HP 120A - Page 18

Sect. II Page 2
Vertical Gain - Adjust the vertical gain with this
control (see instructions for the VERTICAL SENSI 
TIVITY control).
Vertical DC Balance - Adjust this control so that the
trace does not move when the VERTICAL SENSITI 
VITY VERNIER is rotated.
AC - DC Switch - Select either ac or de input cou 
pling for the vertical amplifier with this switch.
Although vertical amplifier response is rated as being
down no more than 3 db at 200 kc, this rating is in 
tentionally conservative. For 20 instruments the av 
erage response at 200 kc vas 86. 6% or -11/4 db and
the lowest response of the 20 was 81%. From these
data it can be seen that the instrument is useable in
many, if not most, cases to view waveforms up to at
least 500 kc. The instrument thus has considerably
more flexibility than might be inplied by simply stat 
ing that the 3 db point is rated at 200 kc. Coupled
with this response characteristic is the fact that the
sweep circuits are designed to trigger from frequen 
cies up to 600 kc.
2-4 SYNCHRONIZATION
The Model 120A oscilloscope is an oscillosynchros-
cope. That is, it may be adjusted to synchronize or
start the sweep in reference to another signal. This
reference or synchronizing signal may be the input
signal (set SYNC control at INT+ or INT-), the pow 
er line frequency (SYNC set to LINE) or from an ex 
ternal signal (SYNC control set to EXT. )
In addition, in the internal synchronization positions,
the slope and trigger level point may be adjusted.
First, the polarity of the presentation on the screen
of the cathode-ray tube may be adjusted by turning
the SYNC switch to start the sweep on either a pos 
itive going (INT+) or a negative going (INT-) part of
the waveform. Then, if automatic sweep triggering
at a zero voltage level (usual case) is not desired,
rotate the TRIGGER LEVEL control out of the AUTO
position. The automatic sweep feature will stop. Fur 
ther rotation of the control will adjust the trigger lev 
el point on the incoming waveform at which the sweep
will start
The 120A features automatic sweep. This special cir 
cuit presents a trace on the face of the cathode-ray
tube in the absence of an input. Thus, even when a
synchronizing signal is absent the Trigger Generator
will trigger itself and present a sweep on the face of
the cathode-ray tube. On an ordinary oscilloscope
some adjustment of controls is usually necessary to
trigger the sweep generator and present a picture on
the screen, On the 120A, however, all adjustments
may be made before the signal is fed into the unit,
As soon as a signal is fed in the input the signal takes
control of the sweep and triggers the sweep generator
at the frequency of the input signal.
A four position (INT+, INT-, LIKE, and EXT) switch
which determines the type of signal used for synchro 
nization. Internal triggering can be accomplished
from a line voltage (LINE) or internally (INT + or -)
from an applied vertical input sigial of sufficient am -
plitude to produce a vertical deflection of . 5 cm or
more on the screen.
Sweep Time - Horizontal Sensitivity -
This eighteen position switch controls the sweep time
and the horizontal sensitivity of the horizontal pre 
sentation. Fifteen positions control the internal sweep
speed and the remaining three positions control the
horizontal sensitivity to an external input. Set to the
SWEEP TIME section to view a signal with an internal
sweep. Select the proper sweep speed with the SWEEP
TIME switch which controls the sweep time per centi 
meter in fifteen calibrated ranges in a 1, 2, 5, 10 . .
sequence from 5 microseconds/centimeter to 200
milliseconds/centimeter. Use the concentric VER 
NIER control to provide continuous adjustment of
sweep speeds between ranges by reducing the sweep
speed. Set the VERNIER in the full clockwise, CAL 
IBRATED, position to obtain ±5% accuracy with the
engraved markings.
The slowest calibrated sweep provided on the in 
strument is 1/5 second/cm which can be extended
with the SWEEP TIME VERNIER to at least 1/2 sec 
ond/cm or 5 seconds for the full 10 cm sweep. If
need occurs for a slower sweep, the range can be
extended indefinitely by connecting a pair of external
condensers into the sweep circuits. Refer to the op 
erating plate, Figure 2-14, to find the placement of
these condensers in the circuit. The condenser men 
tioned in step 1 must be a high-quality type with high
leakage resistance and good recovery characteristics .
No loss of sweep linearity occurs if a capacitor with
these specifications is used. The second capacitor is
connected to the hold-off circuits and does not re 
quire a special condenser.
Sweep Expand - A two position (XI - X 5) switch.
Set this switch in the XI position to view the pattern
without magnification. Switch to the X5 position to
view a 2 centimeter portion of the pattern magnified
five times. Also, a sweep speed of 1 microsecond/sec
may be obtained by means of the X5 expansion of the
sweep on the fastest range.
2-5 SYNC. CONTROLS