First Year LabIntroductory Electronics

• We are Physicists. Why do electronics?• You will probably also end up using computers!• You may end up using optics too.

A small atomic physics experiment here (015 Blackett)

First Year Lab Introductory Electronics

• Aims - to introduce…– The equipment– Good lab book keeping– An awareness of measurement and uncertainties

• Remember…– To use the demonstrators– To colour code your circuits– Be adventurous and inquisitive with your experimentation

Equipment• Benchtop Power Supply – Gives DC power• Digital Multimeter – Measures AC/DC voltage levels, resistance• Function Generator – makes sine, square, triangle oscillating

waveforms.• Oscilloscope• Protoboard• Wire clippers• Resistors/Wire/Banana-banana wires• Headphones• BNC-banana cables (co-axial, two wires in one cable, a sheath

which is usually grounded and a core).

ConductorsInsulators

BNC cableCross-section

TTi Power Supply

On

• Meter– Displays output voltage &

current• Buttons:

– On/off• Knobs

– Coarse and fine voltage adjustment

– Current limit• Connectors

– +V– -V– Ground !??

Digital multimeter

• Buttons– On/off– Measurement type

• Current• Voltage• Resistance

– Measurement range

• Connectors– Common– Volts/ohms– Current

• High (<10A)• Low (<1A)

• Accuracy depends on:– Range (specified in manual)– How recently it was calibrated

• Assume 0.5% + 1 digits– 2.738 reading has error– 0.5% = 0.014– 1 in last digit = 0.001– 2.738 ± 0.015

002.734

Use your digital multimeter to meaure the voltage on your benchtop power supply

• Set power supply to give 5V output• Set multimeter to “DC V”• Connect using banana leads

• Do the digital and analog meters agree?• How accurate is each meter?

Measuring resistance

• Find the 1kΩ resistor in your component box

• Attach banana leads to the common and V/ terminals of your DMM and switch to mode

• Attach your resistor between the other end of the leads using 2 croc clips

• Is your resistor within the stated tolerance?

Resistor colour code

a b c d

a. 1st digitb. 2nd digitc. Power of 10d. Tolerance (accuracy)

0. Black1. Brown2. Red3. Orange4. Yellow10% Silver

5. Green6. Blue7. Violet8. Grey9. White5% Gold

47k±10%

The protoboard

• Rows and columns of holes on the protoboard are electrically connected

• Use your multimeter in resistance mode to check exactly how

• Make simple probes:– Banana lead + croc clip– Short length of single strand

wire

The protoboard

• Rows and columns of holes on the protoboard are electrically connected

• Use your multimeter in resistance mode to check exactly how

• Make simple probes:– Banana lead + croc clip– Short length of single strand

wire

The protoboard

• Rows and columns of holes on the protoboard are electrically connected

• Use your multimeter in resistance mode to check exactly how

• Make simple probes:– Banana lead + croc clip– Short length of single strand

wire

The protoboard

• Rows and columns of holes on the protoboard are electrically connected

• Use your multimeter in resistance mode to check exactly how

• Make simple probes:– Banana lead + croc clip– Short length of single strand

wire

Checking Ohm’s Law

Power supply

R+

A

• When measuring current what do you assume

about the resistance of the ammeter?

Checking Ohm’s law- what you should have in your lab book

• A circuit diagram• Switch meter from “DC V” to “DC A” to measure current I and

voltage V for your different resistors• Record values - include estimates of the error in your measurement

• Calculate resistance from measured I and V • Compare to multimeter measured value of Rmeas

I (/mA) V (/V) R=V/I (/) Rmeas (/)

4.75±0.02 4.78±0.02 1006±6 1001±4

… … … …

… … … …

Tip: Formulae for combining uncertainties are summarised in the inside rear cover of the lab manual.

Function or signal generator

Frequency range (buttons) and value (dial)

Signal shape Signal

amplitude

OutputsVout

Com/0V(Ground)

TriggerDC offset

On/off switch!

Function generator + headphones

• Set the generator to give a 1kHz, 4V peak-to-peak sine wave.• Connect your 3.5mm jack socket to the function generator terminals

and plug in the headphones• What does it sound like?

– Over what range of frequencies can you hear signals?

– Middle C is 262 Hz, what do 131, 524 and 1048 Hz sound like? • An octave in musical terms is a doubling in frequency

– How does the volume change when you change the voltage range• Music is logarithmic!

