CRO Ebook

PAGE 6

USING A CRO

The names of the controls on a CRO have been covered on Page 3 and using a probe is on Page 5. You are now ready for some of the tricks and secrets in getting a waveform onto the screen and how to interpret "what you see."

But first a little background.
The CRO is not the "complete answer" to a design-engineers prayer. It doesn't always help, and sometimes can cause a lot of frustration.
There are three main reasons.
1. It can put a load on a circuit due to the length of the probe-lead plus the resistor and capacitor in the probe (the attenuating circuit). This will change the characteristics of the waveform.
Any circuit operating above about 30MHz will see the lead as an antenna and depending on the amplitude of signal at the test-point, it will draw-off some of the energy and alter the operating conditions.
This effect can be reduced by connecting a very small capacitor (2p to 10p) to the end of the probe to touch the parts on the circuit, but this will affect the amplitude of the wave-form you see on the screen.
2. There are times when the waveform on the screen will show irregularities such as glitches (spikes) and it will be difficult to know if the circuit is responding to them.
3. Some CRO's will not respond to high frequencies because they do not have sufficient band-width and the glitches will be absent from the trace - they will not produce an accurate picture of the waveform.
That's why it is important to buy a CRO with a high bandwidth and a 1X, 10X probe.

BANDWIDTH
Bandwidth is the rating of a CRO.
This is effectively the maximum "cut-off" frequency, at which the amplitude of the waveform on the screen is 70.7% of the true value.
CRO's are available in a wide range of values from 10MHz to 50GHz.
Some of the ratings you will see advertised are: 10MHz, 20MHz, 50MHz, 100MHz, 150MHz, 250MHz, 1GHz, and 10GHz and beyond. The high ratings are extremely expensive, not only because of their capability but due to the added features and the low sales-volume.

The graph above shows how the performance of any CRO drops off, as the signal you are measuring, reaches the capability of the instrument.
In other words, if you have a 20MHz CRO and are measuring a 20MHz signal, the amplitude of the waveform will only be 70.7% of the true height.
But more important is the SHAPE of the waveform. It will not be a true representation, for two reasons. The CRO will put a load on the circuit and change the shape and secondly, the circuitry inside the CRO will not be fast enough to show a true representation.

To determine the bandwidth of a CRO for a particular measurement, there is a "5 Times Rule:"
Bandwidth Required = Highest Frequency Component of Measured Signal x 5
The "tricky" word in the statement above is "component." It means the highest frequency "part of" the waveform you are measuring.
The highest frequency part of a waveform is when the signal changes direction. This part of the signal is equivalent to another signal with a much higher frequency - but this is getting too complex for this discussion. Ideal bandwidth simply means you need a CRO with a bandwidth 5 times higher than your pocket will allow!
Using a CRO with the 5 Times Rule will give greater than +/-2% accuracy in measurements - typically sufficient for today's applications. However, as signal speeds increase, the cost of a CRO to meet these requirements may be prohibitive. Always keep in mind that the higher the bandwidth, the more accurate the reproduction of a signal… and the longer the useful life of the oscilloscope. When it comes to bandwidth, you can never have too much!

The higher the bandwidth, the more
accurate the reproduction of the
signal, as illustrated with a signal
captured at 4GHz, 1GHz and
250MHz bandwidth levels.

Whenever you probe a faulty circuit and look at the waveform, the first thing you must ask is: "Is the fault coming from the point being probed or from elsewhere?"
Don't always assume you have located the fault, just because the waveform doesn't' look correct.
Remember this: A CRO doesn't solve your problems. It merely shows the waveform at a particular point in a circuit.
Sometimes you can be lucky and the CRO will pick up the problem. It certainly is an amazing piece of test-equipment, but the biggest mistake is thinking it will SOLVE your problems.
When designing a project, one of the most successful uses for a CRO is to show the effect of changing the value of various components.
When adding or removing turns on a transformer, or changing the value of a capacitor in a tuned circuit, the CRO will let you know when the most suitable value has been selected.
It's only when the CRO is used for fault-finding that frustration comes in. When everything is running smoothly, the CRO is an enormous benefit to designing.

