^ecknic&L ^iote 287

DISCLOSURES ON

Magnetic Tape Handler

Focus Detector,

Multi-pen Recorder,

Signal Averaging Filter,

Electrostatic RMS Voltmeter,

Back-up Ring for O-rings, and

Low Temperature Solid State Thermometer

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assur- ing maximum application of the physical and engineering sciences to the advancement of technology

in industry and commerce. Its responsibilities include development and maintenance of the national

standards of measurement, and the provisions of means for making measurements consistent with

those standards; determination of physical constants and properties of materials; development of methods for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices

for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Govern- ment; assistance to industry, business, and consumers in the development and acceptance of commercial standards and simplified trade practice recommendations; administration of programs in cooperation with United States business groups and standards organizations for the development

of international standards of practice; and maintenance of a clearinghouse for the collection and

dissemination of scientific, technical, and engineering information. The scope of the Bureau's

activities is suggested in the following listing of its three Institutes and their organizational units. Institute for Basic Standards. Applied Mathematics. Electricity. Metrology. Mechanics. Heat. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radiation Physics. Radio Standards

Laboratory:* Radio Standards Physics; Radio Standards Engineering. Office of Standard Reference

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Information.**

* Located at Boulder, Colorado, 80301. ••Located at 5285 Port Royal Road, Springfield, Virginia, 22171.
NATIONAL BUREAU OF STANDARDS

Technical Note 287

ISSUED APRIL 8, 1966

DISCLOSURES ON

Magnetic Tape Handler

Focus Detector,

Multi-pen Recorder,

Signal Averaging Filter,

Electrostatic RMS Voltmeter,

Back-up Ring for O-rings, and

Low Temperature Solid State Thermometer

Editors: David Robbins and Alvin J. Englert

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, Government Printing Office

Washington, D.C., 20402 - Price 25 cents

TABLE OF CONTENTS

Title

A High Speed Magnetic

Tape Handler

A Multi-Pen Recorder Having

a Time Delay Arrangement

Electro-Optical Focus

Detector for a Point

Light Image

A Multi-Segment Signal

Averaging Filter

Electrostatic RMS Voltmeter

Silicon Radiation Detector

Measures Low Temperatures

High Pressure Back-Up Ring

for O-Rings

Author Page

McRae Anderson 1 Bureau of Census

Washington, D. C. 20233

Noboru Wakai 10

Environmental Science Services

Boulder, Colorado 80301

Jurgen R. Meyer-Arendt 13

Ralph B. Bergman

Environmental Science Services

Boulder, Colorado 80301

Robert J. Carpenter 17

National Bureau of Standards

Washington, D. C. 20234

Ephraim W. Hogue 20

National Bureau of Standards

Washington, D. C. 20234

William R. Dodge 23

Steve R. Domen

Dale R. Hoppes

Alan T. Hirshfeld

National Bureau of Standards

Washington, D. C. 20234

Hobart S. White 26

(formerly with

National Bureau of Standards)

201 Washington Avenue

Elkins, West Virginia 26241

iii
This booklet presents descriptions and drawings of seven

devices, embodying interesting and unusual solutions to problems

frequently encountered in their respective fields: a high speed

magnetic tape handler, an electro-optical focus detector for a point

light image, a multi-pen recorder having a time delay arrangement,

a multi-segment signal averaging filter, an arrangement for measuring low temperatures using a silicon radiation detector, a high pressure back-up ring for O-rings, and an electrostatic RMS voltmeter.

Other disclosures on various subjects may be found in NBS

Technical Notes 237, 253, 263, and 282.

All property rights in the devices methods and systems described in the above disclosures, unless otherwise noted, have

been retained by the United States Government as represented by

the Secretary of Commerce.

Tech. Note No. 287, April 8, 1966

iv
A HIGH SPEED MAGNETIC TAPE HANDLER

McRae Anderson

TO

MOTOR

CONTROL J C'~S- UNIT at IN THIS FHxUffe

TO

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SOURCE

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22

Tech. Note No. 287, April 8, 1966

1
A HIGH SPEED MAGNETIC TAPE HANDLER

McRae Anderson

FROM

COMPUTER

FORWARD

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IN FIG. 1

Tech. Note No. 287, April 8, 1966

2
A HIGH SPEED MAGNETIC TAPE HANDLER

McRae Anderson

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Tech. Note No. 287, April 8, 1966

3
A HIGH SPEED MAGNETIC TAPE HANDLER

McRae Anderson

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Tech. Note No. 287, April 8, 1966

4
A HIGH SPEED MAGNETIC TAPE HANDLER

McRae Anderson

This tape handler may be used to drive magnetic tape at high speeds,

with precise start-stop characteristics. The handler requires a minimum number of components, has a high degree of electrical efficiency,

and can be constructed as a self-contained unit.

