Tektronix TYPE 109, Instruction Manual

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Applications— Type 109
F ig . 3 - 2 . S a m p lin g test setup w h e re the T y p e 1 0 9 is used to d riv e a test d e v ice or fix tu re .
of this sort, the device is connected as part of the charge
line while the output from the Type 109 is applied to the
oscilloscope input. If the impedance of the inserted device
is exactly 50 ohms, it will merely increase the time that
the amplitude of the Type 109 waveform remains constant.
The displayed waveform will also indicate double the de­
lay time for the inserted device.
The test device, such as a piece of coaxial cable, a
connector assembly, or a delay line, can be connected into
the charge line of the Type 109 in the manner shown in
Fig. 3-3. In Fig. 3-4 the Type 109 waveform is shown when
a length of 125-ohm cable is connected into the charge line
in series with two lengths of 50-ohm cable. The portion of
the waveform due to the 125-ohm section is about 60%
as high as the first portion of the waveform due to the
50-ohm cable. The duration of the Type 109 waveform due
to the 125-ohm cable is twice the delay time of the cable
so it is evident from the picture that the true delay time
of the 125-ohm cable used is actually 5 nsec.
In Fig. 3-4 the relative amplitude of the portions of the
waveform bear a definite relationship to the impedance
of the device that generated that portion. The impedance
of an unknown device can thus be measured by compar­
ing the amplitude of the portion of the Type 109 w ave­
form produced by it against the amplitude of the initial
portion due to the 50-ohm system. The method is generally
limited to the first reflection, unless the deviations are
small, due to multiple reflections and reflection losses.
If we call the amplitude produced by the 50-ohm system
V0 and the amplitude produced by the inserted device V x,
then the impedance of the inserted device is given by the
formula:
Z = 50 (2 — — 1)
Vx
3-3