National Bureau of Sta <*

Library, E41 Bldg' DEC 1 8 1969

NBS SPEC. PUBL. 260-18

Standard Reference Materials:

CALIBRATION OF NBS SECONDARY STANDARD

MAGNETIC TAPE (COMPUTER AMPLITUDE REFERENCE)

USING THE REFERENCE TAPE AMPLITUDE MEASUREMENT

"PROCESS A"

QC

160

I Com U.S. Department of Commerce

National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901. Today,

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology. The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and

uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering -—Time and Frequency -—Astrophysics -—Cryogenics.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops, produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office

of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry. THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological

standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation—Building Research-—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis. THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation to the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists- of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data—Clearinghouse for' Federal Scientific and Technical Information ' —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted: mailing address Washington. D.C. 20234.

- Located at Boulder. Colorado 80302.

:l Located at 5285 Port Royal Road. Springfield. Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Standard Reference Materials:

Calibration of NBS Secondary Standard

Magnetic Tape (Computer Amplitude Reference)

Using the Reference Tape Amplitude Measurement

"Process A"

Sidney B. Geller, Paul A. Mantek, and Norman G. Cleveland

Information Processing Technology Division Center for Computer Sciences and Technology

National Bureau of Standards Washington, D.C. 20234

J, Si National Bureau of Standards. Special Publication 260-18,

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-18, 41 pages (Nov. 1969) CODEN: XNBSA

Issued November 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 (Order by SD Catalog No. C 13.10:260-18), Price 50 cents
NATIONAL BUREAU OF STANDARDS JAN 1 2 1970
TABLE OF CONTENTS

PAGE

1. INTRODUCTION 1 2. Theory of Operation of the NBS Measurement System . 2 A. System Components 2 B. Description of Channel Operation 3 3. Calibration and Set-up of the Reference Tape

Signal Amplitude Measurement System 7 A. e versus I Saturation Curves 8 o w

B. Signal Amplitude Read Channel Calibration and

Set-up Procedure 9 C. Signal Peak Averaging Channel Calibration and

Set-up Procedure . 10

4. Note to Reference Tape Users H

5. Additional Circuit Descriptions 13

6. Component List 17

7. Acknowledgments 18

8. Appendix A 19 LIST OP FIGURES

FIGURE NO.

1. NBS Measurement System 21

2. Write Channel Block Diagram 22

2a. and 2b. Write Channel Circuit Diagrams .... 23-24

3- Signal Amplitude Read Channel Block Diagram . . 25

3a. to 3e. Signal Amplitude Read Channel Circuit

Diagrams 26-30

4. Signal Peak Averaging Channel Block Diagram . . 31

4a. to 4d. Signal Peak Averaging Channel Circuit

Diagrams 3 --35

5. Signal Amplitude Read Channel Timing Diagram . 36

6. Family of e

Q versus I Saturation Curves for an NBS Secondary Standard Magnetic Tape .... 37

7. Full Length Recording of the Signal Amplitude

and the Average Signal Peak Value on the Same

Tape Used in Figure 6 38

iii
CALIBRATION OF NBS SECONDARY STANDARD MAGNETIC TAPE (COMPUTER AMPLITUDE REFERENCE)

REFERENCE TAPE AMPLITUDE MEASUREMENT "PROCESS A"

Sidney B. Geller, Paul A. Mantek, and Norman G. Cleveland''' Information Processing Technology Division

Center for Computer Sciences and Technology

National Bureau of Standards

Washington, D. C. 20234

ABSTRACT

This publication describes the design and operation of

the NBS signal amplitude measuring system (Process A) that is

used for calibrating unrecorded Secondary Standard Magnetic

Tapes (Computer Amplitude Reference). The signal level calibration is made with respect to a reference signal level derived

from the NBS Master Standard Magnetic Tape (Computer Amplitude

Reference) that is kept in repository at NBS. The techniques

for measuring and recording the data that accompany each

Secondary Standard Magnetic Tape in the form of strip chart

recordings and saturation curves are described.

Key words: Magnetic tape,- computer amplitude reference, un- recorded reference, Secondary Standard, Master Standard,

signal level calibration, average peak signal level, saturation

curves, standard reference materials-

* Research Associate from IBM at the National Bureau of Standards

1967-69. Present address: Magnetic Tape Laboratory, Information

Records Division, IBM, Boulder, Colorado 80302.

