The Cathode Ray Oscillograph

The cathode ray oscillograph is at present one of the most useful of radio testing devices. Although as yet relatively expensive, its applications are so numerous that it will readily replace a number of measuring instruments of more or less satisfactory characteristics.

A few of the more important applications of the instrument are the study of wave shapes and transients, measurements of modulation and peak voltages, adjustment of radio receivers, comparison of frequencies, the indication of balance in bridge circuits, tracing of vacuum tube characteristics, etc.

I One of its chief advantages over other types of instruments is its freedom from inertia, allowing the observation of very rapid changes of current and voltage without appreciable distortion.

The Cathode Ray Tube.—Since the cathode ray oscillograph is built around the cathode ray tube, it is necessary to understand the principles of the latter before a study of the instrument itself.

A cathode ray tube, shown schematically in fig. 1 consists fundamentally of the following elements:

1. A glass envelope (F) whose purpose it is to maintain a vacuum in the tube.

2. A cathode (L) for the production of free electrons.

3. An electrode (I) whose purpose it is to accelerate the free electrons.

4. A focusing electrode (G) identified as anode No. 1, whose purpose it is to concentrate the liberated electrons into a cathode ray or beam.

Electrostatic Focussing System Cro
of the electro-static deflection type.

5. A high voltage anode (B) known as anode No. 2 for further accelerating the electrons.

6. A control electrode (H) referred 'to as grid No. 1 whose purpose it is to control the beam current.

7. Two sets of electrostatic deflection plates (C) and (D) for deflection of the electron beam.

8. The screen (S) which is coated on the inner surface of the enlarged end of the bulb with a material which shows a fluorescent glow at the impact point of the electron beam.

The collective action of electrodes (L), (H), (/), (G),and (B) are called an "Electron Gun" inasmuch as their function is to generate a beam of electrons and direct it toward the viewing screen (S).

As the electron beam consists of rapidly moving electrons, it constitutes a current having both electromagnetic and electro-

FIGS. 2,3 and 4—Illustrates various fluorescent patterns which the electronic beam traces on the screens of the cathode and ray tube, under various conditions.

FIGS. 2,3 and 4—Illustrates various fluorescent patterns which the electronic beam traces on the screens of the cathode and ray tube, under various conditions.

static properties. Because no material conductor is required to carry the electrons, the beam has negligible mass and inertia.

It is due to this inertialess characteristic that the electron beam can be deflected easily and rapidly by either electromagnetic or electrostatic fields. In the cathode-ray tube shown in fig. 1 the deflecting force produced by the phenomenon under investigation takes the form of an electrostatic field produced by a potential applied across the deflecting plates C. If this be an alternating voltage, the field produced causes the fluorescent spot viewed from the front of screen (S) to move up and down.

This movement of the spot traces a vertical line, as in fig. 2. A "time sweep" voltage of suitable wave form is applied across the deflecting plates D, causing the beam to move back and forth horizontally, as in fig. 3. The combined deflecting forces of the two fields may be caused to produce a pattern such as that in

Cathode Ray Oscilloscope
FIG. 5—Principal elements of a cathode ray tube of the electro-magnetic deflection type.

fig. 4. The fluorescent pattern which the electron beam traces on the screen can be distinguished, measured and photographed.

A type of cathode ray tube in which the deflection of the beam both horizontally and vertically is accomplished by means of two electromagnetic fields produced by two deflecting coils schematically depicted in fig. 5. In this casé no electrostatic plates are used, otherwise this type of tube functions principally the same as the electrostatic deflection type.

Parts of R.C.A. Type T.M.V.-122 B Cathode Ray Oscillograph.—The fundamental parts of this cathode ray oscillograph, shown in figs. 6, 7 and 8 are as follows:

FIG. G—Panel view of cathode ray oscillograph. (Courtesy R.C.A. Inc.)

(1) The cathode ray tube.

(2) Signal amplifier for vertical deflection.

(3) Signal amplifier for horizontal deflection.

(4) A saw-tooth timing axis oscillator.

(5) A low voltage full-wave rectifier for amplifier tube sup plies.

(6) A high voltage half-wave rectifier to supply approximately

1,200 volts to the cathode ray tube.

Oscilloscope 539

FIG. 7—View of RCA oscillograph with cover removed. The parts are as follows: A, power transformer, supplies power for all tubes and rectifier circuits—oversize to prevent stray magnetic fields; B, spring mounted cathode ray tube socket; C, low voltage full wave rectifier—supplies amplifier tube; D, vertical amplifier, high gain, wide frequency range; E, gas triode, "saw-tooth" timing axis oscillator; F, input binding post; G, vertical and horizontal beam centering adjustment—provide a simple means af centering beam on screen; H, high voltage half wave rectifier—supplies 1,200 volts to cathode ray tube; I, horizontal amplifier—high gain, wide frequency range; J, cathode ray tube—3 in. screen; K, binding posts for external horizontal deflecting voltage; L, binding posts for external synchronizing voltage.

FIG. 7—View of RCA oscillograph with cover removed. The parts are as follows: A, power transformer, supplies power for all tubes and rectifier circuits—oversize to prevent stray magnetic fields; B, spring mounted cathode ray tube socket; C, low voltage full wave rectifier—supplies amplifier tube; D, vertical amplifier, high gain, wide frequency range; E, gas triode, "saw-tooth" timing axis oscillator; F, input binding post; G, vertical and horizontal beam centering adjustment—provide a simple means af centering beam on screen; H, high voltage half wave rectifier—supplies 1,200 volts to cathode ray tube; I, horizontal amplifier—high gain, wide frequency range; J, cathode ray tube—3 in. screen; K, binding posts for external horizontal deflecting voltage; L, binding posts for external synchronizing voltage.

