The Oscilloscope
User Rating: / 8
PoorBest 
Written by Bryce Ringwood   

The oscilloscope is an instrument that allows you to view a repetitve (electrical) waveform on a screen. In a prevHameg scopeious article we looked at multimeters. Like a multimeter, the oscilloscope measures voltages, but the result is displayed on the face of a graphical display, such as a cathode-ray tube or graphical liquid crystal display. A graticule is overlaid on the face of the display. This usually has 10 divisions horizontally and vertically, and each division is further subdivided on the X and Y axes with the origin at the center of the screen. Most of the time the instrument is used for taking measurement of AC waveforms, and the instruments vertical amplifiers (see later) are calibrated in Volts per Division. The horizontal sweep is calibrated in seconds (or milliseconds etc.) per Division. From this, it follows that the instrument can be used for measuring frequency and amplitude of an AC waveform. Normally, we are interested in using it to trace the origin of unwanted AC signals, or comparing input and output signals. Of course, it is the only way to see that the waveforms at individual points in an item of equipment are correct. Later in this article (which is being rewritten), I will provide some practical examples of oscilloscope applications.

Here, I am mainly going to discuss the "traditional" cathode-ray tube (CRT) oscilloscope.

Modern oscilloscopes using digital technology and LCD screens are now available for only slightly more money than a new analog oscilloscope, so the choice is yours. If you are just using an oscilloscope for hobby purposes - then since you are reading this on a computer - you may already have a 'scope!

Let's begin by describing the operation of  the traditional 'scope, like the one on the right.

Oscilloscope Circuit Elements

 

Oscilloscope Block Diagram

 

 

There are other bits and pieces, but those are the basics.

The Cathode Ray Tube (CRT)
Cathode Ray Tube
Illustration of a Cathode Ray Tube. From Easterling DW "Oscilloscope Equipment", Norman Price, 1958

Thanks to modern technology, this device is, well, going down the tube.  The CRT (Shown Right) consists of an evacuated glass envelope with a slim end at the base, and a fluorescent screen display. At the base, there is an "electron gun" made up from a heater and cathode (like a radio valve) and several electrodes designed to accelerate and focus a beam of electrons on the fluorescent screen, which forms a writing surface. An important connection is the grid ("brilliance modulator grid"), which can blank the electron beam or just alter the brightness of the display. 

The focus electrode simply alters the size of the spot on the screen.

The X any Y plates are used to deflect the electron beam in the horizontal and vertical directions.

Note the graphite coating - without it the electrons have nowhere to go and the CRT mystically stops working (as happened on my home-made effort when I was a teenager.) 

The horizontal deflection plates are connected to a timebase oscillator, which sweeps the beam of electrons horizontally across the face of the CRT, returning almost instantly to the beginning of the sweep as soon as it has finished.  If no signal is applied to the vertical plates, you will just see a horizontal line on the screen.

In an oscilloscope, the timebase frequency can be varied.

A word of caution - the voltages applied to the various focussing anodes can be quite high - high enough to jump the gap between you and your hand if placed too close. In an oscilloscope, the current is quite low, so you will just get a nasty fright causing you to knock something else over.

The voltages in television sets are not as benign and can be lethal - particularly in colour TVs.

How the CRT displays a trace

The following illustration really explains it all. This is also taken from "Oscilloscope Equipment" :

The power supply

 forms quite a large part of the circuitry, since it must be stable and provide many different voltages for the various CRT electrodes, as well as supplying DC to the various amplifiers, timebases and other components. In valve type oscilloscopes, the high voltage for the CRT was provided by a high-frequency oscillator and EHT diodes, like the EY51. There would also be numerous voltage regulators and time delay relays to make sure the filaments were hot prior to applying HT. 

Timebase

timebase generator provides a sawtooth voltage to sweep the spot across the screen. At the end of the sweep, the screen is blanked until the sweep starts again. The timebase oscillator frequency is controlled to provide so many seconds/milliseconds or microseconds per division. The timebase is triggered to start at the same point on the measured waveform to stop it from dancing across the screen.

Y - Amplifiers

The waveform to be measured is applied to a Y-Amplifier connected to the vertical deflection plates. This is an accurately calibrated DC amplifier with a large number of switched ranges corresponding to the marks on a grid overlaying the CRT face. Normally, the gain would be expressed in so many millivolts per division, or volts per division. My general purpose scope ranges from 5mV/Division to 20V/div. The coupling to the amplifier can be AC or DC - or simply grounded. I normally use the AC position, since I'm not interested in DC values. 

Beam Splitter

Most oscilloscopes are dual-beam, so that you can compare waveforms. The beam-splitter is a very fast square wave generator that makes one beam look like two or more. This means that you will have two or more Y-amplifiers.