IV
I. INTRODUCTION

The characteristics of present day magnetic computer tapes

are the result of long term research and development by the

magnetic tape industry. Technological developments in related

fields and transducer-tape interplay have stimulated continuing improvement of the tape media and their performance. This

has often led to differences in tape characteristics and

marginal operation when tapes are interchanged between systems.

The only tapes previously available for quality assurance purposes have been industry supplied standards.

At the request of industrial users of computer tape,

Government procurement agencies, producers of magnetic computer

tapes, and producers of tape handling equipment the National

Bureau of Standards has developed a tape measurement system and

is supplying unrecorded Secondary Standard Magnetic Tapes

(Computer Amplitude Reference) through the NBS Office of

Standard Reference Materials.

This new material, which is designated as the NBS Standard

Reference Material No. 3200, consists of a 600-foot length of

secondary reference tape accompanied by applicable test and

calibration data, and a description of the equipment and

procedures employed for measurement of the tape. The secondary

tapes themselves are 1/2 inch wide unrecorded magnetic computer

tapes wound on 8-1/2 inch diameter precision reels.

Each tape is calibrated in terms of a 100% signal level

that is derived from the NBS Master Standard Magnetic Tape

(Computer Amplitude Reference) which is kept in repository at NBS. This signal level calibration is done at recording

densities of 200, 556, 800 and 3200 flux reversals per inch

and calibration information In the form of signal output charts

recorded near each edge track and center track accompanies each

secondary tape. A set of saturation curves relating the

reproduce head output voltage (on the first read-aft er-write

pass) to the write current at each bit density of both the NBS

1
master and each secondary tape is also included. The write

current ranges from zero to at least 150% of the saturation

current level.

Presently, the main criteria for the selection of

unrecorded amplitude reference tapes for calibration as secondaries are signal amplitude uniformity and proximity of readafter-write output voltage to that of the master. The instantaneous track-to-track variation in the average signal amplitude

is limited to a maximum of 3% among the outputs from the NBS

test tracks 2, 5 and 8. The location of these tracks is defined

in USASI Document X3 . 22-1967. The maximum variation in the

longitudinal average signal output voltage along each test track

is limited to h% over the 600 foot length of tape. The absolute

value of the average signal amplitude from each test track is within 10% of the NBS Master Standard Magnetic Tape (Computer

Amplitude Reference). These criteria are applied for peak

current recording on degaussed (fully ac erased) tape when

measured on the NBS system in the manner described in this

publication

.

2. Theory of Operation of the FES Measurement System

A. System Components

Figure 1 shows the organization of the system used for

measuring and calibrating the unrecorded NBS Secondary Standard

Magnetic Tapes (Computer Amplitude Reference). The system

consists of a write channel (NRZI mode), a signal amplitude

read channel and a signal peak averaging channel. Various

readout devices and precision voltage supplies are also included

in the system.

The following descriptive material lists the components

that appear in each of the three major channels. One of the

figures that is given with each channel contains the block

diagram of the channel functions and the other figures consist

of the circuit realizations of the block functions. Note that

within most of the individual blocks a figure number is

2
inscribed which identifies the circuit diagram that corresponds

to that block.

1. Write Channel Components (Figures 2, 2a, and 2b):

a. A variable frequency square wave generator G-l whose

frequency ranges from at least 22.5 kHz to 360 kHz,

a pulse counter and a bistable multivibrator circuit

BC-1.

b. A slotted read-write head (IBM Assembly No. 5417960 - Part -No. 2510730), a write head current driver

circuit WD-1 and a write current amplitude control

circuit CC-1.

2. Signal Amplitude Read Channel Components: (Figures 3, 3a

to 3e)

a. A slotted read-write head (IBM Assembly No. 5417960 - Part No. 2510730), a line driver LD-1, a termination

network TE-1 and a preamplifier PR-1.

b. A delay network DLY-1, Inverters 1-1 and 1-2, a post-amplifier PO-1 and peak detector PD-1.

c. A strobe pulse generator SG-1.

d. A sample-and-hold circuit SH-1-

e. An electrometer E-l and a summing amplifier SA-1.

3. Signal Peak Averaging Channel Components: (Figures 4, 4a to 4d)

a. A sample time pulse generator G-2 with power line

frequency control.

b. An inverter 1-3, a linear integrator LI-1, and

electrometer E-2.

c. A sample-and-hold circuit SH-2, an FET-electrometer E-3,

an inverter 1-4 and a summing amplifier SA-2.

