Tektronix TYPE 109, Инструкция по эксплуатации

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Applications— Type 109
Risetimes
The risetime of any particular assembly of the Type 109,
an oscilloscope (conventional or sampling), and accessory
pieces such as coax cables is a variable depending upon
the cable characteristics as well as individual risetimes. The
“ root of the sum of the squares” method can generally be
applied as an approximation method only, as skin effect
losses of the cables do not add properly using this method.
(The root-sum-squares method applies accurately to gaus-
sian systems only.)
As a general rule, if the equipment or signal being
measured has a risetime 10 times slower than the Type 109
and other related measuring equipment, the error is 1% .
This amount is small and can be considered to be neg­
ligible. If the equipment being measured has a risetime
three times slower than the related measuring equipment,
the error is slightly less than 6 % . By keeping these rela­
tionships in mind, the results can be interpreted intelli­
gently.
Basic Precautions
For faithful reproduction of the pulse certain precautions
should be followed. These can be summarized as follows:
(a) Use proper types of cables, terminations, attenuators,
and impedance matching networks. Low-impedance coaxial
cables are used with the Type 109 as signal conductors.
It is important that these cables be terminated in their
characteristic impedance (50Q) to prevent reflections and
standing waves unless you deliberately wish to improperly
terminate the cables. One application for improper termina­
tion would be to boost the signal to an amplifier input by
leaving the end of a transmission line unterminated.
(b) Keep unshielded wires of uncertain impedance short so
that reflection and/or cross-coupling effects are not intro­
duced. Keep ground-return paths short and direct.
(c) Shield measuring equipment leads to prevent unde­
sired coupling to other parts of the circuit. Shielding is
especially required where radiation is a problem and where
high-impedance dividers or circuits are involved.
(d) Choose components which function properly at fre­
quencies and risetimes encountered.
(e) Keep in mind inherent parameters in circuit com­
ponents such as inductance present in capacitors or resistors.
(f) Consider the possible nonlinear behavior of circuit
components due to changes in voltage or temperature
coefficients.
(g) Consider the input impedance of measuring equip­
ment. The impedance may be enough to cause loading
effects, detuning or undesirable reflections.
Connecting the Type 109 to the Device Under
Test
When connecting the Type 109 Pulse Generator output
to the device under test, observe the following precautions:
1. A complete dc-return path must be provided between
the device under test and Type 109 Pulse Generator 50fi
OUTPUT connector.
2. If the pulse is applied to a 50
Cl
load which has a
dc potential across it, the actual amplitude of the pulse is
the voltage set by the AMPLITUDE control less one-half the
dc voltage across the load. Do not allow more than 200
volts dc to be applied to the Type 109 Pulse Generator 500
OUTPUT connector. This limit will keep the internal com­
ponents of the Type 109 from being damaged.
As an example, assume that the Type 109 Pulse Gener­
ator output is connected to a load which has + 10 volts
across it and that the AMPLITUDE control is set to +1
volt. The actual amplitude is found by substituting these
values in the following equation:
VA = V s - VL = (+ 1) - (+10) = - 4 volts
2 2
where V A is the actual pulse amplitude, Vs is the voltage
setting of the AMPLITUDE control, and VL is the dc voltage
applied across the load.
3. If the load will not terminate the 50 Q output of the
Type 109 Pulse Generator (because it is not practical or
possible), then it will be desirable to use a 50-ohm coaxial
lead (between the Type 109 and the load) which is long
enough to delay the load’s reflection until after the time
of interest. The reflection will appear at a time equal to
twice the output lead delay plus the pulse length.
Some representative test systems involving the Type 109
and other related equipment are described and illustrated
in this portion of the manual. The systems to be described,
as mentioned earlier, may be used as a basis for the de­
velopment of other more specialized systems required by
specific applications.
Using the Type 109 With Sampling Oscillo­
scopes
One of the primary applications of the Type 109 Pulse
Generator is to use it for checking and calibrating sampling
oscilloscopes which have internal triggering capabilities.
Since this application is adequately covered in the instruc­
tion manual for the sampling oscilloscope involved, no de­
tailed explanation will be provided here.
In the usual application the Type 109 is used to drive
a test device so the output from the device can be ob­
served and measured on the crt screen of the sampling
oscilloscope. Fig. 3-2 shows how the connections for this
application are made.
In other similar applications, using this setup, the test
device could be a test fixture. An example of a test
fixture that can be used is the Type 290 Transistor Switch­
ing Time Tester, available through your Tektronix Field
Office.
Fig. 3-3 illustrates a sampling test setup where the de­
vice under test is inserted in series with the charge line.
A clear picture of transmission-line characteristics can be
made using this setup.
The presence of discontinuities
along a transmission line can be determined while the line
is under study by means of the oscilloscope display.
The Type 109 when used in conjunction with a sampling
oscilloscope provides an excellent means for measuring the
impedances of certain devices and cables. In an application
3-2