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^m VI o NBS TECHNICAL NOTE 691

*^C4U of S NBS

Pubii - cations

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Clock Synchronization

and Comparison:

Problems, Techniques

and Hardware

QC

100

,U575

NO

1976

691
NATIONAL BUREAU OF STANDARDS

The 'National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to.strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

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- Located at Boulder. Colorado 80302.
i onaf Bureau . Standard Clock Synchronization and Comparison:

Problems, Techniques and Hardware

Mii

L

-t C^CJr

(= ^\

3 1 mi

James E. Gray

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

«** 0F c

j&& (

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward 0. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued November 1976
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 691

Nat. Bur. Stand. (U.S.), Tech Note 691, 12 pages (November 1976)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC. 20402

(Order by SD Catalog No. C13. 46:691). Price 35 cents (Add 25 percent additional for other than U.S. mailinj
CLOCK SYNCHRONIZATION AND COMPARISON:

PROBLEMS, TECHNIQUES AND HARDWARE

James E. Gray

This report summarizes a lecture-discussion of practical problems in measurement

electronics as experienced in a laboratory for precise timekeeping. It includes ' environmental problems, instrumentation problems, procedural problems and ends with

a description of a convenient automatic measuring instrument, the chronograph. KEY WORDS: Chronograph; frequency-stability measurement; instrumentation;

laboratory technique : time measurement.

INTRODUCTION

Experiments are planned, using idealized models of the hardware to be used, and

one of the surprises of laboratory work is the occasional discovery that the

behavior of some instrument is neither so simple nor so ideal as it was supposed

to be. I shall be discussing this class of problems - problems which could have

been foreseen and avoided, but which sometimes are not. The assortment of instruments used in a precise time laboratory presents an assortment of problems. I have grouped the ones I shall discuss in three categories: environmental problems,

instrumentation problems and procedural problems. Finally, I shall describe a convenient automatic measuring instrument, the chronograph.

I . Environmental Problems

A. Time is often kept by digitally counting the cycles of an oscillator

to produce a tick once per second. Occasionally the digital divider

will count some cycles which did not occur in the oscillator - the

clock jumps. In this laboratory, I have most often found this to be

due to electrical noise potentials existing between the "grounds" in

different relay racks. Most of our signals are communicated over

RG-55 cables, grounded at both ends. Although this system is con- venient when using commercial instruments with grounded BNC jacks on

their panels, it does have the disadvantage that ground noise potentials

and their associated currents are in the signal circuits. Several

remedies might be considered. For example, a bifilar choke, a length

of coaxial cable wound on a transformer core, can be placed at the

input of the divider. Depending upon the location of the noise source,

this may cause the noise current to take some alternative route. In

practice, I have found this method rather ineffective if used

alone. In my experience, a heavy copper braid connected between

racks has been most effective. Passing the oscillator

signal through an isolation transformer should also help, though I have not yet tried it, because ground straps have been adequate.

B. In a laboratory that uses the short-term stability of a cesium frequency standard, a surprising amount of isolation may be needed.

Suppose that a 5 MHz sine wave signal is stable to a part in 10 at one second. 10 seconds of phase shift is 314 micro-radians of phase

shift. Thus an admixture of 8 parts in 10 of some other 5 MHz

signal, will cause phase shifts in the desired signal which are comparable to the stability being utilized. That requires an isolation
between 5 MHz signals of 70 dB or better. Many laboratory devices

do not meet that specification. Isolation amplifiers, dividers, and even cables laid parallel in cable races may provide more cross coupling than can be tolerated in some high-precision measurements.

C. Temperature changes may alter the phase delay of a divider. We have

some old dividers with slow rise-time circuits and ripple-carry design

which move 20 nanoseconds from day to day. Some of our newer dividers

have a temperature coefficient of about 0.05 nanosecond per degree,

estimated from the circuit. There seems to be a variety of circuits

on the market; look at the specifications before choosing one.

II . Instrumentation Problems

A. Generation of Ticks. The commercial dividers that I have recently

seen for generating precise one-second ticks generally use "window"

or re-clocked circuits to avoid propagation delay through the dividers,

but I still find one-shot multivibrators used to provide an adjustable

delay. Such convenience is purchased at the cost of added jitter and

temperature sensitivity. One popular clock is specified at 5 nano- seconds jitter. An alternative to using ticks, is to measure the time

of zero crossing of the sine wave coming from the oscillator. A

divider is still needed to identify the correct zero crossing, but

measurements can be made on the sine wave directly. Typical errors

in setting the trigger levels of the counter used to measure the time

of zero crossing can introduce errors of the order of 1 nanosecond at

5 MHz, 7 nanoseconds at 1 MHz and 66 nanoseconds at 100 kHz. These

estimates assume 1.5 volt peak sine wave amplitude and trigger level

set using a ± 2.5% meter on its 2.5 volt scale.

