USEFUL
INFORMATION
FOR
USERS
OF
TEKTRONIX
INSTRUMENTS
NUMBER
11
ACCURATE FREQUENCY
MEASUREMENTS
By
Jerry
Kraxberger,
Tektronix
Field
Engineer
The
sweep-time
accuracy
of
most
Tek-
tronix
Oscilloscopes is specified to be ±
3'/r
on
any
range.
Some
oscilloscope
operators
find it necessary,
at
times, to
make
time
or
frequency
measurements
to much closer
tolerances.
Faced
with
these requirements,
the
oscilloscope
operator
will most likely
1·ely
on
the highly
accurate
and
well-known
method
of
using
Lissajous
patterns
to com-
pare
an
unknown
frequency with a
standard
frequency.
This
method
has
two
drawbacks;
one, it
may
not be suitable
for
the oscillo-
scope
at
hand;
two, it
requires
consider-
able
set-up
time.
You
may
use a
much
simpler, yet equally
accurate,
method
provided
your
oscilloscope
possesses-as
all
Tektronix
Oscilloscopes
do-a
triggered
sweep.
The
accuracy
of
the
instrument
does
not
enter
into the
measure-
ment
and
the
wave
shape
of
either
signal,
i.e.
sine
wave, sawtooth, pulse, etc., is not
important.
To
use this method, connect one signal to
the
external-trigger
input
and
the
other
Lo
the
vertical-amplifier
input
of
the oscillo-
scope.
You
may
use
either
signal as the
standard
but you
must
connect the lower-
f
requency signal to
the
external-trigger
in-
put.
Trigger
the sweep in the normal
man-
ner.
On
Tektronix
Oscilloscopes, set the
TRIGGERING
MODE
switch to
AC
or
DC.
Do
not use the
AUTOMATIC
or
HF
SYKC
modes
for
this application.
Make
certain
the sweep
is
not
free
running
hy
temporarily
removing
the
external-trigger
signal.
Ii
the sweep is
free
running, a
trace
will
remain
on the crt.
Let's
look
at
a specific application.
Sup-
pose
you
want
to
adjust
a 400-cps signal to
an
accuracy
better
than
±0.01%.
\\'e
sug-
gest
the
use
of
a
triggered-type
oscilloscope
and
a
Tektronix
Type
180A
Time-Mark
Generator*
since 400 cps is an
integer
of
the
time-mark
generator's
1-mc ( ±0.001
'/r)
cry-
stal-controlled
oscillator
frequency.
Here
is
a
suggested
procedure:
(see block
diagram,
Fig.
1)
1.
Adjust
the oscilloscope sweep time to
l
msec/cm.
2.
Trigger
the oscilloscope
externally
from
the 10-msec
marker
(100 cps)
of
the time-
mark
generator
(external-trigger-input
of
oscilloscope).
3.
Connect
the
400-cps signal to the
\'erti-
cal-amplifier
input
of
the
oscilloscope.
4.
Adjust
the 400-cps
frequency
precisely
until
it
does
not
move
horizontally
on
the
screen.
PRINTED
IN
U.S.A
Fig. 1
Since the period
of
a 400-cps signal is
1
/400 sec
or
2.5 msec, you should
now
have
4 complete cycles
of
the 400-cps signal dis-
played in
10
cm
horizontally
(
10
msec
oi
time)
on
the screen.
Ii
the
400-cps signal does not move
hori-
zontally,
and
if
the
Time-Mark
Generator
has zero tolerance, the signal
under
test will
be
400 cps ± 0.0.
\\'ith
the
Tektronix
Time-Mark
Gen-
erators
specified in this
article
as
the
stand-
ard
source,
the
signal
under
test can be
ad-
justed
lo
400 cps ± 0.001 %
or
400 cps
±0.004
cps.
K
ote
that
if the signal
were
off
by ±
0.01
cps, one complete cycle would move
past a given point
on
the
crl
screen in
10
seconds.
\Vhen
you
consider
the
accuracy
of
the
specified
time-mark
generators,
this
would be
400 cps
±0.104
cps.
