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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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