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Chapter 25
Test Procedures
This chapter, written by ARRL Techni-
cal Advisor Doug Millar, K6JEY, covers
the test equipment and measurement tech-
niques common to Amateur Radio. With
the increasing complexity of amateur
equipment and the availability of sophisti-
cated test equipment, measurement and test
procedures have also become more com-
plex. There was a time when a simple
bakelite cased volt-ohm meter (VOM)
could solve most problems. With the ad-
vent of modern circuits that use advanced
digital techniques, precise readouts and
higher frequencies, test requirements and
equipment have changed. In addition to the
test procedures in this chapter, other test
procedures appear in Chapters 10 and 11.
object. If a merchant or scientist wanted to
know what his pound weighed, he sent it
to a laboratory where it was compared to
the official pound. This system worked
well for a long time, until the handling of
the standard pound removed enough mol-
ecules so that its weight changed and mea-
surements that compared in the past no
longer did so.
Of course, many such measurements
depended on an accurate value for the
force of gravity. This grew more difficult
with time because the outside environ-
ment— such things as a truck going by in
the street — could throw the whole proce-
dure off. As a result, scientists switched to
physical constants for the determination
of values. As an example, a meter was
defined as a stated fraction of the circum-
ference of the Earth over the poles.
Generally, each country has an office
that is in charge of maintaining the integ-
rity of the standards of measurement and
is responsible for helping to get those stan-
dards into the field. In the United States
that office is the National Institute of Stan-
dards and Testing (NIST), formerly the
National Bureau of Standards. The NIST
decides what the volt and other basic units
should be and coordinates those units with
other countries. For a modest fee, NIST
will compare its volt against a submitted
sample and report the accuracy of the
sample. In fact, special batteries arrive
there each day to be certified and returned
so laboratories and industry can verify that
their test equipment really does mean
1.527 V when it says so.
Basic Units: Frequency and Time
Frequency and time are the most basic
units for many purposes and the ones known
to the best accuracy. The formula for con-
verting one to the other is to divide the
known value into 1. Thus the time to com-
plete a single cycle at 1 MHz = 0.000001 s.
The history of the accuracy of time keep-
ing, of course, begins with the clock.
Wooden clocks, water clocks and mechani-
cal clocks were ancestors to our current
standard: the electronic clock based on fre-
quency. In the 1920s, quartz crystal con-
trolled clocks were developed in the
laboratory and used as a standard. With the
TEST AND MEASUREMENT
BASICS
The process of testing requires a knowl-
edge of what must be measured and what
accuracy is required. If battery voltage is
measured and the meter reads 1.52 V, what
does this number mean? Does the meter
always read accurately or do its readings
change over time? What influences a meter
reading? What accuracy do we need for a
meaningful test of the battery voltage?
Table 25.1
Standard Frequency Stations
(Note: In recent years, frequent changes in these schedules have been common.)
Call Sign
A Short History of Standards and
Traceability
Since early times, people who measured
things have worked to establish a system
of consistency between measurements and
measurers. Such consistency ensures that
a measurement taken by one person could
be duplicated by others — that measure-
ments are reproducible. This allows dis-
cussion where everyone can be assured
that their measurements of the same quan-
tity would have the same result. In most
cases, and until recently, consistent mea-
surements involved an artifact: a physical
Location
Frequency (MHz)
BSF
Taiwan
5, 15
CHU
Ottawa, Canada
3.330, 7.335, 14.670
FFH
France
2.500
IAM/IBF
Italy
5.000
JJY
Japan
2.5, 5, 8, 10, 15
LOL
Argentina
5, 10
RID
Irkutsk
5.004, 10.004, 15.004
RWM
Moscow
5, 4.996, 9.996, 14.996
WWV/WWVH
USA
2.5, 5, 10, 15, 20
VNG
Australia
2.5, 5
ZSC
South Africa
4.291, 8.461, 12.724 (part time)
Test Procedures
25.1
advent of radio communication time in-
tervals could be transmitted by radio, and
a very fundamental standard of time and
frequency could be used locally with little
effort. Today transmitters in several coun-
tries broadcast time signals on standard
calibrated frequencies.
