Tektronix 422, Instruction Manual

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Operating Instructions—Type 422 AC-DC
leads and the BNC to binding post adapter (supplied acces­
sory). This coupling method works best fo r signals below
about one kilohertz and deflection factors above one v o lt/
division. When this coupling method is used, establish a
common ground between the Type 422 and the equipment
under test. A ttem p t to position the leads away from any
source o f interference to avoid errors in the display. If in t­
erference is excessive w ith unshielded leads, use a coaxial
cable or a probe.
Loading Effect of Type 422
As nearly as possible, simulate actual operating condi­
tions in the equipment under test. Otherwise, the equip­
ment under test may not produce a normal signal. The 10X
attenuator and field effect transistor probes mentioned pre­
viously offer the least circuit loading. See the probe instruc­
tion manual fo r loading characteristics of the probes.
When the signal is coupled directly to the input o f the
Type 422, the input impedance is about one megohm paral­
leled by about 30 pF. When the signal is coupled to the
input through a coaxial cable, the effective input capaci­
tance is increased. The actual input capacitance depends
upon the type and length o f cable used and the frequency
of the signal.
Ground Considerations
Reliable signal measurements cannot be made unless
both the oscilloscope and the unit under test are connected
together by a common reference (ground) lead in addition
to the signal lead or probe. Although the three-wire AC
power cord provides a common connection when used w ith
equipment w ith similar power cords, the ground loop pro­
duced may make accurate measurements impossible. The
ground straps supplied w ith the probes provide an adequate
ground. The shield of a coaxial cable provides a common
ground when connected between two coaxial connectors
(or w ith suitable adapters to provide a common ground).
When using unshielded signal leads, a common ground lead
should be connected from the Type 422 chassis to the chas­
sis of the equipment under test. Additional ground consid­
erations must be made fo r safety reasons to provide an
earth (ground) connection to this instrument at all times
when operating from external DC or internal batteries (see
Operating Voltage in this section fo r details).
Input Coupling
The Channel 1 and 2 Input Coupling switches allow a
choice o f input coupling methods. The type of display de­
sired and the applied signal w ill determine the coupling to
use.
The DC Coupling position can be used fo r most applica­
tions. This position allows measurement o f the DC compo­
nent o f a signal. It must also be used to display signals
below about 20 hertz (two hertz w ith a 10X probe) as they
w ill be attenuated in the AC position.
In the AC Coupling position, the DC component o f the
signal is blocked by a capacitor in the input circuit. The
low-frequency response in the AC position is about tw o
hertz (—3 dB point). Therefore, some low-frequency atten­
uation can be expected near this frequency lim it. Attenua­
tion in the form o f waveform t ilt w ill also appear in square
waves which have low-frequency components. The AC
coupling position provides the best display of signals w ith a
DC component which is much larger than the AC compo­
nents.
The GND position provides a ground reference at the
input of the Type 422 w ith o u t the need to externally
ground the INPUT connectors. The signal applied to the
input connector is internally disconnected, but not ground­
ed, and the input circuit is held at ground potential.
Deflection Factor
The amount of vertical deflection produced by a signal is
determined by the signal amplitude, the attenuation factor
o f the probe (if used), the setting o f the V O LTS /D IV
switch and the setting of the V A R IA B LE V O LT S /D IV con­
tro l. The calibrated deflection factors indicated by the
V O LTS/D IV switches apply only when the V A R IA B LE
V O LTS/D IV control is set to the CAL position.
The V A R IA B LE VO LTS/D IV controls provide contin­
uously variable (uncalibrated) vertical deflection factors be­
tween the calibrated settings of the VO LTS /D IV switches.
The V A R IA B LE control extends the maximum vertical de­
flection factor of the Type 422 to at least 50 volts/division
(20 volts position).
When the X I 0 GAIN AC switch in Channel 2 is pulled
out, the deflection factor indicated by the V O LTS /D IV
switch must be divided by 10 to obtain the actual deflec­
tion factor.
Channel 2 X I 0 Gain
The X I 0 GAIN AC switch provides 10 times gain fo r the
Channel 2 amplifier to extend the minimum deflection fa c­
to r to 0.001 volt/division in the .01 position o f the Channel
2 VO LTS/D IV switch. The DC component of the applied
signal is not amplified. For best results when using the X I 0
GAIN AC switch, the Channel 2 Input Coupling switch
should be set to the AC position.
Dual-Trace Operation
Alternate Mode. The A L T position o f the vertical Mode
switch produces a display which alternates between Chan­
nel 1 and 2 w ith each sweep of the CRT. Although the A L T
mode can be used at all sweep rates, the CFIOPPED mode
provides a more satisfactory display at sweep rates below
about 0.5 millisecond/division. A t these slow sweep rates,
alternate mode switching becomes visually perceptible.
Proper internal triggering in the A L T mode can be ob­
tained in either the CH 1 & 2 or the CH 1 position o f the
TRIGGERING Source switch. When in the CH 1 & 2 posi­
tion, the sweep is triggered from the signal on each channel.
This provides a stable display of tw o unrelated signals, but
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