505, to position the trace to within 1.5 divisions of graicu[e center. (See note in step

INTERACTION:

1).

quence, steps 2 and 3 must also

this adjustment is made out of se-

1 mV

GND

control, position the trace to

switch to 20 pV.

performed.

VAR BAL R425.

Set the Type 7A22 controls as in step 1 -a.

Position the trace to graticule center with the POSl-

TION control.

With the VAR (CAL IN) in the out position, rotate the

VARIABLE control from stop to stop. Check for no move-

ment of trace.

Adiust R425 for no movement of trace as the VARlABLE control is rotated from stop to stop. (See Fig. 2-2 for location of adjustment.)

3. STEP ATTEN DC BAL. With zero input voltage and the LF –3 dB POINT selector at DC, if the STEP ATTEN DC BAL is not properly adiusted the CRT trace will shift vertically as the VOLTS/DIV its range.

tive positions.

VOLTS/DIV

VARIABLE [CAL IN)

POSITION HIGH FREQ –3 dB POIN’ LOW FREQ –3 dB POINT AC-GND-DC (+ INPUT) AC-GND-DC (–INPUT) STEP ATTEN DC BAL

CRT trace to the center of the graticule.

STEP

The shift is more noticeable at the most sensi-

Set the Type 7A22 front-panel centrals as follows:

Using the Type 7A22 POSITION control, position the

Set the VOLTS/DIV

Return the trace to graticule

ATTEN DC BAL cantrol.

(Internal adjustment].

switch is rotated throughout

10mV CAL Midrange

1 MHz DC GND GND Midrange

switch to

I o pv.

center by adiusting the

NOTE

The adjustment of the STEP ATTEN DC BAL con-

trol should be checked periodically

of the instrument. If the Type 7A22 is used DC

coupled or in significantly

peratures in the 10 KV/DIV to

the STEP ATTEN DC BAL should be checked quite frequently.

trol and readiust, if necessary, before any critical measurement is made under the above conditions.

It is

good practice to check this con-

during the use

varying ambient tem-

mV/DIV ranges,

4. GAIN

Perform steps 1 through 8 in the First-Time Operation

Procedure.

Adiust the GAIN control for exactly 4 divisions of

display.

NOTE

Accuracy of this adjustment is dependent upon the voltage accuracy of the calibration source.

2-3

TM 11-6625-2749-14&P

GENERAL OPERATING INFORMATION

Trace Drift

The environment in which the Type 7A22 is operated and the inherent characteristics of the Type 7A22 influence trace drift. Therefore, to determine trace drift for vironment refer to the Specification Section. In environment in which the ambient temperature does not vary as an air-conditioned building) the trace drift generally will not exceed 10 ~LV in one hour.

Input Gate Current

When using the .1 mV/DIV to 10 ILV/DIV ranges for measurement with where the source impedance is high input gate current should

adlusted to zero. This is particularly desired at high temperatures (abave 40” Check/Calibration procedures for setting the gate current to zero.

Voltage Measurement

To obtain accurate DC measurements at maximum sensitivity, it is necessary the amplifier lust before making the measurement. This is accomplished by adiusting the STEP ATTEN DC BAL as

described under operational adjustment number 3.

When measuring DC voltages, use the largest deflection

factor (10 V/DIV) when first connecting the Type 7A22 to an unknown voltage source. small to make the measurement, switch to tion factor. If the input stage is overdriven, of current might flow into the input. See CAUTION after item 6 of First Time Operation.

Where only the AC component of

AC and DC components is to GND-DC switches to take advantage of the pre-charging circuit incorporated in the unit. The pre-charging circuit

permits the coupling capacitor to charge to the DC source

voltage when the AC-GND-DC switch is set to GND. Procedure for using this circuit is as follows:

Before connecting the Type 7A22 to

ing

a DC

switch to GND. Then connect the input to the circuit under test.

Allow about one second for the coupling capacitor to

charge.

Set the input coupling switch to AC. The display will remain ured in the usual manner.

d. On

DC switch to GND and short the input connector to ground.