4. The tape transport is a modified IBM 729-VI and the readout

devices consist of a 6-track pen recorder, an x-y recorder and an oscilloscope. Precision voltage sources are used for calibration purposes.

B. Description of the Channel Operation.

1. Write Channel Operation: (Figures 2, 2a and 2b)
a. The signal amplitude measurements are performed at all bit densities on candidate tapes that have

been initially degaussed and then recorded with

all "ones" in the NRZI mode.

b. The recording current signal frequency is controlled

by pulse generator G-l whose output pulses are used

to drive a binary flip-flop BC-1 that has been

designed into the IBM 729-VI transport.

c. BC-1 produces a symmetrical output voltage square wave at one half of the input frequency which is

used to gate the write head driver circuit WD-1.

WD-1 is constructed with matched components so that

it produces equal values of write head current in

both directions.

d. The write current controlling circuit CC-1 is used

to vary the absolute current value in WD-1 over a range of from 0 to 45 mA . The write" current value

is determined from the voltage drop across the 100

resistance in the write head center tap (Figure 2b).

2. Signal Amplitude Read Channel Operation: (Figures 3, 3a

to 3e. The timing for the signals described in steps

a through d is shown in Figure 5.)

a. The signal which has been reproduced by a readafter-write operation from a candidate tape is

passed through a unity gain, line driving amplifier LD-1 which is physically located at the

read head. The line Is terminated by network

TE-1 at the input of PR-1.

b. This signal is then passed through preamplifier

PR-1 that has an adjustable gain of approximately

150 to 300. PR-1 is adjusted to produce a 4.0

volt peak-to-peak output signal when the NBS

Master Standard Magnetic Tape (Computer Amplitude

Reference) is used for the electronics calibration procedure.

4
c. This 4.0 volt peak-to-peak signal is then processed

during its transmission to SH-1 and PO-1 as follows

:

(1) The signal is delayed for a time T^ = 0.75

ys and inverted in 1-2. It is then compared to its inverted but non-delayed signal counterpart

from 1-1 as they enter the peak detection PD-1

system. This comparison process locates the

signal peak and at this instant a strobe pulse

is generated in SG-1 and injected into SH-1.

(2) The 4.0 volt peak-to-peak signal is simultaneously delayed by an amount equal to that in

(1) and is passed through the post-amplifierclipper circuit PO-1. PO-1 has an adjustable

gain capability and its output is half wave rectified and adjusted to consist of negative

polarity pulses having 3.0 volt peak amplitudes.

This is the nominal signal level that has been

chosen for the system.

d. SH-1 will use the above strobe pulse in conjunction with the 3.0 volt negative signal to develop

and hold a dc level equal to the peak value of each

successive incoming negative signal peak from PO-1.

The output of SH-1 is sensed with a very high input

impedance, unity gain circuit E-l in order to prevent information loss due to unwanted discharge

of the "hold" capacitor C^.

e. The E-l output voltage is then fed into a summing

amplifier SA-1 which has a dc offset capability

that is controlled with a calibrated potentiometer.

SA-1 can be adjusted by the offset circuit so that

its output will contain a specific percentage of

the total signal range. For example, a range

consisting of from 90 to 110$ of the total signal

amplitude is charted on a recorder by offsetting

5
the lower 90% of the nominal signal value and then

selecting a recorder range setting that is capable

of encompassing only the next 20% of the total

signal level over its chart limits . It was found

useful to convert the output signal amplitude to

a value that yields a direct relationship between

the major recording chart divisions and the percentage value of the signal under test. Note: This same offset capability of SA-1 can be

used to compare relative signal peak values and to measure the distribution of incoming signal peaks

on a high sensitivity range of an oscilloscope.

This measurement, however, does not require prior

peak detection and samp le-and-holding of the signal,

i.e., the signal is passed directly from PR-1 to SA-1. When the oscilloscope sweep rate is set to display only one half or less (positive or negative)

of one signal cycle per trace then a broad arched

band of light appears on the screen. The broadness of the band is a function of the scope

sensitivity and of the distribution of the signal

peak amplitudes read from the tape. The intensity

of the light band varies across its height with the

brightest level appearing at the amplitude related

to the most prevalent signal peak value. Signal

drop-outs have no effect on this measurement and

it is possible to differentiate between signal peak

amplitudes that differed by less than 5 mV out of

3 volts.