Table 25.1
con-
tains the locations and frequencies of
some of these stations.
In the 1960s, Hewlett-Packard began
selling self-contained time and frequency
standards called cesium clocks. In a
cesium clock a crystal frequency is gener-
ated and multiplied to microwave frequen-
cies. That energy is passed through a
chamber filled with cesium gas. The gas
acts as a very narrow band-pass filter. The
output signal is detected and the crystal
oscillator frequency is adjusted automati-
cally so that a maximum of energy is de-
tected. The output of the crystal is thus
linked to the stability of the cesium gas
and is usually accurate to several parts in
10
−12
. This is much superior to a crystal
oscillator alone; but at close to $40,000
each, cesium frequency standards are a bit
extravagant for amateur use.
A rubidium frequency standard is an
alternative to the cesium clock. They are
not quite as accurate as the cesium, but
they are much less expensive, relatively
quick to warm up and can be quite small.
Older models occasionally appear surplus.
As with any precision instrument, it should
be checked over and calibrated before use.
Most hams do not have access to cesium
or rubidium standards—or need them.
Instead we use crystal oscillators. Crystal
oscillators provide three levels of stability.
The least accurate is a single crystal
mounted on a circuit board. The crystal fre-
quency is affected by the temperature
environment of the equipment, to the extent
of a few parts per million (ppm) per degree
Celsius. For example, the frequency of a
10-MHz crystal with temperature stability
rated at 3 ppm might vary 60 Hz when tem-
perature of the crystal changes by 2°C. If
the crystal oscillator is followed by a fre-
quency multiplier, any variation in the crys-
tal frequency is also multiplied. Even so,
the accuracy of a simple crystal oscillator
is sufficient for most of our needs and most
amateur equipment relies on this technique.
For a discussion of crystal oscillators and
temperature compensation, look in the
Oscillators and Synthesizers
chapter of
this
Handbook
.
The second level of accuracy is
achieved when the temperature around the
crystal is stabilized, either by an “oven” or
other nearby components. Crystals are
usually designed to stabilize at tempera-
tures far above any reached in normal
operating environments. These oscillators
are commonly good to 0.1 ppm per day
and are widely used in the commercial
two-way radio industry.
The third accuracy level uses a double
oven with proportional heating. The two
ovens compensate for each other auto-
matically and provide excellent tempera-
ture stability. The ovens must be left on
continuously, however, and warm-up re-
quires several days to two weeks.
Crystal
aging
also affects frequency
stability. Some crystals change frequency
over time (age) so the circuit containing
the crystal must contain components to
compensate for this change. Other crys-
tals become more stable over time and
become excellent frequency standards.
Many commercial laboratories go to the
expense of buying and testing several ex-
amples of the same oscillator and select
the best one for use. As a result, many
surplus oscillators are surplus for a rea-
son. Nevertheless, a good stable crystal
oscillator can be accurate to 1 × 10
-9
per
day and very appropriate for amateur ap-
plications.
method connects the output of the cali-
brated oscillator into the horizontal input
of a high frequency oscilloscope, and the
oscillator to be measured to the vertical
input. It helps, but they need not be on the
same frequency. By noting how long it
takes the sine wave to travel one division
at a given sweep speed, one can calculate
the resulting drift in parts per million per
minute (ppm/min).
Another technique of oscillator calibra-
tion uses a VLF phase comparator. This is
a special direct-conversion receiver that
picks up the signal from WWVB on
60 kHz. Phase comparison is used to
compare WWVB with the divided fre-
quency of the oscillator being tested.
Many commercial units have a small strip
chart printer attached and switches to de-
termine the receiver frequency. Since
these 60-kHz VLF Comparator receivers
have been largely replaced by units that
use Loran signals or rubidium standards,
they can be found at very reasonable
prices. A very effective 60-kHz antenna
can be made by attaching an audio trans-
former with the low-impedance winding
connected to the receiver antenna termi-
nals by way of a series dc blocking capaci-
tor. The high-impedance winding is then
connected between ground and a random
length of wire. A typical VLF Comparator
can track an oscillator well into a few parts
in 10
-10
. This technique directly compares
the oscillator with an NIST standard and
can even characterize oscillator drift char-
acteristics in ppm per day or week.