The above procedure should

other signal with

If the Type 7A22 is connected to

age source sion,

an AC

companent, set the AC-GND-DC

the screen and

completion of the measurement, set the AC-GND-

the peak charging current (into 0.1 ILF

coupled input, for DC measurements

C).

Steps 7 and 8 in the Performance

Procedure describe the

ground the input and DC balance

the AC

different DC level is connected.

CAUTION

without using the pre-charge provi-

(in

checked and allowed for, or

Then, if the deflection is too

measured, use the AC-

component

followed whenever an-

specific

much (such

excess of 1 Mo) the

check

signal having both

large DC volt-

and adiust

lower deflec-

large amount

signal containinput coupling

can be

en-

meas-

capacitor) source, and this source may be damaged or de-

stroyed.

When 7A22 with the input AC coupled, the input coupling capacitor acquires as

very slowly decaying output voltage. This sequent AC coupled measurements at other DC voltages

and drive the trace off-screen. A period of at least 10 min-

utes, with input set to GND, should

reasonable recovery from polarization, and a longer period

may

connectors are shorted to ground the depolarization process will require less time.

Signal Input Connectors

When connecting signals to the +INPUT and –INPUT connectors pling that will sometimes source, particularly when is monitored at of these factors are missing, it becomes increasingly important to use shielded signal cables. In all cases, the signaltransparting leads should

When making single-ended input measurements, to establish test and the Type 7A22. The shield of normally used for this purpose.

In some cases differential measurements require mon ground’, ference by ground loop currents. Some problems with stray magnetic coupling into the signal transporting leads can

also single-ended measurement. cussed later in this section under Differential Operation.

It is always important to consider the signal-source loading and resulting change in the source operating characteristics due to the signal-transporting circuit of the Type 7A22. The circuit at the input connectors can

normally ground paralled by 47 pF. A few feet of shielded cable may increase the parallel capacitance In many cases, the effects of these resistive and capacitive loads may them through the use of

Attenuator probes not only decrease the resistive and capacitive loading of measurement range of the Type 7A22 to include substantially higher voltages. Passive attenuator probes having attenuation factors of 10X, 100X, and lOOOX, as well as other special-purpose types are available through your Tektronix Field Engineer or Field Office.

Some measurement situations require put to the Type 7A22 with very littIe source loading or sig-

‘The DC plus to the chassis the levels listed in Section 1 under Voltage reiection ratio

will be limited only by the signal

large DC voltage has been applied

low voltage, high impedance voltage source with

characteristics.

charge due

necessary for critical measurements. If the input

the Type 7A22, consider the method of

used to connect the Type 7A22 to

common ground between the device under

and therefore are less susceptible to inter-

minimized by using

represented by

too great and it may

peak

potential

and

exceed the input

dielectric polarization and acts

used. Ordinary unshielded test leads

high level, low-frequency signal

low impedance point, However when any

kept as short as practical.

These considerations

attenuator probe.

signal source, but also extend the

voltages

of the Type

Higher

on the test points with respect

levels will

differential rather than

leads and

a 1

megohm resistance ta

desirable to minimize

7A22

Maximum

degrade

voltage rating

the Type

can

offset sub-

allowed to assure

cau

coaxial cable is

are dis-

the

100 pF or more.

high-resistance in-

should be limited to

Common-mode Input

the common-mode

of the unit.

can

signal

sure

com-

input

2-4

TM 11-6625-2749-14&P

not attenuation. In such

probe cannot by using an vision of the Type 7A22.

High Input Impedance

TIM

coupled signals which permit the use of 10mV through 10 pV”psitions no input attenuator is used at these switch positions, the

internal gate return resistor olone establishes the 1 megohm input resistance.

The high input impedance is obtained by unsoldering the wire strap nal gate return resistance. The signal source must then provide

a DC

csetive

high input impedance provision applies only to DC

the VO”LTS/DIV switch, (DC coupled). ‘Since

(see

Fig. 2-3] between the input line and the inter-

path for the FET gate current.

situcstion

used. However, this problem may

probe or the high input impedance pro-

passive attenuator

solved

Display Polarity

Single-ended signals or —INPUT connector. If the + INPUT is chosen, positivegoing changes in the input signal will cause the trace to be deflected upward, and negative-going changes will cause the trace to

chosen, input-to-display polarity relationship will as shown previously in Fig. 2-1.

