HP 120A - Page 17

Sect. II Page 1
SECTION II
OPERATING INSTRUCTIONS
2-1 CONTROLS AND TERMINALS
The panel of the Model 120A has been designed to be
as self-explanitory as possible. You may be able to
operate the 120A without instructions if you have op 
erated oscilloscopes before. However, a complete
description of the functions of the controls follows:
2-2 POWER AND CRT CONTROLS
Power and Intensity Control - Turn the oscillos 
cope on by turning this control just out of the ex 
treme counter-clockwise position. After the os 
cilloscope has warmed up, adjust the intensity of the
trace by turning this control further. Rotating this
control clockwise increases the intensity. In most
cases the oscilloscope will show a trace on the screen
with only this adjustment. If no trace shows put the
controls exactly as shown of the facing two color
fold-out sheet, and follow the numbered steps.
Scale Light - Rotate this control clockwise to in 
crease the scale illumination.
Focus - Rotate this control for optimum sharpness
of the trace.
Vertical Position - Rotate this control clockwise to
move the trace upward.
Horizontal Position - Rotate this control clockwise
to move the trace to the right.
2-3 VERTICAL PRESENTATION CONTROLS
Vertical Sensitivity - This six position switch con 
trols the sensitivity of the vertical presentation. It
has six positions marked OFF, 10 and 1 VOLTS/CM,
100 and 10 MILLIVOLTS/CM and CALIBRATOR, re 
spectively. The 10 and 1 VOLTS/CM, and the 100 and
10 MILLIVOLTS/CM positions adjust the sensitivity
of the vertical presentation.
In the CALIBRATOR position a square wave of 60 mv
amplitude is applied to the vertical input for calibra 
tion purposes. When in this position the pattern should
be adjusted for a height of six centimeters with the
VERTICAL GAIN control (make sure VERNIER is in
CALIBRATED). Use the VERNIER control concen 
tric with the VERTICAL SENSITIVITY control, for
continuous adjustment between ranges. Set the VER 
NIER control in the CALIBRATED position for the
engraved sensitivity markings to be accurate (±5%).
Each VERTICAL SENSITIVITY position can be re 
duced in sensitivity by a known amount by the use of
the calibration square wave. Switch the VERTICAL
SENSITIVITY switch to the CALIBRATOR position and
the VERNIER to the CALIBRATED position. The
square wave pattern should be six centimeters in
height. However, this sensitivity may be reduced to
a known fraction of the engraved sensitivities with
the VERNIER control. For example, if you wish the
sensitivity to be one-half of that shown on the panel
markings set the VERNIER control so that you get a
pattern height of only three centimeters. As long as
the VERNIER is left in this position the sensitivities
should be multiplied by two to get the true readings .
Note that the original calibration can be recovered
simply by returning the VERNIER to its CALIBRATED
position.
For balanced or differential input (10 mv/cm) remove
the shorting jumper between the bottom red terminal
and the black (ground) terminal. Feed the balanced
signals into the red terminals and the common re 
turn to the black (ground) terminal. Balanced input
wiH'be found useful in application where it is desir 
ed to amplify the (out-of-phase) differential signal
applied to both grids and attenuate the (in-phase)
common-mode signal (see Figure 2-15). Thus you can
reject to a considerable degree any common-mode
signal while at the same time passing and amplify 
ing the differential signal. If there is any pickup of
hum or noise in the equipment under test or the test
leads, such extraneous signals will be attenuated
while the desired signal will be amplified.