USING OSCILLOSCOPES.pdf

USING OSCILLOSCOPES

by Harry Lythall - SM0VPO

I have recently been asked if I would provide some information regarding the use of an Oscilloscope, so I

have put together this document as a start for those who wish learn a little of the use of the instrument in

Amateur radio. If you have any sugestion for improving this document then please give me a call; my

addresses will appear at the foot of this page. This information is not intended as a definitive work regarding

the use of Oscilloscopes, merely a little information to get you started and show some of the possibilities.

The Oscilloscope (often called simply a "scope") is nothing more than an instrument for displaying electrical

signals in the TIME domain. In other words, you can view waveforms on it. Some of the uses of the scope

may not be obvious but if you already posess one, then you will most probably regard it as the most useful

tool on your workbench.

CONTROLS

A basic (and fictitious) Oscilloscope is shown below.

This is a Dual-Beam oscillloscope although most of the information presented here is also applicable to a

single-beam Oscilloscope. The basic controls are:

• BRIGHT (Brilliance) adjusts the intensity of the display.

• FOCUS Adjusts the focussing of the display.

• GRAT (Graticule) Illuminates a scale used to measure a trace.

• TRACE (Channel etc.) Selects which trace is to be displayed.

• TRIGGER Selects a trigger source.

• TRIGGER LEVEL Selects the point of the waveform used to trigger.

• TIMEBASE Selects the speed the trace moves across the tube face.

• INPUT LEVEL Adjusts the input level.

• POS (Position) Sets the position of the trace on the display.

The Scope will also have an input socket for each input channel, situated on the front of the instrument.

There will most probably be more controls on your scope, and I will try to cover some of these later on.

BRIGHT

This is self-explanatory, it controls the intensity of the display. It is well worth remembering that one cannot

execute screen-saver programs on an oscilloscope, so leaving the scope with a high brilliance will certainly

burn the trace into the tube face. If you must leave your scope switched ON for any length of time then turn

the brilliance control down.

FOCUS

The focus control is also quite self-explanatory but most "scopes" seem to adjust a single spot from a short

horizontal to a vertical line. A single horizontal line is therefore not the best waveform to adjust the focusing,

this is best done whilst viewing a waveform.

GRATICULE

(Scale) Illuminates a scale used to measure a trace. This is usually just a clear plastic sheet placed over the

tube face and shows a matrix of 1cm squares. By comparing the matrix with the waveform, it is possible to

measure such things as Frequency and voltage. If this control is turned down then the scale will not be seen.

TRACE

(or CHANNEL) Selects which trace is to be displayed. There are usually two or more selections possible:

• A - Selects trace A only (single channel).

• B - Selects trace B only (single channel).

• A+B - Selects both trace A and trace B (dual channel).

• ADD - Both channel inputs are added and displayed as a single trace.

TRIGGER LEVEL

A trace displaying a waveform without the use of TRIGGER will roll in much the same way as a TV with

the horizontal hold set wrongly. Trigger action will stop the trace from starting, until a determined part of the

waveform occurs. This will make each "sweep" of the tube face occur in exactly the same place and the

display will appear to be stationary. The trigger level control is used to select that point of the waveform.

TRIGGER SELECTOR

Selects the trigger source. Most dual beam scopes may be triggered from either Channel A or channel B.

Many oscilloscopes have the facility to trigger from an external source, in this case an extra TRIGGER input

on the front panel will be provided.

TIMEBASE

The speed the spot travels across the screen of the tube may be varied by means of the TIMEBASE selector

switch. This is calibrated in Seconds (S), MilliSeconds (mS = 0.001 seconds) or MicroSeconds (uS =

0.000001 seconds).

INPUT LEVEL

Adjusts the input level of each channel so that the trace display will fit on the screen. This is a switch

calibrated in Volts per centimetre (V/cm).

POSITION

Sets the vertical location of the trace on the tube so that the display is positioned in a manner more

comfortable to read. For example, you may wish to set zero volts to the graticule mid-position for AC

measurements, or to the bottom of the tube face for DC measurements.

BASIC OPERATION

By way of example, connect a lead to CHAN-A input then hold, with your bare hand, the centre conductor

of the wire. You will see some rubbish on the screen which is the 50 Hz (60 Hz in some countries!) Mains,

which is being received by your body. Set the timebase to 10mS / Div and the CHAN-A input level to a

setting that does not go off the screen. You should see a wave-form such as is shown below.

On the trigger input select CHAN-A. Adjust the TRIGGER control slowly and at some setting of the control

the display will become stationary. If the TRIGGER control has an AUTO position then select it and you do

not need to adjust anything.

The waveform you see will not look exactly like that shown above, as there will be loads of distortion. This

is due to many things mainly because you are picking up spurious signals radiated by household electrical

equipment such as TVs, flourescent lamps etc. All these items distort signals in one form or another.

If you look at the waveform you will see that there are two horizontal red divisions on the graticule scale

between two consecutive peaks. Since the timebase is set to 10mS/Div, it will take the spot 20mS to travel

between the two divisions. The PERIOD TIME of the waveform is therefore 20mS, (or 0.02 seconds). The

FREQUENCY of the waveform is 1 divided by 0.02 = 50 Hz.