B. Impedance Problems. The sine wave from an oscillator is often carried

to the site of measurement by RG 55 cable. In addition to cross talk

mentioned above, consider impedance matching of these cables. Any

mismatch along such a cable will cause a reflection. The reflected

sine wave adds to the original one to produce a resultant with altered

phase. Although it is not necessary to maintain a perfect match along

such cables, it is necessary to remember that any impedance change is

capable of shifting the phase being measured. In addition to the effects

of reflected waves on the cables, load impedance changes affect the

phase of the output from some oscillators. This load sensitivity is

large enough to be important in many applications. For example, I observed the following phase shifts when I connected an additional load

in parallel with a 50 Q resistive load on the 5 MHz output of a commercial cesium frequency standard:
Additional Load Phase Shift (nanoseconds)

100£3 + 2.6

47ft +4.8

24ft + 7.8

330pf -13.3

680pf -21.2

lOOOpf -27.6

Phase shift with load is not one of the specified characteristics of

these frequency standards, so that the effect is probably different

for other instruments, but these observations suggest the approximate

magnitude of the problem. 47 ft might represent an additional 50 ft cable paralleled on the output of the oscillator; 680 pf might

represent about 5 meters of 50 ft cable terminating in a high impedance

device

.

Similar considerations apply to cables carrying pulses. For example,

we observed erratic results when the arrival times of pulses were measured with a time-interval counter after they had travelled between

buildings via underground cable. There appeared to be a discrete

error. Sometimes the pulses were late; when they were, the delay was always the same. An oscilloscope paralleled with the counter showed

the rising edge of the pulse as in figure 1. Slight errors in setting

the trigger level of the counter resulted in some measurements being

taken above the step, an error. Investigation showed that someone had

run a few meters of cable from the source of each tick to an open jack on a nearby patch panel for trouble shooting use. This was in addition

to the long, underground cable. The two cables presented an initial

load of 25 ft to the source which expected 50 ft. Thus the pulse only

rose part way. Later, when the pulse returned to its source after

reflection from the open jack, the voltage at the source rose to its

full height. The resulting stepped rise was causing an intermittent

error equal to the round-trip transit time for the pulse on that few

meters of open-ended cable to the patch panel (about 10 nanoseconds

per meter of cable)

.

FIG. 1 Stepped rise due to cable reflections.

3
Internal Errors of Instruments. The errors of time interval counters

appear to be rather well described in the counter specifications.

Resolution and trigger errors of the order of a nanosecond are available. Other errors that depend upon the length of the interval being

measured, can also be of the order of a nanosecond for intervals of

about one second. These errors include time-base accuracy and interpolator accuracy. In using such counters for precision measurements,

one usually needs to use a matched load at the end of the cable to

avoid ringing which can trouble the trigger circuits of the counter.

The chronograph is a less well-known method of time keeping which

deserves to be mentioned because of its ability to measure the relative

phases of the oscillators in an ensemble with high precision. The

chronograph is an instrument which mixes the sine-wave signals from

pairs of radio frequency oscillators and records the times of zero crossing of the resulting low-frequency beat. Although the chronograph was developed for other purposes, it can be used to take the

data needed to construct a time scale with an ensemble of oscillators.

For example, the following procedure might be used.

All the five-megahertz oscillators, except number one, are set

low in frequency by one part in 10 so that each will have a 20 second

beat period when mixed with number one. Oscillator number one is

used to drive a 1/20 pps divider. The ticks of that divider are used

to start a time interval counter. The zero crossing of the beat

between oscillator one and oscillator i is used to stop the counter.

Call the measured time interval B.

.

i If all the oscillators started in phase with one another and were noiseless and on frequency, then all the zero crossings would coincide

with the ticks of the 1/20 pps divider. In fact, they occur at other

times as measured by B.. Thus B. represents the accumulated phase error between oscillator one and oscillator i. From another point of

view, B. represents the phase error of oscillator one as measured by

oscillator i. From this data (the B.'s), a time scale computation

can be made which will give a value for the error of oscillator one

as measured by the ensemble of oscillators. Oscillator one can also be

used to drive conventional hours, minutes, seconds displays and put

out 1 pps ticks if desired.

