Tektronix 1S1 Instruction Manual page 21

Operating Instructions – Type 1S1
Passive Probes. The Tektronix P6034 10X Probe and
the P6035 100X Probe are moderate-resistance passive
probes designed for use with 50-ohm systems. They are small
in size, permitting measurements to be made in miniaturized
circuitry. Power rating is 0.5 watt up to a frequency of 500
Mc. Momentary voltage peaks up to 500 volts can be per-
mitted at low frequencies, but voltage derating is required
at higher frequencies. Characteristic data is given in the
probe instruction manuals.
The P6034 10X Probe places 500 ohms resistance and less
than 0.8 pf capacitance in parallel with the signal source at
low frequencies. The probe bandpass is dc to approximately
3.5 Gc, and risetime is 100 psec or less (10% - 90%). At 1 Gc,
the input resistance is about 300 ohms and the capacitive
reactance is about 450 ohms.
The P6035 100X Probe places 5k ohms resistance and
less than 0.7 pf capacitance in parallel with the signal source
at low frequencies. Bandpass of the probe is dc to ap-
proximately 1.5 Gc, and risetime is 200psec or less (10% -
90%). At 1 Gc the input resistance is about 2 k ohms and
the capacitive reactance is about 450 ohms.
The P6026 Passive Probe, also designed for use with 50-
ohm systems, has a bandpass of dc to approximately 600
Mc when dc-coupled, and a risetime of 600psec or less.
The probe consists of a coax cable, connectors, ac-coupled
and dc-coupled 50-ohm terminations, and seven attenuator
heads with attenuation factors from 5 to 5000.
Cathode-Follower Probes. The Tektronix P6032 Cathode-
Follower Probe is a high-impedance high-frequency probe for
Tektronix sampling systems. Bandpass is dc to approximately
850 Mc and risetime is 400 psec or less. Seven attenuator
heads are provided, with attenuation factors from 10X to
1000X for the combination of probe and attenuator. Input
resistance is 10 megohms at dc, and the parallel capacitance
ranges from 1.3 pf to 3.6 pf, depending on the attenuator
head used. At 1 Gc, the capacitive reactance is about 100
ohms and the input resistance is about 100 ohms for the 10X
attenuator and 2 k ohms for the 1000 X attenuator.
The advantage of the cathode-follower probe is the high
input resistance and low capacitive loading at moderately
high frequencies. Dynamic characteristic data is given in
the probe instruction manual.
Built-in Probes. Another satisfactory method of coupling
fractional nanosecond signals from within a circuit is to de-
sign the circuit with a built-in 50-ohm output terminal. With
this method, the circuit can be monitored without being
disturbed. When the circuit is not being tested, a 50-ohm
terminating resistor can be substituted for the test cable. If
it is not convenient to build in a permanent 50-ohm test
point, an external coupling circuit, which may be considered
a probe, can be attached to the circuit.
Several factors must be considered when constructing such
a built-in signal probe. A probe is designed to transfer
energy from a source to a load, with controlled fidelity and
attenuation. Both internal and external characteristics affect
its operation. It must be able to carry a given energy level,
be mechanically adaptable to the measured circuit, and be
equally responsive to all frequencies within the limits of
the system. The probe must not load the circuit significant-
ly or the display may not present a true representation of
the circuit operation. Loading may even disrupt the opera
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tion of the circuit. When it is necessary to ac-couple the
probe, the capacitor should be placed between the series
resistance and the probe cable to minimize differences
between the input characteristics with and without the ca-
pacitor. In this 50-ohm environment, stray capacitance to
ground has a shorter and more uniform time constant than
if the capacitor were placed at the signal source where
the impedance is usually higher and of unknown value.
Fig. 2-8. Built-in probes for coupling to a test circuit. (A) Para-
llel method;(B) series method; (C) reverse-terminated parallel
method
.
Fig. 2-8A shows the parallel method of coupling to a cir-
cuit under test. Resistor R
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ohm input cable to the Type 1S1, placing R
across the impedance in the circuit. This method usually re-
quires the use of an amplitude correction factor. In order to
avoid over-loading the circuit, the total resistance of R
+ 50 ohms should not be less than 5 times the impedance of
the device (R in parallel with Z
L
tion. The physical position of R
coupling.
is connected in series with the 50
+ 50 ohms
S
) requiring a 20% correc-
0
will affect the fidelity of the
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S