Another fairly direct method compares
an oscillator with one of the WWV HF
signals. The received signal is not im-
mensely accurate, but if the oscillator of a
modern HF transceiver is carefully com-
pared, it will be accurate enough for all
but the most demanding work.
The last and least accurate way to cali-
brate an oscillator is to compare it with
another oscillator or counter owned by you
or another local ham. Unless the calibra-
tion of the other oscillator or counter is
known, this comparison could be very
misleading. True accuracy is not deter-
mined by the label of a famous company
or impressive looks. Metrologists (people
who calibrate and measure equipment)
spend more time calibrating oscillators
than any other piece of equipment.
Time and Frequency Calibration
Many hams have digital frequency
counters, which range from surplus lab
equipment to new highly integrated instru-
ments with nearly everything on one chip.
Almost all of these are very precise and
display nine or more digits. Many are even
quite stable. Nonetheless, a 10-MHz oscil-
lator accurate to 1 ppm per month can vary
±10 Hz in one month. This drift rate may
be acceptable for many applications, but
the question remains: How accurate is it?
This question can be answered by cali-
brating the oscillator. There are several
ways to perform this calibration. The most
accurate method compares the unit in
question by leaving the oscillator operat-
ing, transporting it to an oscillator of
known frequency and then making a com-
parison. A commonly used comparison
25.2
Chapter 25
DC Instruments and Circuits
This section discusses the basics of ana-
log and digital dc meters. It covers the
design of range extenders for current, volt-
age and resistance; construction of a
simple meter; functions of a digital volt-
meter (DVM) and procedures for accurate
measurements.
instrument normally makes the pointer
move in direct proportion to the current.
The second section is an integrator. It is
usually based on an operational amplifier
that is switched by a timing signal. The
timing signal initially shorts the input of
the integrator to provide a zero reference.
Next a reference voltage is connected to
charge the capacitor for a determined
amount of time. Finally the last part of the
timing cycle allows the capacitor to dis-
charge. The time it takes the capacitor to
discharge is proportional to either the in-
put voltage (V
in
, after it was scaled into
the range of 0 to 1 V) or 1 minus V
in
,
depending on the meter design. This dis-
charge time is measured by the next sec-
tion of the DVM, which is actually a
frequency counter. Finally, the output of
the frequency counter is scaled to the se-
lected range of voltage or current and sent
to the final section of the DVM — the digi-
tal display.
Since the timing is quite fast and the
capacitor is not used long enough to drift
much in value, the components that most
determine accuracy are the reference volt-
age source and the range multiplier resis-
tors. With the availability of integrated
resistor networks that are deposited or dif-
fused onto the same substrate, drift is
Digital Multimeters
In recent years there has been a flood of
inexpensive digital multimeters (DMMs)
ranging from those built into probes to
others housed in large enclosures. They
are more commonly referred to as digital
voltmeters (DVMs) even though they are
multimeters; they usually measure volt-
age, current and resistance. After some
years of refining circuits such as the “suc-
cessive approximation” and “dual slope”
methods, most meters now use the dual-
slope method to convert analog voltages
to a digital reading. DVMs have basically
three main sections as shown in
Fig 25.1
.
The first section scales the voltage or
current to be measured. It has four main
circuits:
• a chain of multiplier resistors that re-
duce the input voltage to 0-1 V,
• a converter that changes 0-1 V ac to dc,
• an amplifier that raises signals in the 0-
100 mV range to 0-1 V and
• a current driver that provides a constant
current to the multiplier chain for resis-
tance measurements.
Basic Meters
In measuring instruments and test
equipment suitable for amateur purposes,
the ultimate readout is generally based on
a measurement of direct current. There are
two basic styles of meters: analog meters
that use a moving needle display, and digi-
tal meters that display the measured val-
ues in digital form. The analog meter for
measuring dc current and voltage uses a
magnet and a coil to move a pointer over
a calibrated scale in proportion to the cur-
rent flowing through the meter.