The random noise and frequency offsets of commercial cesium beam

frequency standards in such an ensemble are small enough so that there

would generally be a period of several days before the phase of an oscillator has changed by one cycle. Thus, daily measurement would

be sufficient to keep track of all the oscillators even if only

number one had a divider.
Suppose that the time-interval counter had a resolution of 0.1

microsecond so that the phase comparisons were made with a resolution

9 of 0.1 microsecond out of 20 seconds or five parts in 10 . This is a resolution of one femto second in the phase at 5 MHz. Using commercial

double-balanced mixers and operational amplifiers, the zero crossings

are probably not detectable with this precision, but a resolution of one picosecond seems reasonable.

Ill . Procedural Problems

A. Redundancy. Useful protection can be achieved against the effects of

human errors and equipment failures by redundant measurements. In a timekeeping laboratory it is useful to have the relative phase of each signal

recorded at several different reference planes, each under fully

specified conditions of measurement. At times of equipment failure

and repair, the phase history of an oscillator after the equipment

repair can be related to that before, because not all of the reference

planes will be affected by the failure and repair work. For example,

on a portable clock trip, if the divider fails or jumps, the data

can still be obtained by measurement of the times of zero crossing

of the sine-wave outputs of the clock, providing that tick-to-phase

time intervals are known for that portable clock. The time of zero crossing of the 5 MHz output can be measured with a precision of about

one nanosecond and the zero crossings of the 100 kHz output are far

enough apart to make it reasonably easy to identify the correct cycle.

A second example: when we placed in service a new pulse cable

from one laboratory to another, we calibrated its delay by carrying

a portable quartz clock from one end to the other. But we made a redundant measurement by observing the return of the pulse, reflected

from the open-circuited end of the cable. I paralleled an oscilloscope across the sending end of the cable and saw something like

figure 2a, a completely unexpected result.

ITLn

ly SECOND

_n

ly SECOND

FIG. 2 Voltage at sending end of a pulse cable

a) faulty connector

b) open-circuit load

c) short-circuit load

d) matched load
Investigating the find the cause, we found a badly installed connector

at the sending end, which did not contact the outer conductor. When

we replaced the connector, the oscilloscope trace looked like figure

2b. A short circuit at the far end gave figure 2c, and a matched load

gave figure 2d. 2b, 2c, and 2d are just what I would expect to observe.

The redundant measurement not only gave us an independent measurement of the

cable delav. but also permitted us to correct a fault before it caused troublt

B. Rate Effect. A time-interval counter is often thought of as a device

that measures the instantaneous phase relationship between two

oscillators. Actually it compares the phase of one oscillator at the

instant it starts counting with the phase of the other oscillator at

the instant it stops. Thus the measured value includes any phase

change between the two oscillators during the measurement interval.

That phase change is often negligable but in some circumstances it can cause trouble if overlooked. If the oscillators are offset 300

parts in 10 , and the measurement interval is one second, the error will be 30 nanoseconds.

C. Remeasurement error. In constructing our time scale, we measure the

time of each clock daily, using a time-interval counter with a resolution of one nanosecond. If the counter makes an error of one- half nanosecond every day, by the end of the year, a few tenths of a microsecond error will have accumulated. This is an acceptable error

in our application, but if we were to measure once every hour rather

than once a day the remeasurement error would be 24 times as great

and no longer acceptable.

D. Portable clock trip. A portable clock is also a non-ideal device.

Many of the imperfections of portable clocks are unpredictable and

merely set limits to the precision of the time transfer. An interesting exception is a rubidium clock developed at NBS . Its frequency is

a known function of barometric pressure. Even an approximate record

of barometric pressure during the trip will permit adequate corrections

to the time of such a clock. For example, this clock was recently

taken from NBS Boulder to the United States Naval Observatory and

gave a result within 0.1 microsecond of that obtained a few days

later with a cesium clock. The approximate barometric record is

shown in figure 3. At Boulder pressure and aircraft cabin pressure

which happened to be the same, the clock went late 0.78 microseconds

and at sea level pressure, it went late another 0.27 microseconds as calculated from the data shown in figure 3. This total correction of

1.05 microseconds was applied as a clock correction. Assuming a time

of descent of pressure of one-half hour, the error due to the un- certainty in the shape of the curve during that period is of the order

of one hundredth of a microsecond and can be neglected.

6
PRESSURE,

PASCALS

0.79E5

BOULDER

1E5

1 .96E4

TRAVEL TIME, SECONDS

CLOCK

RATE

5.7E-11

USNO

3.9E-1 1 2.06E4

FIG. 3 Barometric record of portable clock trip.