With reference to Fig. 2, the clutch-brake control unit 10 receives

command signals from the computer. When a forward command signal is

received, unit 10 applies a control signal to flip-flop 11, which in turn

applies signals to clutch-brake assembly 12 to engage the clutch and

release the brake in the latter assembly. Motor 13 will then drive capstan 14 (Fig. 1) in a clockwise direction, and tape 20 is transported

from supply reel 21 through vacuum chambers 22 and 23 to take-up reel

24. The tape is guided by idler pulleys 2 5 and passes in front of erase

head 26 and read or record heat 27 and between the components of photocell unit 28.

When a rewind signal is received from the computer, tape 20 is

transported from reel 24 to reel 21. Toward the end of the tape, photocell unit 28 senses a clear leader and transmits a signal to photocell

detector 35 (Fig. 2) which then applies a stop signal to control unit 10.

A series of events are then initiated to disengage the clutch in assembly

12 and thereby stop the counterclockwise rotation of capstan 14 and the

movement of tape 20.

Sensing ports 36 (Figs. 1 and 3) are positioned in vacuum chamber

23 and are coupled by means of tubes to pressure-differential switches

A to E as illustrated in the figures. The switches are tied to motor control unit 40 as shown in detail in Fig. 3. One end of tube 37 is open to

the atmosphere, and one end of tube 3 8 is connected to a vacuum source,

not shown. Port 39 is also connected to the vacuum source so that the

pressure in the chamber below the tape is approximately 18" of water

while the pressure above the tape is that of the atmosphere. Any one of

the switches A to E is open when loop 20 is positioned in such a way

that both sensing ports, related to the switch, are at the same pressure,

and the switch is closed when the pressure on one of the ports is atmospheric while the pressure on the other is 18" of water.

Tech. Note No. 287, April 8, 1966

5
Sensing ports 42 are positioned in vacuum chamber 22 and are coupled to pressure-differential switches A' to E' in the manner shown

in Fig. 1. One end of tube 43 is open to the atmosphere, and one end

of tube 44 is connected to the vacuum source. The vacuum source is

also connected to port 45 to maintain the same pressure differential in

this chamber as in chamber 23. Motor control unit 46 is identical to

unit 40 and switches A

1 to E' are tied to the former control unit in the

same way as A to E are tied to 40.

Perforated metal stops 47 and 48 are positioned in chambers 22

and 23, respectively, to prevent the loops in tape 20 from entering

either vacuum port 39 or 45.

When the loop in tape 20 is moved from the center of chamber 22,

motor control unit 46 applies power to motor 50 to drive reel 21 and

return the loop to the center of the chamber, where under ideal operating

conditions the loop should be. The power is applied to the motor in

three stages, and the magnitude of power in each successive stage is

increased as the distance between the loop and the center of the chamber

is increased. Likewise, when the loop is moved from the center of

chamber 23, motor control unit 40 applies power to motor 49 which in

turn drives reel 24 so that the loop is returned to the center of the chamber.

Motor 50 is identical to motor 49, which is shown schematically

in Fig. 3 and is an AC torque motor with balanced windings 56 and 57

having a common point connected to AC power supply 58. Control of

the direction of the motor is obtained by phase-shift capacitor 59,

coupled across the windings.

As an example of operation, assume that tape 20 is being driven by

capstan 14 from supply reel 21 to take-up reel 24 and that one loop is in

the center of vacuum chamber 22 while the other is in the center of chamber 23. Assume further that clutch-brake control unit 10 (Fig. 2) receives

a command signal from the computer to move the tape backward, i. e. , to reverse the direction of tape movement. Clutch-brake assembly 12 will

then cause capstan 14 to reverse the direction of movement of tape 20.

However, due to inertia, reels 21 and 24 will continue to rotate clockwise for a short period of time and the loop in chamber 2 2 will move

toward the bottom of chamber 22 while the one in chamber 2 3 will move

toward the top of the chamber.

Tech. Note No. 287, April 8, 1966

6
Now assume that a loop is located above the first sensing port 36

from the top of chamber 23. Differential-pressure switch A (Fig. 3) is

then closed and during the positive half-cycle of the AC voltage applied

to motor 49, diode 61 conducts through tapped resistor 62 and diode 63

to the AC neutral. The first positive pulse of voltage Vf (Fig. 4) then

appears across the tapped resistor, and since switch A is closed voltage

V , represented by curve 64 (Fig. 5), is applied through potentiometer

65 and diode 66 to the gate of silicon-controlled rectifier 67. The firing

potential of this rectifier is indicated by dotted line 68. When the

magnitude of curve 64 exceeds the magnitude of line 68, the rectifier

conducts and continues to conduct as long as positive potential is fed

through winding 56 to the rectifier's anode. 'When the rectifier conducts,

a circuit is completed through winding 56 and rectifier 67 to AC neutral so that pulse 70 represents the voltage V appearing across rectifier 67

(Fig. 6) and region 71 represents an area in which power is applied to

motor 49.

