Oscilloscope

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