Transcript BASIC INSTRUMENTS

BASIC INSTRUMENTS
- OSCILLOSCOPES
•
An oscilloscope is an
instrument that allows
observation of constantly
varying signal voltages,
usually as a twodimensional graph of one
or more electrical potential
differences using the
vertical or 'Y' axis, plotted
as a function of time,
(horizontal or 'x' axis).
1. TYPES OF WAVEFORMS
2. WAVEFORM MEASUREMENTS
3. OSCILLOSCOPE CONTROLS
• Vertical controls
• Horizontal controls
• X-Y Mode
4. OSCILLOSCOPE PROBES
•
•
Passive Probes
Active Probes
TYPES OF
WAVEFORMS
Triangular or Sawtooth Wave
Sinewave
Pulses
Square or Rectangular Wave
A sudden change in the amplitude of
the signal and back to the previous
value
WAVEFORM
MEASUREMENS
AMPLITUDE
Measured from the maximum peak to the minimum peak of the voltage
(peak-to-peak voltage)
Can also be useful to measure the RMS voltage
PERIOD AND FREQUENCY
A period, T : amount of time the signal takes to complete one cycle and
it is expressed in seconds
A frequency: number of times the signal repeats itself in one unit of
time (1/T) unit is Hertz (Hz)
PHASE SHIFT
When you have two similar waveforms, the oscilloscope can measure
the time difference between them. This is measured in degrees or
radians.
RISE TIME
Time for the signal to go from 10% to 90% of its peak-to-peak
voltage. But the percentage can be changed arbitrarily
FALL TIME
Similarly, the time for the signal to go from 90% to 10% of its peakto-peak voltage.
Determine the pulse amplitude, frequency, rise time and fall time
of the waveform in the figure below
pulse amplitude = 4 x 2 V = 8V
T = 5.6 x 5 μs /div = 28 μs
Frequency, f = 1/T = 1/28 μs = 35.7 kHz
rise time, tr = (0.5 ) x ( 5 μs /div ) = 2.5 μs
Find the phase difference between the two sine waves as shown below.
From the figure, observe that 1 complete cycle takes 8 divisions = 360
One division = 45 
The phase difference = 1.4 x 45  = 63
OSCILLOSCOPE
CONTROLS
VERTICAL CONTROLS
Position: Controls vertical positioning of oscilloscope
display.
DC and AC Coupling:
DC coupling is the most used position on a scope because it
allows the scope to display both AC and DC voltage signals
present in the circuit.
When AC coupling is selected, a capacitor is placed into the
meter lead circuit, which effectively blocks all DC voltage
signals but allows the AC portion of the signal to pass and be
displayed
REF: http://hackaday.com/2014/11/26/scope-noob-probing-alternating-current/
2 V DC offset
0 V DC offset
HORIZONTAL CONTROLS
Position: Controls horizontal positioning of oscilloscope
display.
Controls the time scale and position.
50 Hz  0.02 s
100 Hz  0.01 s
0.02 / 4 = 5 ms / div
0.01 / 8 = 1.25 ms / div
X-Y MODE
X-Y Mode is generally used to find out what is the phase
difference between two waveforms.
This measurement technique involves in putting one signal
into the vertical system as usual and then another signal into
the horizontal system
The waveform that results from this arrangement is called a
Lissajous pattern
Only focus on
signals with same
ratio frequency
2. At 45
1. At 0
3. At 90
4. At 135
5. At 180
Sin  = Y1 / Y2 = X1 / X2
For example, a circle shape:
Y2 = Y1
Hence, sin-1 (Y1 / Y2 ) = 
sin -1 (1) = 90 
X2 = X1
What if the amplitude of one of the signal is higher but the
frequency between them stays the same?
Y2 = Y1
X2 = X1
The phase difference is still 90
But the ratio between Y and X
equal to ratio of amplitudes
between the two waveforms
REF: http://lissajousfigures.tk/
OSCILLOSCOPE
PROBES
Test leads are likely to pick up interference or noise, so they are not
suitable for low level signals. Furthermore, the leads have a high
inductance, so they are not suitable for high frequencies.
So to solve this, we use coaxial cables
The coaxial cable consists of an insulated central
conductor surrounded by a braided circular
conductor which is covered by an outer layer of
insulation
The central conductor carries the input signal,
and the circular conductor is grounded so that it
acts as a screen to help prevent unwanted
signals being picked up by the oscilloscope input.
Using the coaxial cable, oscilloscope probes may be categorized into two
main types, and they can fall into one of two main areas:
Passive oscilloscope probes:
This type of probe is the one that is in most widespread use.
It only includes passive elements and may provide 1:1, i.e. straight
through connectivity from the point under test, to the scope input.
Other types may provide a defined degree of attenuation.
1X scope probe ( 1: 1 attenuation ratio )
• The input impedance of the oscilloscope
at the front panel is typically Ri in parallel
with Ci.
• The coaxial cable connecting the probe to
the oscilloscope has a capacitance (Ccc)
(typically 90 -100 pF) which can overload
a high-frequency signal source.
• Hence, this probe is suitable to be used
for signals from 6 MHz to 10 MHz
• The value of Ri and Ci can be found at the front panel.
•
Value of Ri is normally 1 M and value of Ci ranges from 15 – 30 pF
At high frequency or high impedance signal, loading effect
occurs.
Basically what happens is that the output is no longer a
correct representation of the input waveform
The total impedance offered by the coaxial cable and the
oscilloscope input should always be much larger than the signal
source impedance. When this is not the case, the signal is
attenuated and phase shifted when connected to the oscilloscope.
Vin / Vout
0.707
1
f3db
10X scope probe ( 10: 1 attenuation ratio )
Hence, the 10X scope probe is introduced in order to control the attenuation
factor.
12.2 pF
90 pF
Time constant,
RC = C1 x 9 M
1.1 x 10-4 = C1 x 9 M
C1 = 12. 2 pF
20 pF
Time constant,
RC = (20 + 90) pF x 1 M
RC = 1.1 x 10-4 = 110 ms
Example
Xcc = 50 
Total impedance = 450 
Calculate the capacitance of the adjustable capacitor if the capacitance
of the coaxial cable is 106 pF and given that the reactance is 50 
Answer = 11.8 pF
It is important that every probe be correctly adjusted when it is first
connected for use with a particular oscilloscope - CALIBRATION
Value of adjustable capacitor is
too low
Value of adjustable capacitor is
too high
It is generally accepted that for generalpurpose mid-to-low-frequency (less than
around 500-MHz) measurements, the
10:1 probe is the most suitable option.
The factor of 10 loss in voltage is not a problem as long as the
voltage that is being measured is not so small that dividing it by
10 makes it unreadable by the scope.
This means that the scope's sensitivity and the signal voltage may
be factors in deciding whether to use a 10:1 probe.