On the negative half-cycle of the AC voltage applied to motor 49, a negative potential is transmitted through diode 7 2, tapped resistor 62

and diode 7 3 to neutral. Referenced to the cathode of silicon-controlled

rectifier 74, a positive potential appears at the top end of resistor 62 as

drawn in Fig. 3 and is applied through diode 7 3 to the anode of rectifier

74. The voltage appearing across the resistor is represented by the

second positive pulse of voltage v\. in Fig. 4. Voltage V , illustrated by

curve 7 5 (Fig. 5), is positive relative to the cathode of rectifier 7 4 and

is applied through switch A, potentiometer 65 and diode /6 to the gate

of this rectifier. When the magnitude of curve 75 reaches the firing

potential indicated by line 68, the rectifier conducts and continues to

conduct as long as the anode of the rectifier is positive relative to the

cathode. As the rectifier conducts current flows through winding 56,

and region 79 represents another area in which power is applied to motor

49. Pulse 80 represents the voltage V developed across reactifier 67.

When power is applied to motor 49, as described above, reel 24

is rotated counterclockwise, moving the loop in tape 20 toward the center

of chamber 23. As the loop enters the area between the first and second

sensing port from the top of the chamber, switch B closes. Since switch

A remains closed, a portion of tapped resistor 62 is short circuited and

curves 85 (Fig. 5) illustrate voltage V , pulses 86 (Fig. 7) the voltage

V and regions 87 the power applied to motor 49.

Tech. Note No. 287, April 8, 1966

7
As the loop enters the area between the second and third ports

from the top of chamber 23, switch A opens and B remains closed.

Curves 88 then represent voltage V , pulses 89 (Fig. 8) the voltage V

and regions 90 the power fed to motor 49. Thus it is seen that as the

loop moves closer to the center of the chamber, the power transmitted to

the motor decreases.

When the loop in tape 20 is in damping zone 93, switch C is closed

and a circuit is completed from positive potential source 95 through brake

coil 96, current limiting resistor 97 and resistor 98 or capacitor 99 to

neutral. If the loop remains in zone 93 for a comparatively long period

of time, the current flow through this circuit will have a value I, that is

dependent upon the resistance values of coil 96 and resistors 9/ and 98.

The voltage V,, developed across capacitor 99, will then be the same as

that appearing across resistor 98. In the discussion that follows the

inductance of coil 96 is considered to be negligible.

When the loop leaves the damping zone 93, as in the example

above, switch C is opened (Fig. 9) and the voltage V, (Fig. 10) across

capacitor 99 is discharged. Since in this example, the loop is out of

the zone for a comparatively long period of time, the voltage on the

capacitor approaches zero. When at time t, the Iood is returned to the

damping zone 93, switch C is closed, connecting one side of capacitor

99 to neutral so that current flows from positive potential source 9 5 through the braking coil 96 in the manner illustrated in Fig. 10. This

applies a braking force to motor 49 of such a magnitude that the loop

will normally come to rest in the damping zone. Meanwhile the voltage

across the capacitor increases in magnitude, and if the loop remains in

the damping zone for a comparatively long period of time, the voltage

across the capacitor will equal V, and the current through braking coil

96 will equal I,. Thus, a "soft" oraking force will be applied to motor

49.

L

If the loop in tape 20 overshoots the damping zone 93 and approaches the bottom of chamber 23, switches D and E, potentiometer 100,

diodes 101, silicon-controlled rectifiers 102 and bridge circuit 103

cooperate with motor 49 to return the loop to the center of the chamber.

Tech. Note No. 287, April 8, 1966

8
Assume in the present example that the loop overshoots damping

zone 93. As the loop leaves the zone at time t ?

, switch C opens and

capacitor 99 starts to discharge through resistor 98. During the overshoot the loop is out of the zone for a shorter period of time than when

it first left the zone. As the loop re-enters the zone at time t~, switch

C closes and capacitor 99 starts to charge as shown in Fig. ID. Because

a greater potential was stored on the capacitor at time t~ than at time t,

,

the value of current flow into the capacitor is smaller at t~ than at t,

.

If the loop overshoots several times, switch C will open and close as

shown in Fig. 9, and the current and voltage waveforms in Figs. 10 and

11, respectively, will be generated until the loop comes to rest in the

damping zone. It is noted that after each successive overshoot it takes

longer for the loop to pass through the damping zone 93 and each successive overshoot occurs in a shorter period of time. It is also noted that

the longer the loop is out of the damping zone, the greater the current

flow through coil 96 on the loop's return to the zone and the greater the

braking force applied to motor 49.