The most common dc analog meter is the
D’Arsonval type, consisting of a coil of
wire to which the pointer is attached so that
the coil moves (rotates) between the poles
of a permanent magnet. When current flows
through the coil, it sets up a magnetic field
that interacts with the field of the magnet to
cause the coil to turn. The design of the
Fig 25.1 — A typical digital voltmeter consists of three parts: the input section for scaling, an integrator to convert voltage to
pulse count, and a counter to display the pulse count representing the measured quantity.
Test Procedures
25.3
automatically compensated because all
branches of a divider drift in the same di-
rection simultaneously. The voltage
sources are generally Zener diodes on sub-
strates with accompanying series resis-
tors. Often the resistor and Zener have
opposite temperature characteristics that
cancel each other. In more complex
DVMs, extensive digital circuitry can in-
sert values to compensate for changes in
the circuit and can even be automatically
calibrated remotely in a few moments.
Liquid crystal displays (LCD) are com-
monly used for commercial DVMs. As a
practical matter they draw little current and
are best for portable and battery-operated
use. The usual alternative, light emitting
diode (LED) displays, draw much more
current but are better in low-light environ-
ments. Some older surplus units use gas
plasma displays (orange-colored digits).
You may have seen plasma displays on gas-
station pumps. They are not as bright as
LEDs, but are easier to read. On the down
side, plasma displays require high-voltage
power supplies, draw considerable current
and often fail after 10 years or so.
The advantages of DVMs are high input
resistance (10 M
measure voltage. In the same way, a mea-
surement of both current and voltage will
obviously yield a value of resistance.
These measurement functions are often
combined in a single instrument: the volt-
ohm-milliammeter or VOM, a multirange
meter that is one of the most useful pieces
of test equipment an amateur can possess.
Accuracy
The accuracy of a D’Arsonval-move-
ment dc meter is specified by the manufac-
turer. A common specification is ±2% of
full scale, meaning that a 0-100
A meter,
for example, will be correct to within 2 µA
at any part of the scale. There are very few
cases in amateur work where accuracy
greater than this is needed. When the in-
strument is part of a more complex measur-
ing circuit, however, the design and
components can each cause error that accu-
mulates to reduce the overall accuracy.
µ
Fig 25.2 — This test setup allows safe
measurement of a meter’s internal
resistance. See text for the procedure
and part values.
is purchased at a flea market or is taken
from a commercial piece of equipment, for
example). Unfortunately, the internal re-
sistance of a meter cannot be measured
directly with an ohmmeter without risk of
damage to the meter movement.
Fig 25.2
shows a method to safely mea-
sure the internal resistance of a linearly
calibrated meter. It requires a calibrated
meter that can measure the same current
as the unknown meter. The system works
as follows: S1 is switched off and R2 is set
for maximum resistance. A supply of con-
stant voltage is connected to the supply
terminals (a battery will work fine) and R2
is adjusted so that the unknown meter
reads exactly full scale. Note the current
shown on M2. Close S1 and alternately
adjust R1 and R2 so that the unknown
meter (M1) reads exactly half scale and
the known meter (M2) reads the same
value as in the step above. At this point,
half of the current in the circuit flows
through M1 and half through R1. To
determine the internal resistance of the
meter, simply open S1 and read the resis-
tance of R1 with an ohm-meter.
The values of R1 and R2 will depend on
the meter sensitivity and the supply volt-
age. The maximum resistance value for R1
should be approximately twice the ex-
pected internal resistance of the meter. For
highly sensitive meters (10 µA and less),
1 k
Extending Current Range
Because of the way current divides be-
tween two resistances in parallel, it is pos-
sible to increase the range (more
specifically to decrease the sensitivity) of
a dc current meter. The meter itself has an
inherent resistance (its internal resistance)
which determines the full-scale current
passing through it when its rated voltage
is applied. (This rated voltage is on the
order of a few millivolts.) When an exter-
nal resistance is connected in parallel with
the meter, the current will divide between
the two and the meter will respond only to
that part of the current that flows through
its movement. Thus, it reads only part of
the total current; the effect makes more
total current necessary for a full-scale
meter reading. The added resistance is
called a “shunt.”