Probes

Because we are measuring dynamic voltages, the normal test leads used for a multimeter will not work accurately, except at very low frequencies. Oscilloscope probes normally contain a small capacitance, and are connected to the Y amplifier(s) via co-axial cable and a BNC connector. The probes have a X1 and a X10 position. The X10 position confusingly divides the input voltage signal by 10. The idea of this is so that the circuit under test doesn't get loaded - it sees a nice high input impedance - and less of the cable capacitance too. If possible, always use the X10 position. There is also a small capacitor adjustment. Before using the probe, it should be attached to the oscilloscopes' calibration circuit and adjusted so that the square wave on the screen really does look nice and square - not a series of funny looking rounded humps or kinky spikey things. 

You can also use the calibrator to make sure you are really getting 1 Volt per division - there's normally an adjustment on the scope - but don't use the front panel calibrated/uncalibrated pots. (See Later)

Controls

Timebase Switch  -This will provide you with set sweep frequencies - usually lots of them. In the centre of the switch, there's a pot that allows you to vary the frequency from the calibrated frequency. I have seldom used the centre pot.

Y-Amp Gain- There is one of these for each Y amplifier. Again, lots of switch positions to ensure your waveform fits on the screen. Start with the largest value so you will at least see something. Once again, there is a centre pot for fine adjustment of the size - but since you'll lose calibration, there's not much point to using it.

Y-Position - This makes sure you can move the trace on to the desired part of the screen

X-Position - Use to centre the trace.

Trigger - Various adjustments for triggering. Often inlude for LF and HF signals. Also a pot adjustment to adjust where on the trace you want triggering to start. Maybe there will also be the option to trigger on slope or level.

If you don't see anything on the screen - it may be the trigger adjustment.

Dual/Single - There will be switches for dual/single mode and whether to trigger on the A or B trace. Also an option to ADD the signals or chop them.

In the beginning

Don't get too complicated. Get used to the scope with the inputs grounded. Get a feel for the various controls. Begin by using the built-in calibrator and adjust the probes.  As a next step, connect to an audio oscillator/function generator and observe the signals you see.

Be careful with grounding and be extra careful with AC/DC sets - you will soon find how easy it is to trip the earth-leakage. Maybe NOW you will take my advice and use an isolation transformer!!!

What use is it?

In my own case I use it to view the signals as they progress through each stage of a radio receiver. This is not really a good enough justification for purchasing an oscilloscope and the initial reason I purchased it was to try and trace an annoying signal in a load-cell amplifier. The 'scope is an essential tool for  diagnosing faults in electronic equipment where you know what the shape of the signal should be - but its not as it should be. I use it to analyse the signals in inverter circuits and true enough, you can look at an audio amp and see at what point distortion is taking place.

I don't do TV receivers, but an oscilloscope is an essential for making sure the waveforms are all as they should be.

Then there are the odd things. You need an oscilloscope to repair another oscilloscope. Without an oscilloscope, my transistor curve tracer could never have "been made whole".

So, for what its worth, you don't need one for repairing old radios, but you do need one for just about everything else.  For this reason it gets ranked fourth in my list of essential instruments.

Taking Measurements with an Oscilloscope

(I left this out - deliberately, but on of my readers has taken me to task)

Switch on and set the intensity and focus controls to a comfortable level with the sweep set to untriggered. 

Y- Calibration

The oscilloscope will have a calibration output. This might be a square-wave with an amplitude of 5.0 volts at a frequency of 1kHz. There may be additional output voltages and possibly frequencies. Connect the probe from the amplified you want to calibrate to the output with the gain set to 5.0 volts/division. Be sure to set triggering for the channel you want to calibrate. Set the timebase frequency to .1 milliseconds per division, so that you get a train of 10 square waves.

The square waves may not look truly square because the probe needs frequency compensation. Twiddle the small adjustment on the probe with the trimming tool supplied until those square waves look really square. The height of the waves should be exactly one vertical division. 

If not, consult your oscilloscope manual to correct the gain of the amplifier. Be sure the amplifier is set to its calibrated position first.

Using an oscilloscope as a Voltmeter

You will require a 9 volt battery.

Turn down the beam intensity, because we will not be using the sweep yet - sweeping the spot across the screen requires a higher beam intensity.

Set the probe to X1 and set the gain of the amplifier you are using to 5volt/division (normally the Y2 amplifier). Set the amplifier to DC input. Set the other (normally Y1) amplifier input to ground on, say 5 volts per division.

Set the oscilloscscope to X-Y mode, and use the controls to centre the spot on the screen.