VIEWING A SINGLE SIGNAL
This is called single-channel mode or single-trace mode.
We have already gone though the name of each control and its function. And we have covered the probe with its 1X, 10X feature. We have also produced a waveform on the screen by using the "cal" test-point on the front of the CRO.
Now we come to discussing the "setting-up."
Any waveform on the screen can be increased in height (or decreased) by changing the Volts/div control.
The Volts/div setting is a scale factor. For example, if the Volts/div setting is 5, each vertical division represents 5 volts. If there are 8 vertical divisions on the screen, the entire screen can show 40 volts from bottom to top. If the setting is 0.5 Volts/div, the screen can display 4 volts from bottom to top. The maximum voltage you can display on the screen is the Volts/div setting times the number of vertical divisions.
When the probe is switched to 10X, the Volts/div must be MULTIPLIED by 10. This means the maximum reading for the first example is 400 volts.
Now it's a matter of selecting a project and seeing what is happening at various points on the circuit.
The best for this is a DIGITAL design. An audio circuit is going to be much more difficult to analyse. A digital circuit will have a waveform that repeats at a constant rate and will allow the CRO to produce a steady trace on the screen.
Talking Electronics has produced a number of suitable projects such as: PIC LAB-1, and
5x7 Display. Or you can visit the complete list of projects: FREE Projects.
When you have decided on a project, place it on the work-bench, near the CRO and supply it with a voltage. It is best to use a battery for the supply.
Connect the probe to Ch 1 and make sure the setting on the probe is 1x (x1).
If the project is battery powered, the voltage at any given point will not be much higher than 9v, so a setting of 2v/div for the Volts/div will be ok.
The trace should be adjusted to run across the centre of the screen and the Time/div (called the Horizontal Sweep) should be set to 1mS.
Make sure the AC-GND-DC switch is set to DC. This sets the type of Input Coupling from your test circuit to the oscilloscope. The coupling can be set to AC or DC, or the signal can be removed by switching to GND (ground).
AC coupling blocks the DC component of a signal so you see the waveform centered at zero volts.
DC coupling shows all of an input signal.
The AC coupling setting is handy when the entire signal (alternating plus constant components) is too large for the Volts/div setting. (The waveform will be above the screen!)
The ground setting (GND) disconnects the input signal from the CRO, which lets you see where zero volts is on the screen. With grounded input coupling and auto trigger mode, you see a horizontal line on the screen that represents zero volts. Switching from DC to ground and back again is a handy way of measuring voltage levels with respect to ground.
The following two diagrams show the effect of Input Coupling DC and Input Coupling AC. The signal is 2v DC with 1v ripple:

Input Coupling DC and Input Coupling AC

Back to setting up the CRO for our experiment:
Set the Trigger to AUTO.
Set the Trigger Source to INT (internal)
Touch the probe on the "cal" test-point and you will see a square-wave on the display.
Everything is now ready for a measurement.

MANUAL TRIGGERING
The aim of this exercise is to get a steady waveform on the screen by using manual triggering.
At the moment, the Trigger control is set to Auto (The Trigger control is pulled out) and this makes almost any waveform steady on the screen. This is the purpose of Auto Trigger.
We will use our skill to produce a steady trace using manual Triggering.
You will need to produce one or two complete cycles on the screen by clicking the rotary switch called: Time/div. This is to get the waveform as large as possible so the detail can be observed.
Now, the experiment starts:
Push the Trigger control, so Auto Trigger is off.
The trace may disappear.
Now rotate the Trigger control until the trace re-appears on the screen. You will see (+) and (–) on the Trigger control to indicate the CRO is now triggering on the positive portion of the waveform of the negative portion. You are adjusting the trigger level so the CRO starts the trace at a point on the waveform determined by the setting of the Trigger Control.
Depending on the type of signal you are detecting, it may be easy to manually show a waveform on the screen or the trigger control may be very sensitive when you rotate it left or right. This will depend on the shape of the waveform and its regularity. When the waveform is steady, click the Time/div rotary switch until one complete cycle occupies the space between a grid line and the next and get a steady trace.
You can now work out the frequency of the waveform by using the following table:

SWEEP-TO-FREQUENCY
50mS 20mS 10mS 5mS 2mS 1mS 0.5mS 0.1mS	20Hz 50Hz 100Hz 200Hz 500Hz 1kHz 2kHz 10kHz
50µS 20µS 10µS 5µS 2µS 1µS 0.5µS 0.2µS 0.1µS	20kHz 50kHz 100kHz 200kHz 500kHz 1MHz 2MHz 5MHz 10MHz

When one cycle = one grid spacing,
use this table to work out the
frequency of the waveform.

