A.2. Oscilloscope 743
the time information. In this way, the value of the signal at any instant in time can
be obtained.
Both vertical and horizontal deflections are carried out by letting the electron
beam pass through electric potentials. The potential difference at the vertical plates
depends on the input signal voltage and the range set by the user. The range is
usually set by a knob provided on the front panel and is available in units of volts per
division, where division is a large grid division on the screen. Typical available range
set points ordeflection coefficientsare 10mV /div,20mV /div,50mV /div,0. 1 V/div,
0. 2 V/div, ... , 5. 0 V/div. Some oscilloscopes also provide fine range settings between
such coarse steps. The deflection coefficient set by the user can then be simply
multiplied by the height of the signal trace to determine the absolute voltage at that
time.
In almost all of the modern oscilloscopes there is also a possibility of letting the
system adjust the deflection coefficient automatically. In this case it is not possible
to determine the absolute voltage level. This setting is useful when determination of
absolute voltage is not required and the signal amplitude has unexpected variations.
All oscilloscopes have at least two input channels with BNC connectors and are
able to display two traces at the same time or alternately. These input channels of
most oscilloscopes are terminated with 1MΩ resistor in parallel with 20pFcapacitor.
This high input impedance ensures least distortion of the input signal for proper
acquisition. However some of the high frequency oscilloscopes are equipped with
50Ω impedance inputs.
Most oscilloscopes provide different ways in which multiple channels can be dis-
played on the screen.
ALT Mo de: In thisalternatemode the display is switched between channels
after each sweep.CHOP Mode:In this mode the switching between channels is done in a small
time step instead of the full sweep as in ALT mode. The alternating frequency
can be very high (500kHzor more), creating the illusion that both channels
are being simultaneously displayed.
ADD Mode:This mode can be used when the instantaneous sum of the two
channels is required. The resulting waveform is simply the sum of the two
waveforms at each instant in time.So far we have only discussed how the amplitude of the signal is displayed. We
mentioned waveform but didn’t explain how the time component of the waveform
is generated. This is done by the horizontal deflection system, which is used to
deflect the electron beam in horizontal direction, thus producing a chart of the signal
amplitude with respect to time. There are two main components of this system: a
sweep generator and an amplifier. The purpose of the sweep generator is to start
generating a sawtooth wave as soon as it receives a trigger initiated by the trigger
system, which we will shortly discuss. The sawtooth wave is then amplified and
connected to the horizontal deflection plates allowing the electron beam to sweep
from left to right. The shape of the sawtooth waveform ensures that rate of this
sweep remains constant till the beam reaches the end of the display. The beam is
then switched off till the trace comes back to the start position on the left, as set
by the user.