We must know the meter’s internal re-
sistance before we can calculate the value
for a shunt resistor. Internal resistance
may vary from a fraction of an ohm to a
few thousand ohms, with greater resis-
tance values associated with greater sen-
sitivity. When this resistance is known, it
can be used in the formula below to deter-
mine the required shunt for a given multi-
plication:
on most ranges), accu-
rate and precise readings, portability, a
wide variety of ranges and low price.
There is one disadvantage, however: Digi-
tal displays update rather slowly, often
only one to two times per second. This
makes it very difficult to adjust a circuit
for a peak (maximum) or null (minimum)
response using only a digital display. The
changing digits do not give any clue of the
measurement trend and it is easy to tune
through the peak or null between display
updates. In answer, many new DVMs are
built with an auxiliary bar-graph display
that is updated constantly, thus providing
instantaneous readings of relative value
and direction of changes.
Ω
Current Ranges
The sensitivity of an analog meter is
usually expressed in terms of the current
required for full-scale deflection of the
pointer. Although a very wide variety of
ranges is available, the meters of interest
in amateur work give maximum deflec-
tion with currents measured in microam-
peres or milliamperes. They are called
microammeters and milliammeters, re-
spectively.
Thanks to the relationships between
current, voltage and resistance expressed
by Ohm’s Law, it is possible to use a single
low-range instrument (for example, 1 mA
or less for full-scale pointer deflection) for
a variety of direct-current measurements.
Through its ability to measure current, the
instrument can also be used indirectly to
should be adequate. For less sensi-
tive meters, 100 Ω should suffice. Use no
more supply voltage than necessary.
The value for minimum resistance at R2
can be calculated using Ohm’s Law. For
example, if the meter reads 0 to 1 mA and
the supply is a 1.5-V battery, the minimum
resistance required at R2 will be:
Ω
R
m
−
(1)
R
=
n
1
where
R = shunt resistance, ohms
R
m
= meter internal resistance, ohms
n = the factor by which the original
meter scale is to be multiplied.
1.5
R
2
=
0.001
Often the internal resistance of a par-
ticular meter is unknown (when the meter
R2 (min) = 1500 Ω
25.4
Chapter 25
In practice a 2- or 2.5-kΩ potentiometer
would be used.
resistance means that less current is drawn
from the circuit and thus the circuit under
test is less affected by connection of the
meter. Although a 1000-
as multipliers. Look for a series of four or
five resistors that add up to 10 M
:
0.9,9,90,900,9,000,90,000 and 900,000 Ω.
There is usually another 1-M
Ω
Making Shunts
Homemade shunts can be constructed
from several kinds of resistance wire or
from ordinary copper wire if no resistance
wire is available. The copper wire table in
the
Component Data and References
chapter of this
Handbook
gives the resis-
tance per 1000 ft for various sizes of cop-
per wire. After computing the resistance
required, determine the smallest wire size
that will carry the full-scale current, again
from the wire table. Measure off enough
wire to give the required resistance. A
high-resistance 1- or 2-W carbon-compo-
sition resistor makes an excellent form on
which to wind the wire, as the high resis-
tance does not affect the value of the shunt.
If the shunt gets too hot, go to a larger
diameter wire of a greater length.
/V meter can be
used for some applications, most good
meters are 20 kΩ/V or more. Vacuum-tube
voltmeters (VTVMs) and their modern
equivalent FET voltmeters (FETVOMs)
are usually 10-100 M
Ω
resistor in
series to isolate the meter from the circuit
under test. A few of these high-accuracy
resistors in “odd” values can help calibrate
less-expensive instruments.
Ω
Ω
/V and DVMs can
DC Voltage Standards
For a long time NIST has statistically
compared a bank of special Weston Cell
or cadmium sulfate batteries to arrive at
the standard volt. By using a special
tapped resistor, a 1.08-V battery can be
compared to other voltages and instru-
ments compared. However, these are very
high-impedance batteries that deliver al-
most no current and are relatively tem-
perature sensitive. They are made up of a
solution of cadmium and mercury in op-
posite legs of an “H” shaped glass con-
tainer. You can read much more about
them in
Calibration—Philosophy and
Practice
, published by the John Fluke Co
of Mount Lake Terrace, Washington.