Various designs may vary slightly, although principally the number of elements employed are the same.

In commercial types of cathode ray oscillographs, the deflection of the cathode ray beam is caused by an electrostatic field produced by an impressed alternating current applied across either of two plates (C) or (D), fig. 1.

Two types of cathode ray tubes commonly employed for radio servicing are 906 or 908. The construction of the two are similar, the only difference being in the screen coating employed.

The use of No. 906 in a typical oscillograph circuit is shown in fig. 9. In this circuit the electrode voltages are obtained from the bleeder circuit connected across the high voltage supply.

Regulation of spot size and intensity can be accomplished by the variation of No. 2 anode current and voltage. The current to anode No. 2 may be increased by reducing the bias voltage applied to the control electrode (grid No. 1). An increase in the No. 2 anode current increases the size and intensity of the spot. An increase in the voltage applied to anode No. 2 increases the speed of the electrons, which increases spot intensity and decreases spot size.

In applications involving extremely accurate measurements, the No. 2 anode current should be reduced to the minimum value consistent with the desired brilliance of pattern. Where high brightness is an important consideration, the No. 2 anode voltage may be increased to the maximum rated value. This procedure, however, is not always desirable since the greater electron speed causes reduced deflection sensitivity.

The maximum input power to the fluorescent screen should not exceed 10 mw. per sq. cm. except for short-interval operation. The use of screen-input power in excess of this value will adversely affect the fluorescent coating, depending on the mag nitude and the duration of the power input. The resultant injury to the screen may be a temporary loss of sensitivity, or a permanent destruction of the active screen material.

ANODE No. 2

ANODE No. 2

FIG. 9—Schematic diagram of cathode ray tube connection in an oscillograph.

A high intensity spot should be kept in motion by applying voltage to the deflecting system, in order not to exceed the maximum fluorescent screen input rating. Until this voltage is applied, the fluorescent screen input power should be kept low, either by applying a high negative control-electrode bias or removing the voltage from anode No. 2.

TYPE 879

MICRO-AMMETER (0 - 200 MICI10-A)

TYPE 879

MICRO-AMMETER (0 - 200 MICI10-A)

FIG. 10—Typical circuit for cathode ray oscillograph.

CATHODE RAY TUBE

SWITCH

SWITCH

MICRO-AMMETER

FIG. 11—Front view arrangement of oscillograph whose circuit is shown in fig. 10-

MICRO-AMMETER

FIG. 11—Front view arrangement of oscillograph whose circuit is shown in fig. 10-

Time sweep circuits are of various types. The choice of circuit depends upon the type of phenomena under observation as well as upon the type of cathode-ray tube used. For recurrent phenomena, a periodic sweep with a repetition frequency adjustable to a simple multiple relation with the frequency qf the phenomena is generally employed. For transient phenomena, a single sweep of the electron beam across the screen is ordinarily desirable; the starting of this sweep essentially coincident with the starting of the transient can be controlled manually, or automatically by electrical circuits, depending upon the application.

Supreme Model 546 Cathode Ray Oscillograph.—The front view and connection diagram are shown in figs. 12 and 13 respectively.

All the controls are located on the front panel for instant and convenient use. It has an intensity and focus control, also vertical and horizontal spot centering control. The vertical and horizontal amplifiers are built in, each using a 6C6 tube with graduated gain control.

The input impedance is 500,000 ohms at less than 20 micro-micro-farads input capacitance. Both amplifiers are flat from 15 to 90,000 cycles. The linear saw tooth time base or sweep voltage is developed by an 885 gas triode oscillator which covers a range of from 15 to 30,000 cycles in seven over-lapping steps, which make it possible to observe frequencies of up to approximately 300,000 cycles. A vernier control allows accurate setting of any frequency desired.

Another important feature is the high speed return trace eliminator which takes out the return sweep at high time base frequencies and eliminates distortion when observing high frequency patterns on the cathode ray screen.

FIG. 12—Panel view of Supreme Model 546 cathode ray oscilloscope.
Cathode Ray Oscilloscope Circuit Diagram
TABLE OF SYMBOLS AND RESISTANCE VALUES

r

ohms

r

ohms

r

ohms

c

mfds.

c

mf;ds.

r • i

25 m

r • 11

4.0 meg.

r • 21

1000

c • 1

4.0

c ■ 11

0.05

r ■ 2

10 m

r - 12

0.13 "

r • 22

0.5 meg.

c • 2

4.0

c • 12

50 mmfd.

r ■ 3

10 m

r • 13

0.10 "

r • 23

1000

ç • 3

0.5

c • 13

0.2

r ■ 4

220

r • 14

1.0 »

r ■ 24

1.0 meg.

c • 4

8.0

c • 14

0.04

r ■ 5

0.25 meg.

r ■ 15

0.5 »

r • 25

4.0 meg.

c ■ 5

0.05

c ■ 15

0.01

r ■ 6

0.75 '

r • 16

1.0 •'

r • 26

0.1 meg.

c • 6

. 0.05

c ■ 16

0.0025

r • 7

0.50 »

r - 17

1.0 "

r • 27

0.1 meg.

c ■ 7

0.1

c - 17

600 mmfd.

r - 8

0.10 »

r • 18

4.0 «

r - 28

500_r\_

c ■ 8

0.1

c • 18

200 mmfd.

r ■ 9

0.20 »

r ■ 19

1.0 •

c ■ 9

0.05

c • 19

0.5 mfd.

r • 10

4.0 "

r ■ 20

15 m

c ■ 10

0.05

FIG. 13—'Schematic wiring diagram of Supreme Model 546 cathode ray oscilloscope.

The operator has a choice of internal or external synchronization, and of either the internal linear sweep or any external

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