LabScope Basics
Application:
A LabScope, or Digital Storage Oscilloscope (DSO), performs only one basic function. It displays voltage over a period of time. Think of your LabScope as a voltmeter, connected to a VCR, which in turn is connected to a television.
Like a voltmeter, it receives voltage inputs. Like a VCR, it records and stores the inputs. Like a Television, it displays the inputs. Because it records and stores inputs before displaying them, electrical events that occur in less than a thousandth of a second can be displayed in detail.
Although the LabScope only displays voltage, it can be used in conjunction with various accessories and a good working knowledge of electricity to measure amperage, dynamic resistance, frequency, temperature, pressure and a variety of other things.
Terminology:
If you’ve never used a LabScope, there are a few new terms you’ll need to become familiar with. Most will be covered in more detail in later chapters.
1. ................................................................................... Amplitude: vertical height (voltage amplitude)
2. .................................................................................................................... Capture: displayed input
3. ................. Coupling: setting that allows the constant component of a signal to be blocked, or not
4. .................................................... Current ramping: the process of capturing amperage waveforms
5. ................................................. Cursors: movable lines used to measure time or voltage on screen
6. ........................................................................................Division: segment of either height or width
7. ..........................................................................................Duration: Horizontal width (time duration)
8. ........................................................................................... Ground Line: horizontal line at zero volts
9. ........................................................................................................ KV: kilovolt (one thousand volts)
10. ................................................................................ mS: millisecond (one thousandth of a second)
11. ........................................................................................... mV: millivolt (one thousandth of a volt)
12. .............................................................................Signature: defi ning characteristic of a waveform
13. ...................................................................................................Slope: direction of voltage change
14. ............................................................... Time base: length of time set for display (Pattern sweep)
15. ...................................................................................................... Trace: image displayed (Pattern)
16. .............................................. Trigger: setting that determines when scope begins to draw a trace
17. ................................................................................................Trigger Level: voltage level of trigger
18. ................................................................................... uS: microsecond (one millionth of a second)
19. ...................................................... Voltage Scale: range of voltage set for display (Voltage Range)
20. .............................................................................................Wave form: trace of a particular signal
Settings:
Settings are the controls that allow us to tell the scope what we want to see on the display and how we want to see it. Because we will be dealing with signals that vary in voltage range and occur over a variety of time frames, it will be necessary to adjust Voltage Scale and Time Base to properly view different signals. You may also need to tell the scope when to begin drawing the trace. This is called Triggering. With certain types of Triggers, you will need to set a Trigger level (voltage level for the trigger). You must also tell the scope if you want to block the constant component of a signal or not. This is called Coupling.
Voltage Scale:
Component operating range needs to be considered when setting Voltage Scale. To determine the voltage range a component operates within we need to look at a few things:
Supply Voltage: If we’re looking an average actuator, supply voltage is usually very close to system voltage, or around 14 volts. If we’re looking at an average sensor on most systems, supply voltage is the same as VREF, generally about 5 volts.
Inductive Kick: Ignition coils, fuel injectors, solenoids and other components with wound coils produce a magnetic fi eld around the windings when powered up. When the circuit is opened and the fi eld collapses, a voltage is induced.
We call this inductive kick, or voltage spike. Induced voltage can be several times higher than supply voltage.
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Voltage generators: Conventional O2 sensors operate in a range between 0 and 1 volt. AC generating sensors vary in amplitude, but are usually under 10 volts.
The scope will display up to the selected Voltage Scale level. (10 Volt setting will display from 0 volts to 10 volts) In most cases you’ll want to select a Voltage Scale slightly higher than expected voltage so the trace doesn’t range off of the display. Voltage Scale may be adjusted once the trace is on the screen, as can other settings.
Time Base:
This setting will determine how much time is represented on the display. There are some things we need to consider when selecting a Time Base.
First, we need to consider the duration of the event. How long does it take for the event we want to see to occur?
Suppose we want to look at a fuel injector at warm idle. An average pulse width should be around 3 mS. If we wanted to see a lot of detail in the trace we’d use a relatively short Time Base, just slightly longer than the duration of the event. If we want to leave room for change in pulse width, we’d use a longer Time Base.
Next, do we want to see the event repeated several times across the display? If so, a longer Time Base would be necessary.
Last, do we want to see a single event or a chain of related events? If we want to see a chain of events we would need some idea of the time required for it to take place.
Don’t be too concerned if your Time Base selection isn’t right the fi rst try. Like all settings, it can be adjusted once you have the trace on the screen.
