Introduction to oscilloscopes
• Triggering
• 10x probes
• DC coupling vs AC coupling
• X-Y mode
Triggering
Trigger Slope:
Trigger mode:
Positive/negative: rising
Normal: stops if cannot
or falling edge
Trigger source:
Ch 1
Ch 2
Ext/Ext 5
Line:
detect trigger
Auto: keeps going even if
cannot detect trigger
Single: run/stop button
Triggering
y = sin(t)+0.4cos(10t)
1.5
ch1
1
0.5
A
B
0
-0.5
-1
-1.5
0
2
The “sync” output of
function generator
provides a clean
square wave at the
same frequency as the
output
2
4
6
8
10
6
8
10
ext
1.5
1
0.5
0
-0.5
-1
0
2
4
Importance of external trigger:
recovering signal from noise by averaging
(a)
40
Signal(mV)
20
Blue = a single trace
0
Red = 262144 averages
-20
-40
0
0.5
1
1.5
2
Time(mS)
(b)
4
Signal(mV)
3
262,144 averages
2
1
0
0
← initial slope
0.5
1
Time(mS)
1.5
2
Coupling: DC vs AC
DC coupled
AC coupled
Removes all DC information
To observe small AC on top of large DC
oscilloscope
oscilloscope
zc = 1/jωC
f3db = 1/2πRC
Coupling: DC vs AC
Low frequency waveforms can be severely distorted by the high pass filter
AC coupled
DC coupled
1
1
0.5
0.5
0
0
-0.5
-0.5
-1
-1
-1.5
-1.5
-0.1
-0.05
80 Hz
1.5
10 Hz
voltage(V)
volta ge(V)
1.5
0
Time(s )
0.05
0.1
-0.1
-0.05
0
Time(s )
0.05
0.1
Oscilloscope probes
circuit
oscilloscope
30 pF/foot
R1
Vout
Ccable
Vac
For scope:
Cin
Rin
Rin = 1 M
Cin ~ 20 pF
R1
Z1
Vac
Rin
Cin + Ccable
Z2
Z1 and Z2 are
frequency
dependent and
changes for
each circuit
Oscilloscope impedance and stray capacitance can load the circuit
R1
Vac
R1
ω=0
Vac
Cin + Ccable
Rin
ω=∞
Rin
Vac
R1
Example:
Cin + Ccable R1 = 10k
C = 50 pF
f3db = 1/2πRC
= 320 kHz
dB
Low pass filter
The signal is distorted!
log(f)
Advantages of 10x probe
1. Input impedance 10 times larger (reduce loading)
2. Frequency independent (almost, if tuned correctly)
Resistive part:
R1
Vac
oscilloscope
9M
probe
1M
Cstray
For dc and low frequencies,
the 10x probe act as 10:1
divider.
= Ccable + Cin
~50pF
Input resistance is 10x higher.
How to eliminate frequency dependence?
9M
Add capacitor to probe
1M
Cstray
~50pF
Consider the resistive and capacitive
parts separately
9M
oscilloscope
1M
probe
Cstray
~50pF
9M
9Xstray
1M
Both dividers are 1:10, independent of frequency
Xstray
They give the same output voltage
Remember
Quiz 1:
Xstray
Xstray+ 9Xstray
1/jω Cstray
1/jω Cstray+ 9/jωCstray
Both dividers give the same output voltage Î can join the output
Xc =1/jωC
9M
9Xstray
9Xstray
9M
Cprobe ~ 50pF/9
= 5.6 pF
1M
Xstray
1M
Xstray
probe
9M
oscilloscope
1M
Cprobe ~5.6pF
Cstray ~50pF
Cstray
~50pF
10x probe divides signal by
10. To display the actual
signal, we need to tell the
scope to multiply its
measurement by 10.
Tuning the probe
The oscilloscope provides a square wave output
on its front panel, labeled as “probe adjust”
A square wave can be Fourier decomposed into a
sum of many frequencies.
If the probe and scope attenuates all frequencies
by 10, we should get back a square wave.
Tune probe capacitance until the scope shows
a good square wave.
Display:
YT: displays Ch1 and/or Ch2 as a function of time
XY: displays Ch1 as a function of Ch 2
x(t ) = A cos(ω1t − δ1 )
y (t ) = B cos(ω2t − δ 2 )
Example:
ω1 = ω2, δ1 = δ2
2
2 1
1 0
0
-1
XY
YT
XY
-1
-2
y = B/A x
YT
-2
0
0
2
2
4
4
6
6
8
8
10
10
2
ω1 = ω2, δ2 = 0, δ1 = π/2
x(t ) = A cos(ω1t )
y (t ) = B sin(ω1t )
1
0
-1
-2
0
2
4
6
8
10
Lissajous Curves
x(t ) = A cos(ω1t − δ1 )
y (t ) = B cos(ω2t − δ 2 )
Checklist for oscilloscope operation
1. Make sure probe compensation is set to the correct
value (1x, 10x)
2. If you cannot get signal on screen, press “autoscale”
3. Check DC/AC coupling
4. Check trigger source