HOW TO VIEW "RIPPLE"
Ripple is a waveform (also called an AC signal) on a DC voltage. Ripple is always "on-top-of" the DC voltage and dia A below shows a 100mV ripple on a 5v DC voltage:

To view the ripple, you have to go through three stages of adjustment on the CRO:
Firstly you need to see the "whole picture." The whole picture is shown in dia B. The waveform is 5v above the 0v line and contains squiggles that are very difficult to identify:

Set up the CRO to see this waveform and use the AC-GND-DC switch to go from DC to GND to DC to check the height of the wave, to make sure it is 5v. This is the 5v DC component of the waveform. You are simply preventing the signal entering the CRO, to check the position of the 0v of the trace. This is shown in dia C:

Once you have determined the value of the 5v component, you need to look at the ripple.
Move the AC-GND-DC control to AC and only the ripple component will show on the display.
The result is shown in Dia D on the diagram below. The DC component is removed and that's why the waveform shifts down the display.

The ripple is very small on the screen. To increase the amplitude, adjust the Volts/div control to 50mV/div. This is shown in dia E:

To get the waveform to fill the screen, change the Volts/div control to 10mV/div. The result is shown in dia F:

This procedure applies to all situations where a small AC component (called the "signal" or "ripple") is contained within a DC component.

VIEWING TWO SIGNALS
This is called DUAL-channel mode or DUAL TRACE mode.
Dual Trace Oscilloscopes have the capability of drawing two traces on the screen at the same time. (Actually they only draw one at a time, but the image persists on the screen long enough so you cannot see any flicker and it appears as if there are two steady traces.)

Setting the CRO for DUAL TRACE operation:

Connect Probe A to channel 1(the probe with the white identification dot)
and probe B to channel 2.
Each channel has its own Volts/div control, so you can adjust the height of each waveform separately, however they share the same timebase setting (Time/div).
Set the Display mode to either ALT or CHOP.
In general, the ALT setting is used when using a fast timebase (100mS to 0.1mS) and the CHOP setting is used for a slow timebases (50mS to 5mS).
For intermediate timebase settings, the traces produced using either mode settings look essentially the same.
The Trigger source can be set to either Ch A or Ch B. Since both channels have a signal, either can be used to display the trace on the screen.
Connect the tip of each probe to the "cal" test-point on the front of the CRO and adjust waveform A to be at the top of the screen (via the Position up/down control or Shift up/down control) and move waveform B to the bottom. You can adjust the Volts/div setting for either channel to get the waveform to fill the screen.

You should now be able to see two waveforms on the screen.
Remove the probes from the "cal" point and you are ready to test a project. You can use one of the digital projects from Talking Electronics.

There are two situations with DUAL TRACE mode. The signals can have the same frequency or different frequencies.

(A) TWO SIGNALS OF THE SAME FREQUENCY

The only way to get signals of exactly the same frequency is to derive them from the same source.
The source can be a function generator or a project-under-test.
When both signals come from the same project, the CRO will find is easy to "lock on" to the signals and produce a steady waveform on the screen.
This will make it easy to view each and make comparisons. By placing the second waveform below the first, you can see exactly when certain irregularities such as glitches or peaks, are occurring.
And by placing the waveforms on top of each other you can detect minûte changes or distortions.
When viewing two waveforms you can use the Mode Change switch to view Ch 1, Ch 2, Dual or ADD.
The ADD mode produces a single waveform that is the combined value of the two waveforms.
This function and all the other functions of a CRO are fully explained in our interactive
CRO Simulation. This is an amazing feature. It will teach you how to operate a CRO before touching the real thing.

(B) TWO SIGNALS OF DIFFERENT FREQUENCY
When the two signals are of different frequency, it is impossible to make a stable trace for both at the same time. If one is used for the trigger source, then the other will be free running.
This mode of operation is rarely used, as it is difficult to get the second waveform to appear steady on the display.
To see the difficulty in using this mode you will need two separate signals, preferably from two different projects.
Set the mode control to either ALT or CHOP.
Set the trigger source to Ch1.
Adjust the Volts/div control for each input to get a suitable amplitude for each waveform..
Note that the waveform for channel 1 is stable (you may have to adjust the trigger level), but that channel 2 is free-running.
Switch the trigger source to Ch 2. Note that channel B is now stable while channel A is free running.

The next two pages have interactive CRO demonstrations. The next page shows how two waveforms produce different patterns on the screen, depending on the frequency of each and the phase-difference. The following page has a CRO Simulation program to show you exactly how to set up a CRO and read the waveforms.

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