3. Signal Peak Averaging Channel Operations: (Figures 4, 4a to

4d)

The output of SH-1 is transmitted to the average peak

voltage measuring system which continuously processes this

signal and returns its average peak value to one of the re- corder tracks. The operation of the averaging system is as

6
a. It has been determined theoretically that if an equispaced pulse train is sampled-and-held and is then

injected into a linear integrator circuit whose RC product is equal to a sampling time T, then the

final output voltage of the linear integrator at each time T is equal to the average peak value of

the train over that sampling interval.

b. The sample-and-hold signal output from SH-1 is

transmitted via E-l and 1-3 to the input of the

linear integrator LI-1 whose RC product has been

set equal to RC = T = 0.270 seconds. The average

peak value of a sample consisting of approximately

50,000 pulses under the recording conditions of

3200 frpi and 112.5 ips tape speed is thus derived by the linear integrator each 0.270 seconds.

c. These average peak values are strobed out each

T seconds and stored in the sample-and-hold circuit

SH-2 while LI-1 is reset preparatory to the next sampling interval.

d. The output of SH-2 is then transmitted to a recorder track via 1-4, SA-2 and an extremely high

input impedance FET E-3 circuit which has its own gain and level setting capability.

e. A pulse generator that uses the 60 Hz power

frequency as its basic clock source controls the

sampling time intervals of the averaging system

(Figure 4a)

.

3. Calibration and Set-up of the Reference Tape

Signal Amplitude Measurement System

The procedures will be discussed in three sections:

e Q versus 1^ saturation curves. Signal amplitude read channel calibration and setup procedure Signal peak averaging channel calibration and setup procedure
It is to be noted in all three sections that the output

voltage from the tape is always obtained during a first readafter-write pass of a previously degaussed tape.

A. e Q versus Iw saturation curves.

1. An x-y recorder is used for producing curves showing the

reference tape read head signal e

Q versus the write head current

I (Figure 6). They are produced from the tape by plotting a wcontinuously varying write current level versus the read head

output voltages that these levels produce during a first readafter-write operation. Individual plots are made at each bit

density. These curves are used to determine the saturation

write current level at each recording bit density in order to set up the system for the amplitude measurements.

2. In order to produce the e

Q versus I curves on the x-y recorder, the full read signal amplitude (zero SA-1 offset

voltage applied) Is made available at the output of summing

amplifier stage SA-1. The gain of this stage is set so that

it converts the nominal 3.0 volt, 100% signal level of the

system to 5.0 volts prior to its insertion into the y axis of

the recorder. The y axis gain is then adjusted so that the

y axis ordinate values are given in terms of the actual peakto-peak read head output in millivolts as measured by an oscilloscope at the output of the line driver LD-1.

3. The drive to the x (current) axis is derived from the

voltage drop across a 101] resistance in the center leg of

the write head. These voltage drops are converted by an x axis

gain adjustment into abscissa values which represent the

write head current in mi lliamperes . The range of applied

write current is 0-45.0 mA and the resulting head output voltage range has been found to range between approximately 12.0

mV at 3200 frpi to 25.0 mV at 556 frpi.

4. If the saturation write current level is considered to be

the first level at which the absolute maximum value of output

read head voltage is encountered, then it has been found at 3200 frpi that the average saturation write current level is

8
15.0 mA and that at 556 frpi the saturation current level is

approximately 35.0 mA. These average current values were derived from the industry tapes that were sampled in this first

program phase. The saturation write current levels are used for

producing the three track strip charts that will now be described.

B. Signal amplitude read channel calibration and set-up procedure (Figure 3).

The first four steps are accomplished while using a "working" reference tape in order to make the initial ccarse adjustments. This avoids any unnecessary wear of the Secondary

Standard Magnetic Tapes:

1. Set the frequency generator G-l to the required bit rate.

2. Set the write head current to the saturation level which

has been obtained by the method described in Section 3, A.

3. Adjust the gain of the preamplifier PR-1 to obtain an initial output signal level of 4.0 volts peak-to-peak.

4. Adjust the gain of the post-amplifier-clipper circuit PO-1

for a 3.0 volt negative peak signal output.

5. Replace the "working" reference with the Secondary Standard

Magnetic Tape and readjust the write current saturation level as in (2). The remainder of the calibration procedure is accom- plished using a sensitive chart recorder. (For example, one with at least a 200 mV/cm resolution.)