As indicated in the first example of operation presented above,

when the direction of tape movement is toward reel 24 and this direction

is reversed, the loop in tape 20 moves toward the bottom of chamber 2 2. Motor control unit 46 (Fig. 1) will then operate motor 50 to return to the

loop to the center of the chamber.

Tech. Note No. 287, April 8, 1966

9
A MULTI-PEN RECORDER HAVING A TIME DELAY ARRANGEMENT

Noboru A/Vakai

Tech. Note 287, April 8, 1966

10
A MULTI-PEN RECORDER HAVING A TIME DELAY ARRANGEMENT

Noboru Wakai

This device employs several pens to record a plurality of varying information signals on a suitable medium with the same time ordinate. The

signals may be recorded either simultaneously or with a desired time

delay between two or more of the signals.

The two input information signals and a reference timing signal are applied to modulator unit 10 where each signal is modulated with an

appropriate modulation frequency. The signals are then transmitted to

amplifier unit 11 where they are amplified and are then applied to recording heads 12. One of the information signals is recorded in track 14 and

the other is recorded in track 15, while the timing signal is recorded in

track 13. The tracks are located in magnetic tape 16, which is driven

by motor 20 through shaft 21 and drum 22 and is guided by drum 23 and

shaft 24. The track is driven in the direction indicated by the arrow in

the figure, and consequently eraser 2 5 will erase previous recordings on

the tape before the tape reaches the recording heads.

Reading heads 30, 31, and 32 sense tracks 13, 14, and 15, respectively, and the output of the heads are applied to demodulator unit 3 3. The latter unit provides three control signals, and each signal is fed to a related servo system in servo-amplifier unit 3/ where the signals are amplified and are then used to drive motors 3 8, 39, and 40. Thus a servo signal, that is dependent upon the timing signal, drives motor 38

while a pair of servo signals, each dependent upon one of the information signals, drives motors 39 and 40, respectively.

A roll of recording paper 45 is driven in the same direction as

magnetic tape 16 by motor 20 through shaft 21 and drum 46. The paper

is guided by drum 47 which is positioned on shaft 24.

Each of the ink pots 50, 51, and 52 is located on one of the rails

53, 54, or 55 and supplies ink to a pen that is positioned to record data

on paper 45. The pots are controlled by strings 56, 57, and 58 which,

in turn, are operated by motors 38, 39, and 40, respectively, in a conventional arrangement that is not shown in detail in the figure.

Tech. Note No. 287, April 8, 1966

11
In a typical operation, assume that the input information signals

are sinusoidal and that the input timing signal is a pulse. Assume

further that the distance between ink pots 50 and 51 is equal to the

distance between reading heads 30 and 31 and that the distance between

pots 51 and 52 is equal to that between heads 31 and 32.

Under these conditions, the pen attached to ink pot 52 will, at a predetermined time, start to record sinusoidal curve 62, and after a certain time delay, the pen attached to pot 51 will start recording sinusoidal curve 61. After another period equal to that of the time delay,

the pen on pot 50 will record timing dot 60. Due to the spacing between

adjacent ink pots relative to the spacing between adjacent reading heads,

the time delay in recording on paper 45 are compensated for when the data

is read from magnetic tape 16, and the time relations among dot 60 and

curves 61 and 62 is the same as those among the information and timing

signals. It will be apparent that a desired time delay could be introduced

between any two of the input signals recorded on paper 45, or among all

three signals, by proper adjustment of the distances between adjacent

ink pots relative to those between adjacent reading heads. With this

type of staggered pen and time delay arrangement, it is possible for the

resulting recorded signals to appear in their real time relationship to

each other. Each pen could be allowed to traverse the full width of the

recordable area on the chart, thus overlapping, without mutual mechanical

interference.

Tech. Note No. 287, April 8, 1966

12
ELECTRO-OPTICAL FOCUS DETECTOR FOR A POINT LIGHT IMAGE

Jurgen R. Ralph B. Meyer-Arendt

Bergman

Tech. Note No. 287, April 8, 1966

13
ELECTRO-OPTICAL FOCUS DETECTOR FOR A POINT LIGHT IMAGE

Jurgen R. Meyer-Are ndt

Ralph B. Bergman

OJ5

N)l

Tech. Note No. 287, April 8, 1966

14
ELECTRO-OPTICAL FOCUS DETECTOR FOR A POINT LIGHT IMAGE

Jurgen R. Meyer-Arendt

Ralph B. Bergman

This detector may be used to provide information as to when a stationary or moving object is in correct focus.

Its essential equipment is a scanning drum 10 whose outer surface

is shaped in the form of a spiral and may be in the form of another suitable curve, e. g. , of a helix. The drum contains several equally spaced,

narrow, rectangular slots 11 of equal width. The light is received from

the object through lens 12, which may be part of a telescope. As the

drum is driven by motor 13, the light passes through the slots and falls

on photocell 14 to develop a series of pulses, which are applied to pulse

shaper 15.

