Basic Precautions - Tektronix 1S1 Instruction Manual

Operating Instructions – Type 1S1
ANALOG-RECORDER OPERATION
The output horizontal and vertical sampling signals avail-
able at the Type 1S1 HORIZ OUTPUT and VERT OUTPUT
jacks can be used for driving an external recording device.
The Type 1S1 thus permits the recording of waveforms with
picosecond risetimes or gigacycle frequencies.
To use the Type 1S1 in this manner:
1. Set up the equivalent-time sampling display in the
usual manner with the Type 1S1 and oscilloscope.
2. Set the oscilloscope Time/Cm switch to 10mSec.
3. Free run the oscilloscope time-base generator to provide
a sawtooth output waveform. (Leave the oscilloscope set
for external horizontal deflection.)
4. Connect a patch cord from the oscilloscope Sawtooth
Output jack to the Type 1S1 EXT HORIZ INPUT jack.
5. Set the Type 1S1 DISPLAY MODE switch to EXT HORIZ.
6. Adjust the EXT HORIZ ATTEN control for a sweep
length of approximately 10cm.
7. Set the oscilloscope Time/Cm switch for the desired
X-axis rate of the recording device. Most oscilloscopes that
will accomodate the Type 1S1 provide sweep rates down to
5 sec/cm or 50 sec/sweep. If slower rates are required,
an external sweep voltage (up to 150 volts in amplitude)
may be applied to the EXT HORIZ INPUT jack.
8. Connect the Type 1S1 HORIZ OUTPUT jack to the
horizontal-axis input of the recorder. (The patch cord to the
oscilloscope may be left in place to monitor the display.)
9. Connect the Type 1S1 VERT OUTPUT jack to the verti-
cal-axis input of the recorder.
PULSE TESTING
When used with a fast-rise pulse generator, the Type 1S1
can be used for making observations and measurements of
delay line transit time, impedance of an unknown source,
or characteristics of response signals from a test device.

Basic Precautions

Certain precautions should always be observed when
connecting a pulse signal to a test device or to the Type 1S1:
1. Use high-quality coax cables and connectors for all
signal connections.
2. Make sure that all connections are tight and that all
connectors are tightly assembled.
3. Keep signal cables as short as possible to preserve the
signal quality.
4. Use attenuators as needed to limit the signal amplitude
into the sampling system and other sensitive circuits.
5. Use terminations and impedance-matching devices to
suit the application.
Risetime Characteristics
When using a fast-rise pulse to determine the risetime of
a test device, the risetimes of the pulse and of the Type 1S1
may sometimes have to be taken into consideration. As a
2-28
general rule, if the risetime of the test device is at least 10
times as long as the combined risetime of the input pulse
and the Type 1S1 and input cables, the error introduced into
the measurement by the testing system will not be more
than 1% and therefore can be considered negligible.
If, however, the risetime of the test device is less than
10 times as long as the combined risetime of the testing
system, the observed risetime will not give a true measure-
ment of the test device risetime. In this case, the actual
risetime of the test device will have to be determined from a
determination of the effects produced by the various com-
ponents making up the system. Normally the overall risetime
of the system can be found by taking the square root of the
sum of the squares of the individual risetimes. Conversely,
the risetime of the test device can be found from the same
relationship if all of the risetimes in the system are known
except that of the test device.
Impedance Measurement
As a signal travels down a transmission line, each time it
encounters a mismatch, or different impedance, a reflection
is generated and sent back along the line to the source. The
amplitude and polarity of the reflection are determined by
the value of the encountered impedance in relation to the
characteristic impedance of the cable. If the mismatch im-
pedance is higher than that of the line, the reflection will be
of the some polarity as the applied signal; if it is lower than
that of the line, the reflection will be of opposite polarity.
The reflected signal is added to or subtracted from the
amplitude of the pulse if it returns to the source before the
pulse has ended. Thus, for a cable with an open end (no
termination), the impedance is infinite and the pulse am-
plitude would be doubled. For a cable with a shorted end,
the impedance is zero and the pulse would be canceled.
Measurement of an encountered impedance can be made
either by observing the change in amplitude of the applied
pulse or by measuring the reflection alone after separating
the reflection from the pulse with delay cables. The ampli-
tude of the reflection (p), expressed as a decimal fraction of
the applied pulse amplitude, is related to the characteristic
impedance of the system (Z
(Z ) in the following manner:
L
−
Z Z
; Therefore:
ρ
=
L 0
+
Z Z
L 0
The load impedance can thus be calculated from the
amplitude of the reflection.
For example, consider the following situation: a +2-volt
pulse is sent down a 50-ohm cable and encounters an un-
known impedance; the reflection returns to the pulse source
after the pulse has ended and the resulting amplitude is
+500 mv. This is the reflection amplitude; therefore,
= +500/+2000 = +0.25
. 1 00
=
and Z 50
L
. 1 00
By connecting the pulse test system to the SIGNAL IN
connector of the Type 1S1, this reflection signal can be
observed on the oscilloscope screen, and the impedance
measurement can be calculated directly from the display
amplitude.
) and to the load impedance
0
ρ
+
1
=
Z Z
L 0
− ρ
1
+
. 0 25
=
83 3 . ohms
−
. 0 25