6. An accurate calibration voltage source is used to inject

3.0 volts into electrometer E-l. This dc signal then passes

into the summing amplifier SA-1. The gain of SA-1 is set to produce 5.0 volts at its output when the nominal 3.0 volts

input is applied under zero offset conditions. At NBS the

useful strip chart width is 5 cm and typically a total tape

signal range of 20$ {i.e. 90 to 110$) is recorded using the 200

mV/cm scale. Under these conditions an input of from 0.0 to 1.0 volt to the recorder will be linearly distributed across

the chart with 0.0 volts (90$) at the lower edge, 0.50 volts

(100$) at the center and 1.0 volt (110$) at the upper edge. In order to relate the 5.0 volt input signal to these values the

9
dc input to SA-1 is adjusted so as to offset this signal downward by 90$ (4.5 volts). This shifts the 4.5 volt level of the

signal to 0.0 volts and causes the higher voltages to shift to their respective values, i.e. 5-0 volts shifts to 0.50 volts

(100$ level) and 5-5 volts shifts to 1.0 volt (110$ level)-

7. The 3.0 volt calibration signal is now removed and the

Secondary Standard Magnetic Tape is run. The preamplifiers

PR-1 and/or the pos tamplif ier PO-1 are now fine adjusted during

a first read-after-write pass so that the average peak voltage

output from two outer tracks and the center track of the tape

will fall at the 100$ chart level if the tape is a 100$ level

tape. If the tape is not a 100$ level tape but has been precalibrated with the NBS Master Standard Magnetic Tape (Computer

Amplitude Reference), then the adjustments are made to that

precalibrated percentage level.

This completes the calibration of the peak amplitude

measurement portion of the system.

C. Signal peak averaging channel calibration and setup

procedure

.

1. A 3.0 volt dc calibration signal from E-l is fed into the

signal input terminal of the linear integrator LI-1. This is interpreted by LI-1 as being the output of a sample-and-hold

circuit whose input consists of a train of equal amplitude pulses

each having the peak value of 3.0 volts. Since the average peak

value of the dc signal is also equal to 3.0 volts, LI-1 is adjusted (with the variable R; C is fixed in this system) so that its instantaneous voltage level at each strobe time T

(= ,270 s) is also equal to exactly 3.0 volts. This is done by

using a 2 channel oscilloscope and adjusting the sawtooth output

of LI-1 (which is the result of the linear integration of the

dc input signal) so that it just equals the input level at each

time T. When the adjustment is correctly made then RC = T and

the sawtooth voltage peak value will continue to equal the input

dc level at each time T as the input voltage is varied over a

1 See Section 4,D.

10
range of from approximately 0.5 V to 4.5 V.

2. When the calibration of the linear integrator is completed,

a 1.0 volt dc signal is injected into LI-1 and the output

voltage of the FET-electrometer E-3 is measured. A coarse adjustment of the voltage level control in E-3 is made to bring

its output voltage to approximately 1.0 volt. The dc calibration voltage from E-l is now increased to 4.0 volts and the

output of E-3 is measured. The "gain" control of E-3 is

adjusted to bring its output to within 10 mV of the input voltage. All of the preceding steps in (2) are repeated until E-3

displays unity gain over a range of approximately 1.0 V to 4.0 V

After this has been completed, the E-3 voltage level control

may be finely adjusted so as to set both ends of the voltage

range to their exact absolute value.

3. The output from E-3 is passed through inverter 1-4 and SA-2

SA-2 has an adjustment that offsets the average peak signal so that its 100% level coincides with the 100% chart position in

the same manner as described for the peak signal in 3,B,6.

The calibration of the system is now complete.

i| . Note to Reference Tape Users

A. Important : The NBS Secondary Standard Magnetic Tapes

should be used sparingly so as to minimize physical changes to the tape and to assure continued performances as near to the

supplied calibrations as possible.

B. A family of e^ versus I saturation curves (with bit

o w density as the parameter, see Figure 6) will accompany each

NBS Secondary Standard Magnetic Tape (Computer Amplitude

Reference) from which it is derived. In addition, the family

of e versus I saturation curves that was obtained from the o w NBS Master Standard Magnetic Tape (Computer Amplitude Reference)

will also be included in the package. The user will therefore

be able to relate the percentage variations in output at different write current levels between the NBS Master Standard

11
and the NBS Secondary Standard Reference Tapes

.

C. Strip charts derived from 3 head tracks at each bit density

will also accompany the NBS Secondary Standard Magnetic Tape

from which they were derived. Data from each read head will be

recorded on 2 adjacent pen tracks with the signal peak amplitude

appearing on the lower track and the average peak amplitude of

the signal appearing on the upper pen track. Figure 7 shows

tracks B and C as they appear on a typical strip chart recording.