The pulse shaper generates a waveform that will be described with

the assistance of Fig. 2 where, for convenience of explanation, only

seven slots are shown. The ideal plane of focus, which is the theoretical focal plane for the optics concerned, is determined by the design of

lens 12. Variations in the refractive index of the atmosphere between

the lens and the object will change the image distance so that the object

may be in focus either in front or behind the theoretical image plane.

"When the object comes to focus in plane 16, which is the plane

containing the shortest detectable image distance, the slot 11, located

at the largest radius of the drum, will intersect the beam of light and

produce the narrowest pulse. Thus, if the drum is in its initial position

in Fig. 2, the series of pulses generated, as the drum is rotated, is

represented by waveform 30.

When the drum is positioned initially so that the light comes to

focus in plane 18, or the plane of the longest image distance that can

be detected, as the drum is rotated from this initial position, the longest

pulse will appear first and the narrowest pulse will appear last in the

sequence of generated pulses. This sequence is shown in waveform 31.

Finally, when drum 10 is positioned initially so that the light

comes to focus in ideal plane 17, the sequence of generated pulses will

start with a longer pulse and the narrowest pulse will occur when the

drum is midway through its revolution. This sequence is represented by

waveform 3 2.

Tech. Note No. 287, April 8, 1966

15
Disk 42 is positioned on the same shaft as drum 10 and is driven

by motor 13. The disk contains a small magnetized area 43 which is

sensed by reading head 44 whose output is amplified in pulse amplifier

45 and is then used to trigger the horizontal sweep of oscilloscope 38.

The horizontal sweep of the oscilloscope is set to cover screen 46 during

one revolution of disk 42. Thus, one revolution of drum 10 coincides in

time with one horizontal sweep of the screen. The scanning speed of the

drum controls the frequency at which focus detection is accomplished.

The output of pulse shaper 15 is fed to trigger circuit 35 where the

leading edge of each pulse generates a signal that triggers reference

pulse generator 36. The output of generator 36 is transmitted to pulse

length comparator 37 where each pulse developed by the generator is

compared with a respective pulse provided by shaper 15 through pulse

delay network 49. The delay network is provided to equalize delays

caused in circuits 3 5 and 36. When the length of the pulse developed

by the shaper is equal to or less than the one developed by the reference

generator, comparator 3 7 applies a pulse signal to the intensity control

circuit of the horizontal sweep of oscilloscope 38 thereby producing a dot 47 on the screen.

In operation, reading head 44 develops a reference pulse which is

amplified and then triggers the horizontal sweep of oscilloscope 3 8. At

the same time, photocell 14 senses slots 11 to provide a series of pulses

that are shaped in pulse shaper 15 and applied to comparator 37. When

the shaper provides a pulse that is equal to or smaller than the reference

pulse developed by generator 36, comparator 37 puts out a pulse signal

that controls the beam intensity circuit of oscilloscope 38 to produce a dot 47 on screen 46. When the shaper provides a pulse that is larger

than the reference pulse developed by generator 36, comparator 37 does

not produce an output pulse. This system insures that only pulses shorter

or equal to pulses developed by generator 36 are used to produce an "in

focus sensing" by comparator 37 and to produce a dot on the oscilloscope

46 screen. If the dot appears to the left of reference line 48, the image

lies in front of the ideal image plane 17. If the dot appears to the right

of line 48, the image is behind the ideal image plane; and if the dot

appears on the line, the image is correctly in the theoretical image plane

and in sharpest focus.

It will be apparent that a continuous recording film camera could be

placed in front of screen 46 to record the continuous change in the detected image location, or that an arrangement could be provided, responsive

to the output of amplifier 4 5 and comparator 37, to trigger the shutter of a camera, positioned to photograph the object when in the plane of best

focus.

Tech. Note No. 287, April 8, 1966

16
A MULTI-SEGMENT SIGNAL AVERAGING FILTER

Robert J. Carpenter

3P

SIGNAL

SOURCE:

2L

SYNC.

CIRCUIT

iO^

HORIZ.. SWEEP

GENERATOR.

-aM^-

VfAAr30

4-4- 1

29

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25L

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COMPARATOR

RESET 22

TO "ON"

AMD 23 -23

TO "OFF'

PULSED

CURRENT

SOURCE

II 1 1 III

16- 21

\

13

12

20

SWITCH * X

^R.

Tech. Note No. 2 87, April 8, 19 66

17
A MULTI-SEGMENT SIGNAL AVERAGING FILTER

\

Robert J. Carpenter

This filter may be used in analyzing a desired signal that is extremely

weak in the presence of a strong, unrelated one, such as thermal noise

and hum pick-up. The desired signal is repetitively generated and is

applied to a filter which by averaging techniques accentuates the desired

signal and suppresses the unrelated one.