Trigger:
The Trigger setting will tell the scope when to begin drawing the trace. There are times when you’ll just want the trace to scroll across the display. Maybe you’re monitoring a temperature sensor or something that has voltage transitions slow enough to view in real time. Or maybe you’re testing a component that you are unfamiliar with and want to look at the trace before setting a trigger. In either case you would select None. (no trigger)
Many events begin with a change in voltage. We’ll usually want to see an event from the beginning, so we would select a voltage trigger. To trigger on increasing voltage we’d select a positive slope trigger. To trigger on decreasing voltage we’d select a negative slope trigger.
There are also times when you’ll want to trigger on an external event. An external event is an event other than what you want displayed. The fi ring of #1 spark plug is the most common event used as an external trigger and works well for viewing anything that is related to camshaft or crankshaft rotation. Cam sensors, crank sensors, ignition parade patterns and things of that sort would be good examples. This type of trigger will also sync the event to the trigger source.
Trigger Level:
If we’ve selected a voltage trigger, whether positive or negative slope, we’ll need to set a Trigger Level. When a positive slope trigger is selected, the scope will trigger when increasing voltage rises above the set level. With a negative slope trigger, the scope will trigger when decreasing voltage falls below the set level.
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Coupling:
There are two coupling options, AC and DC. The best way to explain coupling is to look at the AC & DC coupling rules:
1. Pure AC voltage will be displayed the same when either AC or
DC coupling is selected. See Fig. 1 (AC Coupled) & Fig. 2 (DC
Coupled)
2. Pure DC voltage will be displayed with 0 volts on the ground line and positive voltage above the ground line when DC coupling is selected. See Fig. 3
3. Pure DC voltage, such as battery voltage, will be displayed on the ground line and varying DC voltage will be displayed above and below the ground line equally when AC coupling is selected.
See Fig. 4
This capture is the same signal shown in Fig. 3, but this time AC coupling was selected. This varying DC voltage is displayed above and below the ground line equally, so we have no real visual reference to ground or voltage level. If we were looking at a constant voltage, or pure DC voltage, it would be displayed on the ground line regardless of voltage level, so we would have no indication of voltage level. In either case, DC coupling would usually be the best choice.
4. When AC coupling is selected the constant component of the signal will be blocked and only the alternating component of the signal will be displayed. See Fig. 5
This is a capture of AC voltage riding on a DC signal. As you can see, only the AC voltage, which is the alternating component of the signal is displayed. The constant component of the signal is the DC voltage level and is not displayed. If we had used DC coupling here, the AC ripple would be displayed at the DC voltage level rather than on the ground line, so we would know what the DC voltage level was.
Ground location:
The Ferret 92 scope offers two options for ground location. With AC coupling, ground is always in the middle of the channel. We would also select this ground location to view any signal where voltage drops below ground. Ground at the bottom of the channel is used when you want added vertical resolution and works well when viewing multiple channels on the display.
Ferret 92 Display:
See Fig.6 - The highlighted bars on either side of the display are called channel labels and show the various settings that have been selected.
If multiple channels were being displayed, they would be stacked vertically and each would have its own channel labels.
On the left channel label, starting at the top, you will see the Channel
Number, Voltage Scale, Ground Location and Coupling selections. If ground at the bottom had been selected the ground indicator would be at the bottom of the label. The settings shown on the left channel label apply to only that channel and may be set differently for each channel being displayed.
On the right label you will see the Time Base and Trigger selections, followed by the Trigger Level setting. The small arrow just below the
Time Base shows the actual Trigger Level on the display. The settings on the right channel label apply to all channels. All channels will have the same time base and use the same trigger. You may have different
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settings for each channel, but only the settings on the active, or highlighted channel will be used. You may move the highlight to another channel by using the Channel Select key. The time base of the highlighted channel will then apply to all channels. To activate the trigger on the highlighted channel press either Trigger key once the channel is active.
Basic Waveforms
Coolant Temp. Sensor:
Fig. 7 is a low speed analog signal, marked by very slow voltage transitions, and is viewable in real time. We used a cursor here to measure the voltage level. As you can see, at 3.5 volts, we’re looking at a coolant temperature of about 50* F.
Fig. 8 is the same CTS as Fig. 7 but, as you can see, the engine is now at about 180*F as indicated by a voltage level of about .7 volts.
Let’s look at the settings and why these settings were used. Voltage
Scale 5 volts. Normal range for this signal is between about .4 and 4.6 volts. DC coupling is used because we want to see the voltage level.
Time Base is set at 5 seconds. Remember, we are viewing this signal in real time, with a slight delay.
When a Time Base of 1 second or more is used on the Ferret 92 scope it goes into what we call rolling mode. This means that the trace will update by scrolling across the display. You’ll notice a break in the trace. The old trace is scrolling off of the right side of the screen while the updated trace is replacing it on the left side of the break. The time base setting will tell you how long it takes for the display to complete an upgrade, in this case it will take 5 seconds. We don’t need a trigger for this type of signal, so the Trigger setting is None and trigger level does not apply.