The saturation write current level that was used when

making these chart recordings is indicated on each e

Q versus

I curve by a dark mark at the peak value. The set of curves shown in Figure 6 are in fact the saturation curves derived

from track Number 5 of the same reference tape used for Figure 7. D. 1. Chart track A corresponds to head track Number 2. 2. Chart track B corresponds to head track Number 5.

3. Chart track C corresponds to head track Number 8. The track numbers are in accordance with USA Standard

Recorded Magnetic Tape for Information Interchange (800 CPI,

NRZI) - USASIX3 . 22-1967 . E. The curves shown in Figures 6 and 7 are derived under

the following test conditions:

1. The tape is bulk ac erased before writing.

2. The dc erase head on the tape transport is not energized

during the measurement process.

3. A slotted read-write head (IBM Assembly No. 5^17960

Part No. 2510730) is used as the transducer.

4. The curves are made during a first read-aft er-write pass.

These are recommended NBS measurement procedures and have

led to consistent and repeatable results.

F. Purchasers of the first groups of Secondary Standard

Magnetic Tapes (Computer Amplitude Reference) will find that

the average signal peak amplitude tracks on the accompanying

strip chart recordings will consist of sequentially sampled

data. This is due to the fact that only one averaging system

existed at the time that these tapes were calibrated. This

12
single averager was commutated from track-to-track and produced an output in the form of pulses of approximately 2.5

seconds duration.

G . PR-1, LD-1, BC-1 and WD-1 are commercially available

units that are part of the test transport read/write electronics

system

.

H. Systems are being developed that are capable of resolving the higher frequency signal components that will exist

on the future types of magnetic tapes.

5. Additional Circuit Descriptions

I. Write Circuits: (Figures 2a and 2b; also Section 2,B,1)

a. The pulse frequencies supplied by generator G-l

when making the signal amplitude tests are as follows

:

200 frpi 22.5 kHz; 556 frpi 62.55 kHz; 800 frpi 90.0 kHz

and 3200 frpi 360 kHz. These repetition rates are then halved by the trigger mode operation of BC-1.

b. The output from BC-1 (Figure 2a) is clipped in the

collector circuit of T^ by the to D-^q diode network. This results in a square wave signal having

an amplitude of + 1.4 volts that is injected into

the Tg - differential current mode switching

circuit (Figure 2b). The circuit containing Tg and

T^ forms a continuously variable current source for

the switch. The current is controlled by the T^q -

- T.^2 Darlington type circuit CC-1 over an adjustable range of 0 - 45.0 mA . This source current

is controlled and switched between Tg and T^ by the

+ 1.4 volt signal input to the base of Tg and passes

through the write head in a direction depending upon

the instantaneous "on" and "off" states of Tg and T^.

The circuit components in both halves of the head

driver circuit WD-1 are matched in order to produce

equal values of write current in both directions.

13
Signal Amplitude Read Channel Circuits: (Figures 3a-3e;

also Section 2 ,B ,2)

a. The output signal from the read head (Figure 3a)

passes through the line driving circuit consisting

of T^^ and T

i4 • The head cable is terminated by

network TE-1 in an impedance of approximately Q6Q . The signal output from PR-1 drives 1-1 and DLY-1

(Figure 3b). Delay line DLY-1 is a lumped L-C line

with a delay of 0.25ys per section.

The direct signal and its delayed signal counterpart from 1-1 and 1-2, respectively, are injected

into the peak detector PD-1 which contains a commercially available integrated circuit high speed

differential comparator IC-1. The comparator

functions in the peak detection process by switching its states within 2.0 mV after the occurrence

of each of the near to the peak crossover points

that occur between the direct and delayed signals

during each operating cycle. This switching

action produces an output pulse from T-^ that

triggers strobe generator SG-1. SG-1 (Figure 3c)

which is a monostable multivibrator then produces an output pulse with an adjustable width range of 0.25 - 1.2 us. Typically a pulse width of 0.4ys is used

in the NBS measurement system. The position of the

strobe pulse from SG-1 is adjusted to coincide with

the peak of the output signal from PO-1 by varying the

gain of 1-1 (Figure 3b). The strobe pulse output

from T^ and the delayed signal component from PO-1

are then injected into the sample-and-hold circuit

SH-1.

