Tektronix Type 82 PLUG-IN Instruction Manual page 13

Operating Instructions—Type 82
refer to the Tektronix catalog. If you decide to use either
or both of these probes, the discussion that follows describes
how they can be properly used with the Type 82.
Use of either the P6034 or the P6035 probe means that
the cable end of the probe must be terminated into 50 ohms
for proper frequency response. !n addition, since maximum
dc voltage rating of the probe is low (compared to the
P6008), external ac coupling can be used to block the dc
component (up to 500 volts maximum) of a signal without
affecting the response of the probe at the high frequencies.
External ac coupling is accomplished by inserting a GR
Type 874-K coupling capacitor between the cable connector
end of the probe and any external resistance paths to ground
such as an attenuator and termination (see Fig. 2-6). An
external capacitor has to be used because the coupling
capacitor in either channel of the Type 82 is not in series
with the de path through the probe resistor and these exter-
nal resistances to ground.
At the low frequencies the approximate 3-db down point
of the P6034 and P6035 probes, when connected to a 50-olim
source and when used in conjunction with the 874-k coupling
capacitor, is 70 ke for the P6034 Probe and 7 ke for the
P6035 Probe.
The probe cable is small in diameter for flexibility and is
made short to minimize pulse distortion due to cable losses.
If the probe is too short to use for your application, you
can extend its overall length by adding good quality 50-ohm
cable such as RG-58A/U. To add the cable, simply connect
a length of the cable, (fitted with GR Type 874 50-olm
connectors) between the coupling capacitor and the other
series-connected 50-ohm components (see Fig. 2-6). Select a
length of cable that will move the reflection on the waveform
out of the area of interest (see Fig. 2-7).
The reflection, which appears as an 8% dip in the wave-
form, originates from the input circuit of the Type 82. It is
caused by the input capacitance of the Type 82 connected
in shunt with the termination. The reflection travels down
the cable and reflects back when it reaches the resistor in
the probe body. The down and back time of the reflection
is equal to twice the delay time of the cable between the
termination and the resistor in the probe. The reflection will
appear behind the leading edge of the pulse delayed by this
amount of time.
As an alternate method of controlling the reflection, a
50-ohm GR 2X T attenuator can be connected between the
added cable and the termination (see Fig. 2-6.) The attenu-
ator decreases the amplitude of the dip by a factor of four.
The probe attenuation, however, will be increased by a
factor of two. Although the probe attenuation factor is
doubled, voltage rating of the probe is still the same.
Therefore, avoid the tendency to apply larger signals to the
probe to obtain a higher amplitude waveform on the crt.
The excessive signal amplitude or dc voltage (if applied]
may exceed the Y, watt power rating of the probe resistor
and damage it.
To avoid having too much bulk extending out from the
front-panel connector of the Type 82 if all components are
used as described, insert a short length of high quality 50-
ohm extension cable between the 50-ohm GR 2X T attenua-
tor and the GR-to-BNC adapter.
Input Coupling
To display both the ac and de components of an applied
signal, set the appropriate AC-DC-GND switch to DC; to dis-
play only the ac component of a signal, set the switch to AC,
In the AC position of the switch, the de component of the
signal is blocked by a capacitor in the input circuit. The
lower frequency limit at —3db for the system is about 15 cps
when the INPUT switch is set to AC. Therefore, some low-
Type P6034 or 50-chm GR 50-ohm BNC
No adjustment required. Type P6035 2X T Atten- Termination
Probe vator {017- (011-049)
GR Type 874-K GR-to-BNC
Probe Coupling Capacitor Adapter
Tip (017-028) (017-064)
vad
TO TEST POINT rN >>— ->>—- >> +> TO TYPE 82
IN DEVICE UNDER >: INPUT CONNEC-
TEST TOR
Probe Use ac coupling, 50-ohm 50-ohm
Resistor when needed, to Extension Extension
protect probe re- Cable Cable
Use, if needed, to reduce reflection
by a factor of 4. Doubles probe
Fig. 2-6. Use of the Type P6034 or the Type P6035 Probe with the Type 82.
2-6