With reference to the figure, voltage 9 is obtained from horizontal

sweep generator 10 and is transmitted to voltage comparator 11. When

the magnitude of 9 is greater than that of voltage 12, appearing across

capacitor 13, the comparator sends a signal through gate 14 to blocking

oscillator 15, triggering the oscillator. The output of 15 is a short

voltage pulse that is converted to a current pulse in the pulsed constant

current source 16.

Since the duration of each pulse generated by oscillator 15 and the

output of source 16 are constant, there is a fixed charge in each pulse

applied to capacitor 13. Since a constant relationship exists between

the instantaneous amplitude of voltage 9 and the horizontal deflection

position on the face of cathode ray tube 46, a predetermined number of

equally spaced pulses are generated by oscillator 15 during the sweep.

The sweep from generator 10 generally is linear vs. time, consequently

the pulses divide the sweep into a number of equal time intervals.

Since the pulses are related to horizontal position, not time interval,

this system will produce the same number of segments for a wide range

of sweep speeds.

At the end of the horizontal sweep a gating pulse 20, obtained

from a gate terminal on generator 10, momentarily closes normally-open

switch 21, and capacitor 13 discharges through the switch. The gating

pulse is also applied to flip-flops 2 2 to 2 5, resetting the first flip-flop

to "on" and the remaining ones to "off. " For convenience of illustration

only four flip-flops are shown although as many as desired may be connected as illustrated in the figure.

Tech. Note No. 287, April 8, 1966

II
The flip-flop 22 is turned on by gating pulse 20 and off by the

first pulse in the series generated by oscillator 15. During the time

interval that the flip-flop is on, it develops a signal that opens diode

gate 29 and a portion of input signal 30 is fed to capacitor 32. The

input signal is developed by signal source 31 and contains the desired

and unrelated signals. When flip-flop 22 is turned off, it provides a signal that activates flip-flop 23, which in turn is de-activated by the

second pulse in the series generated by the oscillator. During the time

interval that flip-flop 23 is on, a second portion of input signal 30 is

fed through diode gate 33 to capacitor 34. In a similar manner in successive time intervals of the horizontal sweep, the remaining flip-flops

24 and 2 5 are turned on and off and successive portions of the input

signal are applied through diode gates 40 and 41 to capacitors 42 and

43, respectively.

When each diode gate 29, 33, 40, or 41 is open, the R-C time

constant, formed by resistor 44 and the related capacitor, is long compared with the duration of the portion of the input signal 30 applied to

the capacitor. Thus, only a small charge can flow into or out of a capacitor when the related gate is open.

Signal source 31 develops a series of input signals 30, and synchronizing circuit 4 5 is used to synchronize these signals with the

horizontal sweep developed by generator 10. Thus, each time one of

the diode gates 29, 33, 40, or 41 is open it will sense the same portion

of one of the input signals 30. When a gate is open the voltage appearing across one of the related capacitors 32, 34, 42, or 43 is applied to

the vertical plates of cathode ray tube 46 in an oscilloscope. Thus,

after a certain number of input signals 30 have been generated, each

capacitor is charged to the average value of the applied portions, and

waveform 47 appears on the screen of the cathode ray tube. Since the

average of the unrelated signals, appearing in signals 30, over a sufficiently long period is substantially zero, waveform 47 represents

the desired signal.

Tech. Note No. 287, April 8, 1966

19
ELECTROSTATIC RMS VOLTMETER

Ephraim W. Hogue

Fieri

TO iT06

2\

7'

Si

18

to m+^—o- 15.

-o—»- TO 4-

TO 14- ——o16'

17'

CWy ZO-

..J

-o—»~ TO IS

Fig ^

Tech. Note No. 287, April 8, 1966

20
ELECTROSTATIC RMS VOLTMETER

Ephraim W. Hogue

This voltmeter provides precise measurements of the root-mean-square

(rms) values of random or repetitive voltages.

As shown in Fig. 1, the voltmeter input terminals 1 are connected

in series with switch contacts 2, 3, a fixed plate 4, and a flexible

metallic diaphragm 5. The closure of contacts 2, 3 connects the input

voltage E across plate 4 and diaphragm 5, causing the diaphragm 5 to

deflect from its rest position towards the plate 4. As can readily be

shown, the average deflection of diaphragm 5 is proportional to the

mean-squared value of E .