Deflection

The amount of trace deflection produced by determined by the signal amplitude, the attenuation factor of the probe, the setting of the VOLTS/DIV switch and the

setting of the VARIABLE control. The calibrated deflection factors indicated by the VOLTS/DIV switch apply only when the VARIABLE control is pushed “in” to the CAL IN position.

The range of the VARIABLE control is at least 2.5:1. It provides uncalibrated deflection factors covering the full range between the fixed settings of the VOLTS/DIV switch. The control can be set to extend the deflection factor to at least 25 volts/div.

Noise

Factor

Can be

deflected downward. If the —INPUT is

applied to either the +INPUT

reversed

signal is

Fig.

2-3. location of wire

return resistor.

The uncompensated gate current is typically less than 100 picoamperes, upon the operating temperature. The signal-source imped-

ance is therefore can important factor since gate current will produce

current through 10 megohms produces

may resu[t in

concern.

Men the wire straps are removed, R111 and R211

are disconnected. The deflection factor in the 20 m V/

DI V to 10 V/Dl V range will be incorrect.

The high-frequency response will

but may

a DC

signcsl-source impedance,

between the source input capacitance, must charge and discharge through that impedance [see Fig. 2-4).

strap between input

several times higher depending

offset. For example,

significant error where small voltages are of

NOTE

since various shunt capacitances

cmd

the 7A22 input as well as the 47 pF

line and internal gate

100 picoampere gate

a 1 mV

also depend upon the

offset; this

To reduce noise and obtain the VOLTS/DIV switch is operated in the 10 wV, 20 wV, and 50 pV positions or when the signal source is “noisy, it is suggested that the HIGH FREQ —3 dB POINT selector to use the lowest bandwidth setting which does not appreciably distort the desired features of the signal under observation. Refer to Fig. 2-6 for the high frequency rolloff for each setting of the HIGH FREQ —3 dB POINT selector.

Bandwidth

In addition to the differential reiection of unwanted signals, many times an undesired signal the bandwidth of the unit. The LOW FREQ –3 dB POINT and HIGH FREQ —3 dB POINT selectors on the front panel of the 7A22 control the low-frequency and high-frequency

–3 dB points of the amplifier. The LOW FREQ –3 dB POINT

selector provides low-frequency response to DC or to ap-

proximate —3 dB points at .1 Hz, 1 Hz, 10 Hz, 100 Hz, 1 kHz,

and 10 kHz. Refer to Fig. 2-5. The HIGH FREQ —3 dB POINT

selector controls the high-frequency rolloff from 1 MHz to

100 Hz in frequency response falls off at to Fig. 2-6.

Varying the bandwidth of the Type 7A22 is useful, for example, when displaying the high-frequency response the noise can, in many cases, be considerably reduced without distorting the desired signal

(see Fig. 2-7). Likewise, undesired line-frequency signals can

filtered out by restricting the low-frequency response of

the unit. When using the LOW FREQ —3 dB POINT and

HIGH FREQ —3 dB POINT selectors, care must be taken not to distort non-sinusoidal waveforms by overly restricting the amplifier bandwidth.

Selection

1-3-10 sequence. Beyond the –3 dB points the

more usable display when

can be

a 6 dB

low-frequency signal. By reducing

attenuated by varying

per octave rate. Refer

set

REV. B, FEB. 1975

2-5

2-6

Fig. 2-4A. 7A22 CMRR at 10 µV/Div to 10 mV/Div. Inputs DC Coupled. Worst-case degradation due to source resistance.

TM 11-6625-2749-14&P

Fig. 2-4B. 7A22 Bandwidth and VOLTS/DIV Error as functions of Source Resistance.

TM 11-6625-2749-14&P

2-7

2-8

Fig. 2-5. 7A22 FREQUENCY RESPONSE as a function of LF -3 dB POINT SETTING (HF -3 dB POINT at 1 MHz)

TM 11-6625-2749-14&P

Fig. 2-6. 7A22 FREQUENCY RESPONSE as a function of HF - 3 dB POINT settings ( LF —3 dB POINT of DC).