Hams often use an ordinary flashlight
battery as a convenient voltage reference.
A fresh
D cell
usually provides 1.56 V
under no load, as would be measured by a
DVM. The Heath Company, which sup-
plied thousands of kits to the ham commu-
nity for many years, used such batteries as
the calibration references for many of their
kits.
Recently, NIST has been able to use a
microwave to voltage converter called a
“Josephson Junction” to determine the
value of the volt. The converter transfers
the accuracy of a frequency standard to
the accuracy of the voltage that comes out
of it. The converter generates only 5 mV,
however, which then must be scaled to the
standard 1-V level. One problem with
high-accuracy measurements is stray
noise (low-level voltages) that creates a
floor below which measurements are
meaningless. For that reason, meters with
five or more digits must be very quiet and
any comparisons must be made at a volt-
age high enough to be above the noise.
go even higher.
Multipliers
The required multiplier resistance is
found by dividing the desired full-scale
voltage by the current, in amperes, required
for full-scale deflection of the meter alone.
To be mathematically correct, the internal
resistance of the meter should be subtracted
from the calculated value. This is seldom
necessary (except perhaps for very low
ranges) because the meter resistance is usu-
ally very low compared with the multiplier
resistance. When the instrument is already
a voltmeter with an internal multiplier,
however, the meter resistance is signifi-
cant. The resistance required to extend the
range is then:
R = R
m
(n – 1) (2)
where
R
m
= total resistance of the instrument
n = factor by which the scale is to be
multiplied
VOLTMETERS
If a large resistance is connected in se-
ries with a meter that measures current, as
shown in
Fig 25.3
, the current multiplied
by the resistance will be the voltage drop
across the resistance. This is known as a
multiplier. An instrument used in this way
is calibrated in terms of the voltage drop
across the multiplier resistor and is called
a voltmeter.
Sensitivity
Voltmeter sensitivity is usually expres-
sed in ohms per volt (
/V voltmeter
having a calibrated range of 0 to 10 V is to
be extended to 1000 V, R
m
is 1000 × 10 =
10 k
For example, if a 1-k
Ω
/V), meaning that
the meter full-scale reading multiplied by
the sensitivity will give the total resistance
of the voltmeter. For example, the resis-
tance of a 1 kΩ/V voltmeter is 1000 times
the full-scale calibration voltage. Then by
Ohm’s Law the current required for full-
scale deflection is l milliampere. A sensi-
tivity of 20 kΩ/V, a commonly used value,
means that the instrument is a 50-µA
meter.
As voltmeter sensitivity (resistance)
increases, so does accuracy. Greater meter
Ω
, n is 1000/10 = 100 and R = 10,000
× (100 − 1) = 990 kΩ.
When extending the range of a volt-
meter or converting a low-range meter into
a voltmeter, the rated accuracy of the in-
strument is retained only when the multi-
plier resistance is precise. High-precision,
hand-made and aged wire-wound resistors
are used as multipliers of high-quality in-
struments. These are relatively expensive,
but the home constructor can do well with
1% tolerance metal-film resistors. They
should be derated when used for this pur-
pose. That is, the actual power dissipated
in the resistor should not be more than
1
/
10
to
1
/
4
the rated dissipation. Also, use
care to avoid overheating the resistor body
when soldering. These precautions will
help prevent permanent change in the re-
sistance of the unit.
Many DVMs use special resistor groups
that have been etched on quartz or sapphire
and laser trimmed to value. These resistors
are very stable and often quite accurate.
They can be bought new from various sup-
pliers. It is also possible to “rescue” the
divider/multiplier resistors from an older
DVM that no longer functions and use them
Ω
DC MEASUREMENT CIRCUITS
Current Measurement with a
Voltmeter
A current-measuring instrument should
have very low resistance compared with
the resistance of the circuit being mea-
sured; otherwise, inserting the instrument
will alter the current from its value when
the instrument is removed. The resistance
of many circuits in radio equipment is high
and the circuit operation is affected little,
if at all, by adding as much as a few hun-
Fig 25.3 — A voltmeter is constructed
by placing a current-indicating
instrument in series with a high
resistance, the “multiplier.”
Test Procedures
25.5
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