HP 120A - Horizontal Amplifier; Calibrator; AC or DC Coupling

Sect, n Page 3
Select the two centimeter portion to be magnified
with the HORIZONTAL POSITION control.
Trigger Level - Normally, a trace will be displayed
in the absence of a signal, since the oscilloscope is
triggered automatically. However, you may select
the point of triggering and stop the AUTOMATIC
triggering with the TRIGGER LEVEL control. Move
this front panel screwdriver control clockwise away
from the AUTOMATIC switch position (extreme coun 
terclockwise) only for special triggering. In the AU
TOMATIC position the sweep is self-triggered in the
absence of a triggering signal. When a signal is fed
in, the signal controls the triggering action. In the
AUTOMATIC position the trigger circuit is adjusted
to optimum setting to trigger on the majority of wave 
forms and at a zero trigger level. As you .rotate the
control clockwise from the AUTOMATIC switch po 
sition the automatic sweep feature will stop. Further
rotation erf this control permits you to adjust the trig 
gering level of the pattern, within limits.
2-6 HORIZONTAL AMPLIFIER
The SWEEP TIME - HOR. SENS, switch also con -
trols the sensitivity of the horizontal amplifier when
it is used with an external input. The three sensiti 
vity ranges of the horizontal amplifier are . 1, 1, and
10 VOLTS/CM. The sensitivity of the amplifier may
be increased to 100 VOLTS/CM by means of the VER 
NIER control.
Even though the circuitry of the vertical and horizon 
tal amplifiers is not identical, the bandshapes and
phase characteristics are identical, at least to well
above 100 kc. As a result the instrument is rated as
having not more than 2° differential phase shifts be 
low 100 kc and is thus well suited to phase difference
measurements. In fact, because the horizontal ampli 
fier has basically an even wider response than the
vertical, an adjustment (C107, Phase Adjustment) is
provided in the horizontal amplifier so that phase
differences can, if desired, be minimized at indivi 
dual frequencies above 100 kc. The adjustment con 
sists of a small variable capacitor which acts to set
the first high-frequency corner of the horizontal am 
plifier for optimum agreement with that of the verti 
cal. At the factory this adjustment is set for opti 
mum agreement at 100 kc.
2-7 CALIBRATOR
An internal square-wave calibrator with an accuracy
of ±2% and a nominal frequency of 400 cps is provided
for checking amplifier calibration accuracy. The cal 
ibrator is connected internally to the input of the ver 
tical amplifier when the VERT. SENSITIVITY switch
is placed in the CAL. position. When in the CAL. po 
sition a 60 millivolt square wave is fed into the ver 
tical amplifier input for calibration purposes. Be 
cause of the inherent stability and medium sensitivi 
ty of the horizontal amplifier, no provision is made
for internal connection between that amplifier and the
calibrator. Horizontal amplifier calibration can be
checked easily, however, since an external signal can
be measured on the vertical amplifier and then applied
to the horizontal as a check. Little adjustment of the
horizontal amplifier is necessary.
In either automatic or trigger operation the sweep
circuits wiU trigger from signals over a range from
600 kc to below 5 cycles (50 cps for automatic opera 
tion). Between 10 cps (50 cps for automatic) and 200
kc the minimum deflection needed for triggering from
displayed waveforms is 0. 5 cm peak-to-peak. Be 
yond this range an increase in this minimum will
occur, but at 600 kc, for example, triggering will
usually occur on waveforms that give 2 cm deflec 
tion. For external trigger signals a minimum am 
plitude of 2-1/2 volts peak-to-peak is required for
the minimum frequency to 200 kc ranges.
2-8 EXTERNAL SYNCHRONIZATION CONTROLS
Switch the SYNCHRONIZATION control to the EX 
TERNAL position. Feed in the external synchro 
nization signal (2. 5 volts or more peak-to-peak)
into the horizontal input terminals at the lower right
corner of the front panel. The external synchro 
nization signal is always ac coupled and synchro 
nizes on the negative slope of the signal. When using
the trigger level control the trace will trigger on the
negative slope of signals within the range of +10 and
-10 volts, approximately (see Figure 2-2).
Figure 2-2. Triggering
2-9 AC OR DC COUPLING
AC coupling permits high gain to be employed with 