2-9

TM 11-6625-2749-14&P

2-7.

Fig.

QUENCY 1 MHz. UPPER —3 dB FREQUENCY, 10 kHz.

Improving

(Al

Lower —3 dB FREQUENCY selector to DC, UPPER — 3 dB FRE-

signal-to-noise

[B) Lower —3 dB FREQUENCY selector to DC,

ratio by setfing

bandwidth.

Voltage Comparison Measurements

Some applications require than the fixed values provided by the VOLTS/DIV One such application is comparison of signal amplitudes by

ratio rather than by absolute voltage.

To accomplish this, apply

put of

the

ABLE control throughout the subsequent comparisons. The settings of the VOLTS/DIV switch to accommodate large ratios. In doing so, regard the num-

bers which designate the switch position as ratio factors rather than voltages.

Type 7A22. Set the VOLTS/DIV switch and VARl-

set of deflection factors other

reference signal to either in-

can be

changed, however,

switch.

Differential Operation

Differential voltage measurements are made by applying the signals to the +INPUT Then, both AC-GND-DC switches should positions: AC or DC, depending

and —INPUT connectors.

set to the same

the method of signal

The Type 7A22 differential input provision may to eliminate interfering signals such as AC line-frequency hum. Single-ended measurements often yield unsatisfactory information because of interference resulting from groundIoop currents between the oscilloscope and the device under test. In other cases, it may voltage by means other than the use of itor which would limit low-frequency response. These limitations af single-ended measurements are virtually eliminated in differential measurements.

A differential measurement is made by connecting of the two inputs to selected points in the test circuit. The input to the amplifier will then of the two selected points

The ability of the Type 7A22 to relect common-mode sig-

nals is indicafed by the common-mode relection ratio (CMRR).

This ratio is at least 100,000:1 at the input connectors for the

lower deflection factors (10

signals between DC and 100 kHz are DC-coupled to the in-

puts. To show the significance of this characteristic, assume

that

single-ended input signal consists of

Hz signal at 1 volt P-P plus

an attempt is made to display the desired signal with the

VOLTS/DIV produce

useful information about the 1 mV signal could

If, however, the same 1 mV signol is monitored differentially so that the 60 Hz signal is ;ommon-mode at the inputs, no common-mode signal will appear in the display. Thus, the desired signal produces only .05 div of interference due to the common-mode signal.

There are

mode rejection. The principal requirement for maximum reiection is for the common-mode signal to arrive at the input FET gotes in presicely the same form. A difference of only

0.01 ~o in the attenuation factors of the input attenuators may reduce the rejection ratio to 10,000:1. Likewise, any difference in source impedance at the two points in the source under test

will degrade the relecfion ratio. Attenuator probes which do

not have an adjustable resistance may reduce the rejection ratio to 100:1 or less.

Outside influences such as electrostatic and Imagnefic fields

can level signals are involved. Electrostatic interference minimized by using shielded signal transporting leads of the

same ty~e to the two inputs, and by twisting the leads to-

gether ‘throughout most of their length. -

Care should signal transporting leads, as any movement of txesence of that lead. Where

avoided, the pickup loop formed by

minir’nized by taping or twisting them together throughout

most of their length. Low-frequency measurements similarly protected by using a

twisted pair of conductors.

switch set to .2 mV, the 60 Hz signal would

deflection equivalent to 5000 div, and thus little

greater than one part

number of factors which

also degrade

the

taken to minimize the movement of

maanetic field, will tend to induce

desirable to eliminate

DC-blocking copac-

the difference in voltage

(see

Fig. 2-9).

/LV to 10 mV

desired signal at 1 mV P-P. If

one hundred thousand of

disploy amplitude af 5 div with

performance, particularly when low

interfering magnetic field cannot

shielded cable which contains

per division) when

can

degrade common-

the

two leads should

each

unwanted 60

obtained.

can be

lead, in the

sianal into

can be

used

the

2-10

Fig. 2-8

TM 11-6625-2749-14&P

DC Offset Operation

By using the DC OFFSET controls, it is possible to use the Type 7A22 differentially in a slide-back mode, to observe small signal whose DC potential difference may be considerable. The offset is continuously adjustable from +1 V to –1 V when the VOLTS/DIV switch is in the 10µV to 10mV positions. In the 20mV, 50mV and .1 V positions of the VOLTS/DIV switch, the 1 V offset is effectively multiplied by

the input attenuator to a range of ±10 V. Table 2-1 sum-

marizes the effective DC offset voltages internally available

for all the VOLTS/DIV switch positions. The table also lists

the input attenuator that is switched into the amplifier circuit for the various VOLTS/DIV switch positions.