– Set the generator to give square and triangle waves• Square and triangle waves contain higher harmonics (multiples of the

fundamental frequency)

Measuring voltage as a function of timeThe oscilloscope:

• Think of groups (horizontal, vertical)• Horizontal = time• Vertical = voltage (2 identical channels)

Channel 1 (vert)

Channel 2 (vert)

Time (horizontal)

Oscilloscope Basics

• e- beam in evacuated tube.• dc voltages applied to X and Y

plates deflect e-.

• Apply sawtooth voltage in time to X-plates (timebase)

• Apply voltage you want to monitor to Y-plates

Y plates

X plates

Phosphor screen

Electron gun

t

Vx

Exploring (some of) the Controls• Turn on `scope, Set CAL knobs fully

clockwise• Set function generator to 4V p-p, 1kHz

sinusoidal.• Set ‘trigger’ control to AC• Check ‘coupling’ is DC, not ground• Input into channel 1 of 'scope (use

banana-BNC cable) • Y-sensitivity knob – multi position rotary

– Sets ‘volts per division’ vertically, 1div=1cm. Set to 1V/div

• Time base knob – multi position rotary– Sets period of saw-tooth, ‘seconds

per div’ horizontally. Set to 0.2ms/div• If you see a mess DON’T PANIC

» Press AT/NM button

2

V/V

t/ms

t

Vx

Screenshot

Trigger to the rescue!

• Input voltage compared with an internally set level – the trigger level

• After a single sweep of the screen the e- gun waits

• When the input equals the trigger level the next tooth of the sawtooth is executed

t

Vx

wait

Reference voltage source internal to ‘scope, set by knob on front panel

– ‘Trigger Level’

Comparator – gives out pulsewhen inputs are equal

Input voltage

Go signal to timebase

Trigger explained

• Sinusoidally oscillating voltage 4V p-p

• For a trigger level at 1.6V, say• As soon as signal goes above

1.6 V the sawtooth triggers• At end of sawtooth, `scope

waits for next trigger event

• Play with the trigger level and see what effect it has on the leading edge of the waveform

– You may need to press the AT/NM button

• Check to see what the ‘slope’ button does

10 ms/div

0.5 V/div

Screenshot

2

V/V

t/ms

1.6

25

Trigger point

Edge of screen for chosen timebase

Trigger point

Wait time

Trigger mode settings• Can trigger off the signal applied to the

channel. – Auto Trigger (AT) always ensures the

trigger level never exceeds the amplitude of the waveform. Always results in a trace.

– Normal Trigger (NM) allows an arbitary trigger level to be set. If greater than the amplitude of the waveform the screen will remain blank.

• Or can trigger off a separate signal – external trigger– e.g. a sig. gen. may simultaneously give

out a TTL (square) pulse train and a sinusoid. Use the TTL pulse as an external trigger

More trigger mode settings• AC trigger mode supports Auto Trigger

and suitable for signals >20Hz.• DC trigger suitable for signals <20Hz,

only Normal Mode trigger is supported.• LF trigger is for low-frequency triggering

(<1.5kHz) and used if the signal is noisy.• TV trigger is used for synchronising to

video signals.• Line trigger (~) triggers from the mains

frequency. Useful for seeing if a ‘noise signal’ is correlated with mains frequency. To activate the line trigger depress both the AT/NM and ALT

buttons. Hold the red banana plug and wave your other hand close to mains cables. The oscilloscope should start triggering off the signal that is picked up from your body!

Other Notes• Cal – ‘Calibrated’

– Change from the calibrated position to make arbitrary sized wave ‘fit’ between grid lines to aid measurement

• Input Coupling – Ground – shorts scope input to ground –

kills signal, allows you to find 0V and set using Vert Position

– DC – the ‘normal’ mode, what you see is what you got

– AC – removes any DC component of a signal, useful for seeing a small oscillating voltage on a big DC background

Output/Input resistances• All instruments possess an effective

resistance, known as impedance when dealing with AC signals.

• The Function generator and Oscilloscope contain complex electronics, but we can approximate the interior electronics with an effective resistance.

• Equivalent circuit for function generator shown on left: ideal source V0 in series with output resistance R0

• Equivalent circuit for oscilloscope shown on right with input impedance R0

• Voltage will divide according to the potential divider VL=Vo RL/(Ro+RL)

Vo

~

Ro

RLVL

Signal generator Oscilloscope

I

Input impedance of headphonesAdjust the function generator to give a

4 V p-p 1 kHz signal• Insert headphones into the circuit• What has happened to the signal

voltage!!?• Connect the headphone jack to the

multi-meter and measure the headphone resistance …if you do not measure about 32 then you are not measuring the right thing! (check the jack plug)

• Can you explain why the voltage measured on the oscilloscope drops when the headphones are connected?

Function generator

VHV0 ~

600

RL

Scope