SH-1 (Figure 3d). SH-1 uses the bilateral switching properties of the FET transistor T

2 g to achieve

sample-and-hold operation. The strobe pulse into

the base of T^^ is regenerated by the T

2 g - T^y

14
current switch and its output is applied to the

input gate (G) of T

2g . When the negative "off" gate

voltage rises towards the zero volt level T^q turns

"on" and begins the sampling process. During this

time capacitor charges or discharges via T

2 g depending upon the relative magnitude of the input

signal on the source (S) terminal and the voltage

on which is tied to the drain (D) terminal.

When the gate voltage returns towards -12.0

volts T

2

g is turned off and C

1 "holds" its final

voltage level until the next sampling interval.

The very high impedance of the FET in its "off"

state prevents discharge of the capacitor voltage

through that device and E-l prevents discharge

in the other direction.

d. The dc offset voltage supply for SA-1 (Figure 3e) uses a Zener diode DZ-1 as a stable voltage source.

R^ and are adjusted to set the upper and lower

offset voltage levels across the precision 10 turn

potentiometer R^. Exact offset voltage values

and chart recorder percentages can then be related

to the potentiometer vernier dial settings.

The operation of the remainder of the read system is described in Section 2 } B,2,e.

3. Signal Peak Averaging Circuit: (Figures 4a-4d; also

Section 2,B,3)

a. Gating Pulse Generator Section (Figure 4a). This

generator uses the 60 Hz power line voltage directly

and converts the input sine waves to fast rising

pulses with TD-1, T^q and T^. The tunnel diode

TD-1 in Figure 4a is loaded by the base-to-emitter

junction of transistor T^q* The positive peak of

the input 60 Hz voltage excursion from the transformer secondary causes the base-emitter load line

to shift so as to switch TD-1 into its low voltage

15
stable state at fractional microsecond speeds.

Is now held in its off state. When the input

signal changes to its negative polarity peak, the

tunnel diode will then switch to its high voltage

state and turns on. The voltage variations

across the collector resistance of causes

to vary its collector current accordingly. The

output voltage from T^ is clamped and shaped by

the diode network in its collector circuit.

Emitter follower T^

2 drives the divide by 16 binary

counter IC-2 to IC-5 with square waves whose

positive excursions occur each 16.7 ms . The out- put of the counter consists of square waves whose

positive excursions occur every 267.2 ms . It is simple to increase the number of pulses averaged

per sample by adding integrated circuits to the

counter thereby dividing further down. The output

from T^^ triggers three similar one-shot (OS)

circuits (only OS-1 is shown in detail in Figure 4b).

The output pulses from the three one-shots are then

fed via ^37 J an<^ ^38 ^ n^° their respective

identical linear integrators LI-1 and sample-hold

SH-2 circuits. Only the circuits driven by T^g are shown in this report.

Gated Linear Integrator Circuit LI-1 (Figure 4c).

LI-1 produces an output that is the integral of

the difference in input voltage between the

positive and negative terminals of OA-7. In this

circuit configuration, the integrator output voltage

appears across capacitor which has one side

grounded. This allows to be discharged by

transistor T^ at the desired sample time intervals.

The signal integration process continues for

T = 0.27 seconds which is the time that the input

16
gating pulse to the base of is at its upper

voltage level. At the instant that the input pulse

drops to its lower level (-4.0 volts) it gates

into its "on" state which discharges C" 2 back

towards zero volts and the integration process is

repeated. The output from is fed to the

signal input (Point G) of SH-2 (Figure 4d) via

the high input impedance circuit E-2. The sampleand-hold voltage across the capacitor is

prevented from leaking during the hold period by

the very high impedance T^-0A-9 circuit. The

operation of the remainder of the circuit con- sisting of and SA-2 is the same as described

in Section 2,B,2,e.

6 . Component List

The mention of specific equipments or components is not

to be construed as an endorsement of these items by NBS to the exclusion of other equivalent devices . There is con- siderable latitude particularly in the choice of semiconductor devices such as transistors and diodes. The

following is a tabulation of the semiconductor devices that were used in the NBS measurement system in each channel.

Only "the components of pulse generator G-2 (Figure 4a) are common to all channels.

Transistor Types

2N3646

2N3638

^Unmarked

2N4222A

2N1700

2N976

T (Number in Figs. ) 10,18,20,29

19,22-27,31-42

1-9,13-17

28,43,44

11,12,21

30

Remarks

NPN-Switching

PNP-Switching

See Footnote

N-Channel FET NPN-Switching

PNP-High-speed

Switching

17
Diode Types

1N916

1N276

^Unmarked

1N1766A

WX822B

D (Number in Figs .