TABLE 2-1

Using the DC OFFSET functions:

Set the Type 7A22 controls as follows:

VOLTS/DIV VARIABLE (CAL IN) CAL POSITION Midrange AC-GND-DC GND

(+ and – INPUT) HF -3 dB POINT 1 MHz LF -3 dB POINT DC

STEP ATTEN DC BAL Adjusted for DC balance

1. Position the trace to graticule center (or some other

convenient reference line) using the POSITION control.

2. Connect a coaxial cable from the signal source to the

+INPUT.

3. Set the + INPUT AC-G ND-DC switch to DC and meas-

ure the DC level to be offset.

4. Set the VOLTS/DIV to the largest deflection factor in an

offset range which will encompass the DC level measured in

step 3. See Table 2-1 and front panel color bands.

5. Set the LF -3 dB POINT selector to DC OFFSET,

10V

2-11

TM 11-6625-2749-14&P

NOTE

If switching into another offset range, the OFFSET controls will need to be readjusted. If switching into

smaller offset range, check that the avail-

able range present (see steps 3 and 4 above) .

sufficient to encompass the DC level

Input Overdrive

The INPUT OVERDRIVE indicator turns

the input FETs approaches the differential dynamic range

of the amplifier. The 7A22 should not a

circuit if this light is on, as this may mean that

voltage is present.

The INPUT OVERDRIVE indicator serves another important function. If the amplifier is direct-coupled at the input, a DC differential signal could overdrive the input stage ond cause a

reduction in gain, The small voltages to

not

distorted, but will sult, amplitude measurements made under such conditions will not

accurate. The Type 7A22 INPUT OVERDRIVE indicator provides lighting before the gain calibration changes by 1

the

sible ways to continue:

DC differential sicinals may then indicated in Table-2-l.

signals up to 1000 V (either input not to exceed 500 be sarily limits the low frequency response to 1,6 Hz (or

0.16 Hz with

The INPUT OVERDRIVE lamp is insensitive mode overdrives, and it is possible to overload the Type 7A22 without lighting the input overdrive light.

In summary following conditions:

of the amplifier

b. An light will remain on even if to

GND.

c. There is

INPUT OVERDRIVE lamp turns on, there are two pos

Switch the LF —3 dB POINT selector

2. Switch the AC-GND-DC switch to AC. DC differential

removed by using AC coupling at the input. This

The input signal exceeds the differential dynamic range

input protective fuse is blown. In this case,

Indicator

reduced in amplitude. As

indication that such

10X probe).

the

overdrive indicator will turn

(see

Section 1 for table of dynamic ranges).

the

circuit malfunction.

‘

AC-G ND-DC switches are set

when the signol

left connected

measured will

signal is present by

~o.

to DC

balanced out as

damaging

OFFSET.

may

neces-

common-

under the

the

re-

Fig,

2-9. Connecting

6. Use the COARSE and FINE controls to move tion of the signal to lished instep

7. If

may

2-12

different size display isneeded, the deflection factor

changed in the same offset range.

differential amplifier

observed to the reference line estab-

across a

circuit.

the

por-

Readout

the

Type 7A22 is

readout capabilities, special probes which correct

out deflection factor for probe attenuation may

Divider probes not having with the instrument, but they will not operate the sensing system, and the deflection factor of on the VOLTS/DIV knob) will

The +INPUT and —INPUT connectors have which is connected allows

the

tion factor from the probe tip for any probe attenuation, For

main-frame readout

to be

used

in an

the

sensing capability may

the readout probe sensing device. This

oscilloscope having

the

plug-in only (as reed

displayed.

display

the

REV.

correct deflec-

B, FE B.

the

read-

used.

outer ring

1975

used

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