Now look at the vertical scale. The centre-line is zero-volts and the waveform moves both 1.8 divisions

above and below the centre line, and if the input level is set to 1volt/cm the level of the input is 1.8v + 1.8v =

3.6 volts PEAK-TO-PEAK. This equates to 3.6v times 0.35 = (about) 1.2 volts RMS, as you would read it

on a voltmeter.

In this way, you can measure the FREQUENCY and VOLTAGE (AMPLITUDE) of just about any

waveform.

OSCILLOSCOPE INPUTS

The input impedance of the oscilloscope is very high so it will not load a circuit under test. Most

oscilloscopes have an AC/DC switch at the input to each channel. When set to DC the channel trace will

move up or down by an amount proportional to the value of the DC voltage. If set to AC, then the DC

voltage component of the input will be removed allowing you to view only the AC component. This feature

is usefull for measuring the ripple that may be present on a power supply. Take for example the following

circuit diagram and test setup:

Here, the oscilloscope has both its inputs connected to the DC output of a home-made 12v DC Power Supply

Unit. Trace B is set to 5v/cm DC and only a single is displayed. The horizontal line will have moved up the

tube face by just over 2cm, but it is straight and shows no deviations.

Trace A input is set to AC (DC block), and the input sensitivity is turned up to about 1mV/cm. The DC

component would give a deflection of 1200cm but the DC has been blocked. The low-level ripple can now

be clearly seen. In practice, with the circuit shown, little or no ripple should be seen, but when a load is

placed on the power supply the ripple may even become quite objectionable, depending upon the component

values chosen. It is also very interesting to see that the ripple frequency will be 100Hz or 10mS between the

peaks. This is because of the action of a full-wave bridge rectifier.

The TIMEBASE selector switch will most probably have an 'XY' position even on the cheapest of

oscilloscopes. This can be used to for a variety of new functions, such as:

• Lisajous figures - 'pretty' frequency measurments, but very acurate.

• Other equipments - such as WOBULATORS and analysers.

• HF TX Modulation - quick and convenient.

The 'XY' timebase selector position disconects the internal generator that moves a spot across the screen.

Input B will now move the spot across the screen, but input A will still make the spot move up and down.

LISAJOUS FIGURES

By way of example, connect two leads to CHAN-A and CHAN-B inputs then hold with your bare hands the

centre conductor of both the wires. You will see the same rubbish as you did before, but this time there will

be differences between the two signals. This will work even better if two people each hold one of the two

wires.

If the two signals were EXACTLY identical, then the spot would move UP and to the RIGHT then DOWN

and to the LEFT. This would be seen as a horizontal line diagonally across the screen. A loop would be seen

if the two signals were exactly the same frequency, but different wave-form or phase. If the two signals were

perfect sinewaves and differed by exactly 90 degree phase then you would see the following waveform:

If you have access to an AF sinewave signal generator, then connect an RC circuits to one input and a CR

circuit to the other input of the sccope:

Notice how the circle slope changes as the frequency is altered.

If the phase were to constantly change (slightly different frequency) then you would see a SQUARE

BLOCK formed by a moving image changing from a left -sloping line - a circle - a right sloping-line - a

circle - then back to a left-sloping line again.

If the two sine-waves were to differ in frequency by EXACTLY 2:1 then you will see something like this:

Notice that in the vertical plane (Y axis = CHAN 2) there is only one peak but in the horizontal plane (X axis

= CHAN 1) there are two peaks. CHAN 1 is therefore twice the frequency of CHAN 2. With other frequency

combinations the waveform may become even more complex, for example 3:2 8:3 2:5 etc.

WOBULATOR

Many circuits have been published for wobulators. A wobulator generates a varying radio frequency that

moves in sympathy with a sawtooth or triangular waveform. If the sawtooth is applied to the Y (horizontal)

input of the oscilloscope, the RF signal can be applied to the X (vertical) input via a receiver IF amplifier

(tuned circuit, filter etc.). The vertical axis will then give a display of the frequency response of the

amplifier:

Here we can see that the left hand peak is not quite as high as the right hand peak which means that the IF

response is not perfect, although it is not bad enough to be noticed by the ear.

Most single beam oscilloscopes have a TIMEBASE output which can be used to make a voltage controlled

oscillator sweep through the frequencies of interest.

I will be posting a simple Wobulator circuit soon. It has been tried and tested over a number of years.

SPECTRUM ANALYSER

It is quite possible to build a simple Spectrum Analyser for use with an oscilloscope, in much the same way

as a Wobulator. A Spectrum Analyser is a device that can display a range of frequencies 'simultaneously' (or

a range of frequencies so fast that it looks continuous). If the local oscillator of a superhet receiver was made

to sweep through a range of frequencies, dictated by the TIMEBASE output of your oscilloscope, and the X

(vertical) axis of the oscilloscope monitored an AM detector at the IF, then you would have a spectrum

analyser. A typical analyser display looks something like this:

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