)

12-14,19,20,27-30,

32-35,37-40

17,18,21-26,31,36,41

1-11,15,16

DZ-1

TD-1

Remarks

Si Switch

Ge Switch

See Footnote

Zener Diode

Tunnel Diode (5mA)

*The devices designated as "unmarked" are located in the com- mercially available PR~1, Wd-1, BC-1 and LD-1 circuits. It was found that the NPN and PNP transistors are replaceable with

2N3646's and 2N3638 T s respectively in the PR-1, BC-1 and

LD-1 circuits. In the WD-1 circuit T^ and T can be replaced

by 2N3831's and T fi and T n can be replaced with 2N3646's.

Integrated Circuit

Types

Type 710

MC358

Number in Figures

IC-1

IC-2 to IC-5

Remarks

High Speed Differential Comparator

(For example yA710C

Fairchild Semiconductor)

J-K Flip-Flop

(Motorola Semiconductor

Products , Inc .

)

Operational Amplifier

PP45U

Number in Figures

OA-1 to 0A-11

Remarks

Philbrick Researches

Inc .

7. Acknowledgments

The assistance of Mr. A. ¥. Ericson (Information Processing Technology Division) with the construction, testing and

operation of the measurement system is gratefully acknowledged.

The assistance of Mrs. R. E. Davis (Information Processing

Technology Division) and Mrs. R. S. Maddock (Analytical

Chemistry Division) with the preparation of the report is also

gratefully acknowledged.

18
8 • Appendix A The design of the signal peak averaging system is based

upon the following mathematical considerations:

The average signal peak amplitude, e , , is defined

p e 3.K as the sum of the pulse peak amplitudes, e^, divided by the

number, n, of such pulses which are in the interval, T 5 being averaged, i.e.,

e , = - t e. (1) peak n 1

The passage of the signal pulse train through a peak

sample-and-hold circuit (SH-1) converts each incoming pulse

into a corresponding voltage step, E, whose amplitude is

equal to the signal peak e and whose duration is At. The

increase in output voltage from the linear integrator LI-1

over each At due to its integration of each incoming voltage

step, for example say the ith pulse, is

i r 1At

%1 = «C J (i-l) At Ei " ^

Since is a constant amplitude over At, therefore the

voltage output from LI-1 increases during the ith interval

by

E. At

e oi

= -i (3) RC

Since the amplitude of each E^ is by definition equal

to each e^ (where, i = 1, 2, *n) we can after inverting

Equation 3 substitute it into Equation 1 to get

19
where T = nAt the total sampling interval.

Equation 4 shows that when the RC product of the elements

of the linear integrator is adjusted to be exactly equal to

RC the sampling time, that is, when '— = 1 , then the final

integrator output at the end of each sampling time, T, is

exactly equal to the average peak amplitude of the n pulses

which occur within the sampling interval. Generally,

Equation 4 shows that the average peak value of a train of

equi-spaced pulses that have been peak sampled- and-held and

then linearly integrated over a sampling time, T, is proportional to the output of the linear integrator system by the

factor jj^-. In summary, the measurement of the average signal peak

amplitude as defined by Equation 1 is accomplished through

the solution of Equation 4 which Is based on linear

integration and sample-and-hold operations.

20
tfPLITUDE HAN N

EL

ro

PEAK i

CHANNEL

SIGNAL

Ar

READ

CI

(Fig >

SIGNAL

AVERAGING

(Fig.

/

itITE

HEAD

3E

TRANSPC

<

HAN N

EL

READ-WF

MAGNETIC

TAf

WRITE

CI

(Fig

CD

00

pq

Pi.

21
22

24
<<

UJ UJ

CC I

25
— . jQ

, , o 21

26
27
28

30
trUJ— <\J Zj < ST

a.

Fig. <

31
32
33

35
Figure 5. Signal Amplitude Read Channel Timing Diagram.

36

' j —?«—

T*. c

1

-j——|

\to+—itoz.

j _i—j —|

j00

i

j iH

i

1

j [

1 1 1 i i ]

' i !'

i :" 1 '" 1

J J j .! _!_ j

j <)0—

j

. . . . h

-|-|—{—

Figure 7. Full Length Recording of the Signal Amplitude and

the Average Signal Peak Value on the Same Tape Used

in Figure 6. 38

I
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