Now place the probe tip on the positive terminal, and the ground clip on the negative terminal. You will, all being well, see the spot move vertically slightly less than two divisions. Some oscilloscopes have tenths of a division marked, so you should be able to measure the battery voltage within a tenth of a volt.

You might like to repeat this with the spot centered on the bottom of the graticule and the gain set to 1.0 volts per division. The spot should move nearly to the top of the screen - and you should be able to measure to 1/100 th of a volt, although that might not necessarily be accurate to that amount. 

In this way, the oscilloscope can be used as a voltmeter to measure DC voltages.

Measuring AC

You will need a small mains 12 volt transformer.

You can't measure AC directly with a moving coil or digital meter. The response of the movement is too slow, but, of course we can put circuits ahead of th movement to interpret the AC voltages and currents.

The oscilloscope uses a stream of electrons (or very fast computing if its digital) to follow the movement of an AC waveform.

Set the gain of the Y2 amplifier to 2 Volts per division and set the input to AC. Apply the probe tip to one lead f the secondary of the transformer, and the ground clip to the other. Be careful to check your connections are correct and use a properly insulated and fused transformer. You should see a vertical line about 8.5 divisions long (+- 4.25 divisions). This is because the total peak to peak voltage is 12.0*1.414 = approximately 17 volts - with 2 divisions / volt, thats 8.5 divisions.

Conversely, the RMS voltage is 17/1.414 = about 12 volts.

Lissajous figures

Now get your other probe and set the second hitherto grounded amplifier to the same settings as the first. Connect the probe tips and grounds together to the - the tips and grounds to the transformer secondary leads. Apply power to the transformer primary.

You should see a circle on the screen. If youplace a 1uF capacitor between the second probe tip and the transformer lead - what do you see?

Measuring Frequency

Nowadays we probably have a multimeter that displays frequency directly. 

Switch the oscilloscope out of x-y mode and put it into single-beam mode. Set the timebase to 10mS per Division and set triggering to the amplifier you are using for input. Set the unused amp to ground. You should see an AC waveform. Note that from the start of the waveform where it crosses the x-axis to the end of one complete cycle should be 2 divisions - or 20 mS.

The frequency is 1000ms/20 = 50 full AC cycles per second, or 50 Hz.

Where to get one

Make one

If your motive for doing so is educational - that's great - go ahead. Even if it doesn't work - you will have learned a great deal. If your motive is to save money - forget it. I'm looking for a 'scope tube (see the wanted ads) and the best quote I have had is over R1500-00. You can almost buy a brand-new scope for that (RSE Electronics had a small 10MHz single channel for less than that fairly recently.)

Download One

There is a nice soundcard oscilloscope here. There are others. You are going to be limited by the performance of the sound card, and, of course, this is a digital scope. For audio work and learning what 'scopes are all about - this is great.

 

Sound Card Oscilloscope

Beware! - You can inadvertently destroy your sound card, computer and everything else with an incorrect connection - such as attaching the probe to the mains to see a sine wave (Extremely foolish example).

It does work really well and I did try it out on an Arduino sine-wave generator. For audio - it gives you the %distortion.

Buy a second hand one

If you buy an old Tektronix from the '70s, it will be as complicated as, well - you can imagine. It will have a minimum of 40 valves, and twice as many transistors - many of which will be difficult to get, or worse, tunnel diodes which are completely Tektronix 547unobtainable. And it won't work - so you'll have to fix it. For which, you will need an oscilloscope.(This is what they call a Catch-22 situation). When you have got it working, it will be big and unwieldly and you'll end up using the 'scope you fixed it with. So these are a labour of love, not a practical proposition.

If you don't buy a Tektronix, you will end up with lots of problems and not much in the way of manuals to help you. (The Tektronix manuals are so good, a three-year old could fix their products. Provided the said three-year-old has read this article first and has a 'scope.)

On the right - Tektronix 547. Nice but very big and heavy. (What are all those knobs and dials for?). Yes it contains a tunnel diode.

Buy a new one

If you need one - just do it. Budget from R3 - 4 000 (Unless you are a research organisation and need a laboratory instrument costing  as much as a large motor car). I think there's even a basic Tektronix in this price range. If you are a "hobbyist" - I wouldn't lash out even that much. There is a box that fits on to a PC - trouble is it costs a couple of grand, and you have to put a PC in your workshop. I guess you will also have the problem of isolating the measurements from the PC - just like the soundcard scope.

Mantech, Communica, RSE Electronics and RS Electronics all sell 'scopes. I'm sure there are others.


References

Hameg HM 203 User Manual

Tektronix 547 (and may others) User Manuals

Horowitz and Hill "The Art of Electronics"

 

 

 

 
Joomla template by a4joomla