IV. The Chronograph

A. Specifications

The chronograph is a versatile measuring instrument designed for time and

frequency measurements. It mixes the sine-wave signals from pairs of

radio frequency oscillators and records the times of zero crossing of the

resulting low frequency beat. From the recorded data, relative frequencies

and stabilities of oscillators can be computed for various sample times, t,

without dead time. I am building a new chronograph to the following

performance estimates:

1)

2)

3)

4)

5)

6)

Ten channels to permit simultaneous measurements on ten pairs of

oscillators

Input 1 volt nominal, broadband from 1 MHz through 60 MHz.

Averaging times x > one millisecond.

Scale factors N = 1, 16, 256, 8192.

(The chronograph will record every Nth zero crossing)

Measurement bandwidths 2 Hz , 20 Hz, 200 Hz, 1800 Hz.

Noise floor a (1 second) 1 x 10 ik

B. General Configuration

Figure 4 shows the overall structure of the chronoqraph.

20 MHz &

1

1 1 SYNCHRONIZING PULSE

MODULE

1 MODULE

2 ETC.

DATE CLOCK, TIME BASE

& MOST SIGNIFICANT BITS

' ' 1

1 '

'

NBS BUS

THREE

FLOPPY

DISCS

MINICOMPUTER

COMPUTER

TERMINAL

FIGURE 4. Chronograph Configuration

7
A group of independent modules plug into a computer-access bus. Each

module accepts the signals from one pair of oscillators and feeds the

time of phase-crossing to the computer. Completely independent modules

facilitate maintenance and permit the use of special purpose modules

for unusual experiments. The data is stored on three floppy discs, one

to receive incoming data, one to permit examination of data taken in

the past (without interupting current measurements) and a third disc

for overflow. The NBS bus is a computer interface for which many

instrument interfaces are available. Consequently, temperature sensors,

etc., can be connected to the bus to permit the measurement of the

environmental sensitivity of oscillators.

The minicomputer supervises data acquisition and in its spare time,

reduces the data to yield convenient measures such as Allan variances.

It controls the modules, setting their bandwidth, scale factors, etc.

This permits the experimenter to adjust his measurement parameters

directly from the keyboard or indirectly under programmed control.

Module Operation

Figure 5 is a block diagram of a typical module.

ISOLATION

AMPLIFIERS

30-kqr

LEVEL

SENSE

0—Wm~\

30-kll

LEVEL | _

SENSE

BANDWIDTH

CONTROL

m

n

9>—667

COMPARATOR

/£"

MULTIPLE ZERO

CROSSING

SUPPRESSOR

20 MHz -—^ COUNTER!

SCALER LATCH

TO NBS

BUS

FIGURE 5. Chronograph Module

The two incoming signals are suitably isolated to protect them from

contaminating each other by cross-modulation or ground loops and are mixed to produce the low- frequency beat. Zero-crossings of this beat

are detected and counted by the scaler. On the Nth zero crossing, the

reading of the time-base counter is strobed into the latch for transfer

to the computer.

8
In addition to the standard mode of operation described above, there are

two special modes. In the slave mode, the chronograph does not record

every zero crossing but rather it records the time of the next zero crossing after it is armed by a command from the computer. It also

records how many zero crossings have occurred since the last one that was recorded. This effectively compares the relative phases of the

oscillators at a time chosen by the experimenter. In the double mixer

mode, each of the two oscillators to be compared is fed to a different

chronograph module, and the remaining two input ports are driven by a common oscillator. The computer then takes the difference between the

times of phase-crossing in the two modules. This subtraction eliminates

most of the noise introduced by the common oscillator. In this mode,

oscillators can be compared even though they are on the same frequency

and therefore cannot beat with one another directly.
IBS-1 1 4A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUHLK ATION OR REPORT NO.

NBS-TN-691

2. Gov't Accession

No.

3. Recipient's Accession No.

I. TITLE AND SUBTITLE

Clock Synchronization and Comparison:

Problems, Techniques and Hardware

5. Publication Date November 1976

6. Performing Organization Code

277.04

\ AUTHOR(S) _ . James E. Gray

8. Performing Organ. Report No.

K PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

10. Project/Task/Work Unit No. 2774246

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

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5. SUPPLEMENTARY NOTES

6. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report summarizes a lecture-discussion of practical problems in measurement

electronics as experienced in a laboratory for precise timekeeping. It includes

environment problems, instrumentation problems, procedural problems, and ends

with a description of a convenient automatic measuring instrument, the chronograph

7. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Chronograph; frequency-stability measurement; instrumentation; laboratory technique;

time measurement.

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