This average deflection produced by E is periodically compared

with an adjustable constant deflection produced by a known dc voltage

from a variable, accurately calibrated dc voltage source 6. Source 6

is connected in series with contacts 7 and 3, plate 4, and diaphragm 5. When the average deflection of diaphragm 5 produced by E equals the

the mean-square of F equals the

ue of E , read from the source 6 x

constant deflection produced by E , , mean-square of E, , whereby the val^v

equals the root-mean-square (rms) of the unknown voltage E

The deflections of diaphragm 5 produced by E and E , are measured in any convenient manner, as by sensing the changes in the

capacitance between diaphragm 5 and a fixed electrode 8. For this

purpose, the capacitor 5-8 is connected in series with a large resistance 9 and a dc voltage source 10, whereby the capacitance changes

produce corresponding voltage changes at the junction between the

capacitor and resistor. These voltage signals are coupled by a capacitor 11 to a high gain amplifier 12 whose output is fed to a transformer

13 having a center-tapped secondary winding 14. The positive and

negative ends of this winding 14 are connected to switch contacts 15

and 16 which are alternately engaged by a moving contact 17. Contact

17 is vibrated in synchronism with the previously-described moving

contact 3, preferably by a common driving means 18. Advantageously,

the amplifier 12 is tuned to the frequency of the driving means 18.

Tech. Note No. 2 87, April 8, 19 66

21
From the foregoing, it will be seen that as contact 3 is alternated

between E and E , , contact 17 is simultaneously vibrated between

X QC

positive and negative voltages having magnitudes proportional to the

deflections of diaphragm 5 produced by E and E , . Consequently,

when the net voltage on contact 17 is zero, as may be determined from

an integrating recorder 19 connected thereto, the deflections produced

by E and E , are equal, and hence E, equals the rms of E . X QC QC X

The operation of the voltmeter is thus readily apparent. The

voltage E , of source 6 is varied until the recorder 19 reads zero. The

value of t, is then read and taken as the rms of E . dc x

It is possible that the vibration of driving means 18 will be

mechanically coupled to the diaphragm 5, causing a spurious deflection

thereof unrelated to the voltages E and E , . It is also possible that

an asymmetrical dwell-cycle in the operation of the vibrating switch

contacts 3,_L7 may aive rise to a spurious deflection of the diaphragm 5 even when E^ and E'f are equal. To circumvent these problems, the

polarities ofthe signals applied to the fixed contacts (2, 7 and 15, 16)

should periodically be reversed, whereby the signal due to mechanical

vibrations will be averaged to zero in the integrating recorder 19. A

suitable switch arrangement for performing the polarity reversals is

illustrated in Fig. 2. The frequency of the vibrating means 20 for the

polarity-reversing switches may be fairly low (e. g. , 1 cps) , in which

case the time constant of the integrator in the integrating-recorder 19

should be a minute or so, to provide a good averaging of the reversals,

particularly over an even number of equal reversal intervals.

It will be obvious that, if desired, the output of the recorder 19

may be used as an error signal to control a servo that automatically

sets E , of source 6 equal to the rms of the unknown voltage E .

The instrument can also be used to measure the rms value of a complex repetitive waveform if none of the components of the wave is

harmonically related to the switching frequency or to the reversing

frequency.

Tech. Note No. 287, April 8, 1966

22
SILICON RADIATION DETECTOR MEASURES LOW TEMPERATURES

William R. Dodge

Steve R. Domen

Dale R. Hoppes

Alan T. Hirshfeld

SEH50R.

, "2,

,

INDICATOR.

Dl

/dy

^

:/.

INTEGRAL

DISCRIMINATOR

3l

SCALER

J

76

PULSE

HEIGHT

PULSE

HEIGHT

TEMPERATURE TEMPERATURE

Fij. Tij.; cr.3

Tech. Note No. 287, April 8, 1966

23
SILICON RADIATION DETECTOR MEASURES LOW TEMPERATURES

William R. Dodge

Steve R. Domen

Dale D. Hoppes

Alan T. Hirshfeld

This adaptation of a conventional silicon radiation detector to low

temperature thermometry is based on the discovery that the normal,

radiation-produced electrical pulses from such a detector essentially

shrink by factors of 2-10 when the detector is cooled to temperatures

in the range of about 5° -30° Kelvin.

This discovery is utilized in the apparatus of Fig. 1, wherein a silicon radiation detector 10, of known design, and a small mass 12

207 210

of Bi , Po , or other suitable radioactive material, are mounted in a casing 13 of copper or the like so as to provide a compact low temperature sensor 14. The sensor output pulses, produced by the detector 10

in response to the particles emitted by the radioactive mass 12, are transmitted over a coaxial cable 15 to a remote temperature indicating

circuit 16. There, the pulses are amplified in a suitable pulse amplifier 17; discriminated as to pulse height by a conventional integral discriminator 18; and, if passed by the discriminator 18, counted by a scaler 19 of known design.

