Asst. Prof. Dr. Prapun Suksompongprapun@siit.tu.ac.th

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Basic Elec. Engr. Lab ECS 204

Lab 5• RC Circuit with Voltage Step Input• Frequency Response of Series RLC Circuit • AC vs DC modes of the Oscilloscope• AC vs DC modes of the DMM• Function generator: Offset

Triggering

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Triggering

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Here, originally, the trigger level was set too high at around 5V so the trigger mechanism can’t “see” the signal. We then use the trigger level knob to adjust the trigger level to around 2.8V which stabilizes the display.

Triggering

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Triggering

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Trigger controls

50 inside the function generator

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Set the (open-circuit) voltage across the output of the signal generator at 6Vp-p.

Connect the generator output across a 100Ω resistor.

Measure the voltage across the generator again.

When nothing is connected across the function generator’s output, Vo = Voc.

Here, Vo is set to be 6 Vp-p. So, Voc is also 6 Vp-p.

When a 100 resistor is connected across the function generator’s output, Voc is split between the 50 inside and the 100.

If you did not adjust the function generator, then Voc is

still at 6 Vp-p and Vo 6 4Vp-p.

50 inside the function generator

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Ch 1: VG

Ch 2: V2

Ch 2 GND

Ch 1 GND

R1

R2

In lab 4, ….

50 inside the function generator

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Ch 1: VG

Ch 2: V2

Ch 2 GND

Ch 1 GND

R1

R2

In lab 4, ….

All ground clips should be together

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Ch 1: VG

Ch 2: V2

Ch 2 GND

Ch 1 GND

R1

R2

Ex: Wrong measurement of V1

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Ch 2 GND

Ch 1 GND

Ch 1: VG

Ch 2: V1?R1

R2

An attempt to use CH2 to measure V1.

Ex: Wrong measurement of V1

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Ch 2 GND

Ch 1 GND

Ch 1: VG

Ch 2: V1?R1

R2

Ex: Correct measurement of V1

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Ch 1

Ch 2

Ch 2 GND

Ch 1 GND

R1

R2

CH1 CH2V VUse to measure the voltage across any pair of nodes in the circuit

while still keeping the ground clips together. Differential Measurement

Part A: RC Circuit

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1k

0.1 F

Voltage across the generator output.

Voltage across the capacitor.

Square wave!Not sinusoid!

RC Circuit with Voltage Step Input

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0out inout

v t v tdC v tdt R

Assume the input voltage vin(t) is fixed at a particular value VS

from time t1 to t2.

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1 1 2,t t

out S out Sv t V v t V e t t t

RC

in

out

(DE)

iCiC iR

iR

Charging vs. Discharging

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1

1 1 2,t t

out S out Sv t V v t V e t t t

RC

1

1 0

1

out

t t

out S

v t

v t V e

Chargingt1 t2

Charging vs. Discharging

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1

1 1 2,t t

out S out Sv t V v t V e t t t

RC

1

1

0St t

out out

V

v t v t e

Dischargingt1

Charging vs. Discharging

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1

1 1 2,t t

out S out Sv t V v t V e t t t

RC

1

1

0St t

out out

V

v t v t e

1

1 0

1

out

t t

out S

v t

v t V e

Charging Dischargingt1 t1

Demo: DC Offset

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no-offset OffsetVoutv t v t

Note: the ground level of the oscilloscope stays at the same place on the screen.

4 Vp-p Square Wave

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2V

-2V

Ground level

4V

Ground level

No DC OFFSET

With 2V DC OFFSET

t

t

DMM: DC vs. AC Modes

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VDC = Measured value of the voltage using DMM in DC mode Theoretically, VDC = Average value = DC offset voltage = DC component

VAC = Measured value of the voltage using DMM in AC mode Theoretically, for “True RMS” DMM,

For non-true-rms DMM, the measurement is calibrated so that the above property hold for sinusoids.

Theoretically,

0

0

DC1V

t T

t

v t v t dtT

0

0

2 2 2 2RMS AC DC

1V V Vt T

t

v t v t dtT

2AC DCV Vv t

DMM: DC vs. AC Modes

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VDC = Measured value of the voltage using DMM in DC mode Theoretically, VDC = Average value = DC offset voltage = DC component

VAC = Measured value of the voltage using DMM in AC mode Theoretically, for “True RMS” DMM,

For non-true-rms DMM, the measurement is calibrated so that the above property hold for sinusoids.

Theoretically,

0

0

DC1V

t T

t

v t v t dtT

0

0

2 2 2 2RMS AC DC

1V V Vt T

t

v t v t dtT

2AC DCV Vv t

Same when VDC = 0.This is why, in lab 4, VAC = Vrms.

Square Wave

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Ground level

Voffset = VDC

VP-PVAC

For square waveform (w/ or w/o DC offset), p-pAC

VV

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“True rms”

p-pAC

VV

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“Rectified average”

Oscilloscope: DC vs. AC Modes

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Input signal:

DC mode: Show

AC mode: Show vAC(t) always have 0 average (theoretically)

when VDC = 0.

v t

DCVACv t v t

DCv t v t

AC DCv t v t

Part A: Find

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Three different methods:

Measure t0.37.

Measure thalf. Then, calculate

Measure R and C. Then, calculate

= RC. 04 V

02 2V 01 .4 7 V e

h a lft 0 .3 7t

half ln 2t

00.37V

1k

0.1 F

0

t

outv t V e

Part B.1

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0V

0 2V

RV

01

2f

LC

R2Sine-wavegenerator

LC

8 Vp-p

22 mH

0.01 F0.47 F [474]0.1 F [104]

OscilloscopeCh-1 Ch-2

100

Part B.2

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f0 f

Peak-to-peak reading from the scope

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Peak-to-peak reading from the scope (1/3)

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Peak-to-peak reading from the scope (2/3)

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Peak-to-peak reading from the scope (3/3)

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Peak-to-peak reading from the scope

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