You
can use the above method
of
fre-
quency
comparison
because
the
400-cps sig-
nal
under
test
and
the
1-msec (1
kc)
time-
mark
signals
are
exact
integers
of
the 1-
mc/sec
frequency
oi
the
time-mark
gen-
erators
crystal-controlled
oscillator.
l\
ote
that
the oscilloscope sweep time does
not
en-
ter
into the measurement. I l is, howe\'er,
an aid
when
making
preliminary
adjust-
ments
of
the signal
under
test.
If
you
use a
dual-trace
preamplifier
(Type
C-A
for
example)**
in the oscillo-
scope vertical channel,
\'OU
can observe
both signals on the
crt
scr;en
simultaneously.
To
do this,
run
another
jumper
from
the
time-mark
generator's
1-msec
output
to the
dual-trace
preamplifier's
B channel.
Set
the
preamplifier
MODE
switch to the
ALTER-
NA
TE,
CHOPPED,
or
ADDED
ALGE-
BRAIC.A.LL
Y position.
The
1-msec
markers
will
remain
stationary
on the
crt
screen but
Lhe
400-cps signal will travel to the
right
or
to the
left
if it is not exact.
To
insure the
accuracy
of
the
Type
180:\
Time-~lark
Generator,
you can
com1~are
or
calibrate its
frequency
with
the
Bureau
of
Standards'
\\'\\'V
station
as
follows:
Tune
a short-\\'a\·e recei\'er to
station
\\'\\'V.
Add
a
short
length
of
wire
to
the
1-µsec
output
of
the
Type
180A
Time-Marker
Gen-
DECEMBER
1961
erator.
The
generator's
1-mc signal will
beat with
the
incoming
'vV'vVV
signal in the
short-wave
receiver's
A.M.
detector.
The
difference
frequency
in cycles will be a
measure
of
the
time-mark
generator's
ac-
curacy
in
parts
per
million.
You
can mini-
mize this
difference
by
adjusting
the
vari-
able-trimmer
capacitor
across
the oscillator
crystal
in
the
time-mark
generator.
Monitor
this
difference
signal
right
off
the
detector
of
the
short-wave
receiver.
Use
an
oscillo-
scope to do this.
One
cannot
hear
the low-
f requency zero heat. Besides, the
low-fre-
quency response
of
the
receiver's
audio
system
may
not
he
:!?,'ood.
*The
following
Tektronix
Time-Mark
Gen-
erators
may
also be used
for
this applica-
tion;
Type
180
with
a
Type
180
Crystal
Oven
Mod
Kit
(Tek
No.
040-252) installed.
Type
181
and
Type
RM
181
with
Crystal-
Oven
Combination accessory
(Tek
No. 158-
007) installed.
**Tektronix
instruments
on which you
may
observe these signals simultaneously
arc;
Type
502
Dual-Beam
Oscilloscope
and
Type
516
Dual-Trace
Oscilloscope.
Any
Type
530,
Type
540,
Type
550-Series Oscillo-
scope with a multiple-trace, letter-type,
Plug-In
Preamplifier
in
the
plug-in com-
partment
(this
would
also include the
Type
580
Series
Oscilloscopes
with
a
Type
81
Plug-In
Adapter).
Type
560
or
Type
561
Oscilloscopes
with
a
Type
72
Dual-Trace
Plug-In
l'nit.
SHIPPING BLOCKS
AND
INSTRUMENT
PACKAGING
A
continuing
problem
for
both
Tektronix
and
our
customers
is
the
damage
suffered
by
instruments
in
transit.
The
most
foolproof
container
a customer
can use
when
shipping a
Tektronix
instru-
ment
is
the
original
carton
in which it was
received.
These
cartons
and
their
attendant
packing
materials
are
the
result
of
much
research
and
some
sad
experiences.
An
in-
strument,
properly
housed in one
of
these
containers,
will come
through
a
normal
ship-
ment
ordeal
in
excellent
condition.
\Ve
earnestly
recommend that,
whenever
possible, the
original
Tektronix
shipping
carton
along
with
the
dunnage
and
any
ship-
ping blocks he
retained
and
stored
for
future
use.
\Ve
would also like to
make
the following
suggestions
for
packaging
Tektronix
in-
struments
for
shipment:
If
you do
not
have
the
original shipping
carton,
contact
your
Tektronix
Field
En-
gineer.
He
very
probabl\'
will be able to
supply yon
with
a
factory-;pproved
carton.
If
the
instrument
ha·s
a shock-mounted
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