Transcript Oscilloscope 3 (vertical deflection)

Vertical deflection system:
* The function of the 
deflection system provides
an amplified signal of the
proper level to derive the
vertical deflection plates
without distorting the signal
* A general type
oscilloscope can accept
as low as a few
millivolts/cm of
deflection up to hundreds
of volts using the built in
attenuator and external
probes.
Here ,the attenuator
sets the sensitivity of
oscilloscope in the
common sequence
(1-2-5)
* the input attenuator provides the correct
sequence attenuation while maintaining a
constant input impedance ,
as well as maintaining the input
impedance and attenuation over the
frequency range for which the
oscilloscope is designed
* Types of
attenuators ::
A) •
Uncompen
sated
attenuator
This type of attenuators (resistive
divider att.)
is connected to an amplifier with 10PF
input capacitors
the impedance changes based on the
setting of attenuator ,,therefore, the RC
time constant and thus the frequency
response of the amplifier are depending
on the setting of the attenuator,
B)
Compensated
attenuator :
This kind has both resistance and
capacitance voltage divider, noting that
the capacitor voltage divider improves the
high frequency response of the attenuator
Practically, all oscilloscopes
provide suitable input coupling
capacitors
Note: this type of attenuator is
needed to provide measurements of
DC signal that are maybe viewed in
the presence of the high DC voltage
* The input impedance of the oscilloscope is in
general 1M ohm shunted with(10PF to 30PF)
* It is usually desirable (for high frequency
oscilloscope)to have an input capacitance much less
than(20-30)PF and this achieved by the use of att.
probes
In the base of the probe at the oscilloscope
connector, there is an adjustable capacitor. This
capacitor is adjusted so that the ratio of the
shunt capacitance to the series capacitance is
exactly 10 to 1.
 The attenuator probe (usually called 10-1 probe)
provides approximately a 10-1 reduction in the
input capacitance. If the setting of the
compensation is incorrect, then the result will be
seen by observing the rise-time-pulse.

Horizontal
system
consist of
:
Time Base
Generator (T.B)
Triggering
Circuit
(T.C)
Horizontal
Amplifier (H.A)
H.D
T.C
T.B
H.A
The time base controls the rate
of which the beam is scanned
The triggering circuit insures that the
horizontal sweep starts at the same
point of the vertical input signal
The sweep generator uses the
changing characteristics of the
capacitor to generate a linear rise
time voltage . The rate of the
voltage is given as I/C
* This rate can be adjusted by the current (I)
or the capacitance (C)
** The sweep generator is capable of
low sweeps (20 µ s/Div – 50 ns/Div)
*** This sweep does not start until a triggering pulse is
initiated
Most Oscilloscopes have a built in
oscillator to trigger the beam when
no input signal is present , so that
one would know the base time,,
Some oscilloscopes have two time
base generator where one would
trigger before the other
This advantage is applied
in a situation where a signal
with longer period is begun
viewed but a small part of
the signal is to be analyzed
…
* The time
base is used
to sweep the
electrons at a
constant
sweep from
left to right
across the
screen
** And then
quickly
returns the
beam to the
left in time
to begin in
the next
sweep
*** The time
base can be
adjusted to
the sweep
time to the
period of the
signal
The sweep does not usually trigger
for each cycle of the vertical input
unless the sweep pulse roll off time
is less than +ve period of the input
…
Oscilloscope Probes
*The goal of the
oscilloscope is to
display the signal
as a function of
time
**To make the
oscilloscope
more useful
device,, different
types of probes
maybe used
***(i.e.
1-1 ratio
,10-1
ratio)
Current Probe :
**This gives the oscilloscope the
ability to measure the magnitude of the
current with a frequency response from
DC(0Hz)- 50 MHz .
*This device can be
clamped around a wire
carrying an electrical
current without any
physical connection with
the probe.
The current sensor consists of a conventional
transformer and a Hall-effect device.
Alternating current in the wire will 
induce voltage in the secondary winding
by the transformer , any direct current
will not appear at the current transformer
secondary circuit, the direct current
passing through the wire will cause the
magnetic flux in the core to increase will
effect the permeability of the material
used for the core (this is undesirable)
To over come this
problem:
,
a Hall-effect
sensor is used to
provide a frequency
response to the DC
A feedback system is arranged with an 
amplifier such that any magnetic field
present in the Hall-effect device will
cause the current to be induced in to the
secondary winding to counteract the
magnetic field induced in the wire
being used in measurement.
Frequency determination:
Δ :
Difference
F=1/T
• ( ΔT/T)
• *360=ɵ
V(t)= Asin(wt)
=Asin(wt+ ɵ)
2- determination of modulation
characteristics :
The oscilloscope can be used to measure the
amount of amplitude modulation applied to a
carrier for adjusting and trouble-shooting
amplitude –modulated transmitters
To display the carrier the oscilloscope
must be capable of covering the carrier
frequency of the transmitter the
horizontal sweep has only to cover the
modulation frequency.
The modulation percentage
can be determined from the
wave form & is calculated as:
Modulation%=
• ((A-B)/(A+B))
• *100%
Where; A: is the
peak of the
modulate
envelope & (B)
is the minimum.
Note : In no case
,,the amplitude
modulation of the
carrier wave
exceed 100% the
reason this should
be avoided is
based on ((
carrier-cut-off))
The carrier in
this case is
actually
completely
gone,,