Fig. 2 illustrates the manner in which the height of the pulses

produced by the detector 10, in response to 480 KeV K-conversion

electrons emitted from a mass of Bi^O' , vary with temperature. The

portion of the curve between points A and B, which represents a large

change in pulse height for a small change (about 3°) in temperature,

has been found to be independent of the parameters of the indicating

circuit 16, and stable with time. Hence the integral discriminator 18

conveniently is set to pass only those pulses having a height approximately midway between the heights of points A and B.

Fig. 3 is a curve similar to that of Fig. 2, except that the pulse

heights are produced by the alpha particles from a mass of Po^lO,

Again, the curve between points A and B is stable, and can be used to

set the discriminator 18.

Tech. Note No. 287, April 8, 1966

24
From the foregoing, it will be evident that in the operation of the

apparatus of Fig. 1, the scaler 19 stops counting when the temperature

of the sensor 14 goes from point A to point B, and starts when the temperature goes from B to A. Hence the apparatus is well suited for incorporation into a low-temperature-servo system.

The median temperature in the range of temperatures from points

A to B, Figs. 2 and 3, can be designed to fall within a range of temperatures about 15° Kelvin, by varying the type and concentration of impurity in the silicon detector 10 of Fig. 1. Therefore, by providing a series

of detectors having incrementally arranged median temperatures, a multivalued low temperature thermometer of high precision and stability may

readily be obtained.

For further details on the phenomena utilized in the present apparatus, see "The Anomaly in the Response of Semiconductor Detectors"

by W. R. Dodge, et al, IEEE Trans. Nuc. Sci . , 11 , 238 (1964); and,

"Investigation of the Anomaly in the Response of Silicon Semiconductor

Radiation Detectors at Low Temperatures" by "W. R. Dodge et al, IEEE

Trans. Nuc. Sci. , 12, 295 (1965).

Tech. Note No. 287, April 8, 1966

25
HIGH PRESSURE BACK-UP RING FOR O-RINGS

Hobart S. White

10

11 -M

1Z, <*

*

Fig:

15 13

17 r

! ,18

TPicf:

Tech. Note No. 287, April 8, 1966

26
HIGH PRESSURE BACK-UP RING FOR O-RINGS

Hobart S. White

Conventional elastomeric O-rings are widely used to provide fluid-tight

seals. When subjected to varying pressures and temperatures, however,

these rings often extrude and cause seal failure. Such extrusion may

effectively be avoided by the use of the back-up ring shown in Figs. 1-

5.

From Figs. 1 and 2, it will be seen that the back-up ring 10

comprises an annular band 11, which may be brass, stainless steel, or other material of suitable elasticity, strength, and thermal properties,

for use at the pressures and temperatures to be encountered. The inner

circumferential surface 12 of the band 11 is made approximately semicircular, to receive a conventional O-ring. The intersections of the

curved surface 12 with the side walls 13, 14 of the band 11 are raised

relative to the body of the band 11, to provide a pair of flexible circular

sealing portions 15, 16. As shown in the enlarged detailed drawings of

Figs. 3 and 4, the sealing portions 15 and 16 may be formed either by

slightly flaring the semicircular surface 12, or by beveling the side

walls 13, 14 so as to form a small angle A.

In operation of the back-up ring 10, the ring and its associated

elastomeric O-ring 17 (Fig. 5) are disposed between two members 18, 19

forming part of an apparatus designed to contain a compressed fluid,

represented by the arrow 20. In accordance with the usual practice, the

members 18, 19 initially are drawn firmly together as by screws or the

like. The resultant compression of the back-up ring 10 causes the

sealing portions 15, 16 to deform to the contours of the adjacent surfaces

of the members 18, 19, thereby closing any minute crevices through which

the elastomeric O-ring 17 might extrude. The apparatus is then pressurized, which causes the O-ring 17 to press the sealing portions, 15, 16

into extremely firm engagement with the members 18, 19. Subsequently,

if the members 18, 19 separate because of differential strains or thermal

displacements in the clamping means, the flexible sealing portions 15,

16 will follow the separations, and prevent any extrusion of the O-ring

17. Hence the back-up ring 10 tends to seal tighter as the pressure 20

is increased.

Tech. Note No. 287, April 8, 1966

27
While the back-up ring 11 of Figs. 1-5 is circular in outline, it will be apparent to those skilled in the art that the ring could be made

in various other shapes, such as oval, slot-like with long parallel sides

and circular ends, or the like, to seal openings having such shapes.

In an exemplary embodiment of the back-up ring described above,

back-up rings of brass and their associated conventional rubber O-rings

were used to seal size No. 3 gage glasses (about 1 3/8" x 6 1/2" x

11/16" thick) at slotted apertures in a dilatometer housing subjected to

pressures ranging from atmospheric to 30, 000 psi, and temperatures

ranging from -40° to +200° C. These rings have been operated successfully for over a year.

Tech. Note No. 287, April 8, 1966

28

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