Throttle Position Sensor
This TPS (Fig. 9) is also a low speed analog signal. In this case we are controlling the voltage transitions by opening and closing the throttle.
This signal is viewable in real time. We have set a voltage cursor at closed throttle voltage, which is about .7 volts.
Fig. 10 is the same capture as Fig. 9, but the cursor is now at WOT, or
4.3 volts. Voltage Scale is set at 5 volts, just like it was for the CTS. We used DC coupling so we can see voltage levels, and Ground Location in the middle, because it allows plenty of vertical resolution.
A positive slope trigger was used because the event begins with increasing voltage when we start to open the throttle. Trigger Level is set at 1.1 volts. Because the scope stores the input it can display what occurred before it actually triggers at 1.1 volts, so we see the voltage transition from the beginning. By setting a trigger we are also telling the scope that we want the trace to remain on the display unless the trigger criteria is met again. If we opened the throttle again a trace of the new event would replace this one.
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Mass Air Flow Sensor:
This MAF Sensor (Fig. 11) is also a low speed analog signal marked by mixed speed voltage transitions. Although the initial voltage transition is fairly fast it is still viewable in real time.
Because this signal is a lot like the TPS used in the previous example we have used the same settings with the exception of Trigger Level.
Normally the same Trigger level could have been used, but this capture was from a cold engine, so idle voltage was higher than the normal of about .6 volts.
AC Voltage:
This AC Voltage signal (Fig. 12) is a high speed analog signal, marked by fast voltage transitions. As you can see, it is not viewable in real time. We need to adjust the Time base to view this signal.
With this setup (Fig. 13) we can properly view this AC Sine Wave.
Voltage Scale is set at 2.0 volts because we expected less than that.
DC Coupling was used, but the trace would have been drawn the same with AC coupling. Ground in the middle as it should be when viewing an AC signal. Time base is set to 100mS and no trigger was set so trigger level does not apply. We could have used either a positive or negative slope trigger and set trigger level at any level within the range of the trace. This would have kept the trace from scrolling across the screen.
Square Wave:
This capture (Fig. 14) is a high speed Digital signal marked by very fast voltage transitions and is not viewable in real time. As you can see, a Voltage Scale of 5 volts is too low. The Ferret 92 scope will show a light blue trace when the signal ranges off of the display.
Fig. 15 shows the same signal as Fig. 14, but with the Voltage Scale adjusted. The cursor voltage shows why it was over ranged in the previous capture. Here Voltage Scale is set at 15 volts. It could have been set lower, but we see everything we need to at this setting. DC
Coupling and ground in the middle work well here. A very short Time base of 20mS was used and allows us to see eight cycles. A little math and we know that the frequency of this signal is a little over 400Hz.
Positive slope Trigger, although negative slope would work just as well, and a Trigger Level very close to the middle of the voltage transition.
This is a good place to set trigger level on most digital signals. If trigger level were set too high or low and circuitry problems kept voltage from achieving the set level the scope wouldn’t trigger so we wouldn’t see the bad events, and wouldn’t know if there was a problem.
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Port Fuel Injector:
Fig. 16 is a capture of a typical saturated switch fuel injector. It is a high speed digital signal and is not viewable in real time. As you can see the relatively short Time Base and ground at the bottom gives maximum resolution. Time cursors were used to measure the pulse width at 2.99mS.
Fig. 17 is a capture of the same injector as Fig. 16, but with a much longer Time Base allowing plenty of room for change in pulse width if we decide to snap the throttle to watch the system respond. As you can see, a Voltage Scale of 50 volts was used here, allowing us to see the voltage spike at 44.3 volts amplitude.
This injector has power to one side of the winding when the ignition is turned on and the driver completes the ground side of the circuit to pulse it. The proper way to connect your scope leads here is negative to a good ground and positive to the control, or negative, side of the injector. We used a negative slope trigger because the driver pulls the control side of the circuit to ground to pulse the injector. This means that voltage will decrease when the injector is pulsed. Trigger level is set fairly close to the middle of this voltage transition. If excessive resistance is present in the control side of the circuit, voltage may not be pulled to ground. If trigger level was set too low the voltage level might not fall past the set level, in which case, the scope would not trigger.
Peak & Hold Injector:
Fig. 18 is a peak and hold injector. We have our scope leads hooked up the same as we did in Fig. 17 and used similar settings. You can see that the waveform for this injector is different than the saturated switch injector we just looked at. it has two voltage spikes. This type of injector is used on CFI and TBI systems where it must open much more often that port fuel injectors. Because of this it must open faster, so it has a very low resistance winding. If it were controlled the same as a port injector the high current draw would produce a lot of heat which would cause the driver to fail. To overcome this obstacle two drivers are used, One driver switches the circuit to ground to snap the injector open and shuts off after a very short time, a little over 1 mS.
This gives the driver plenty of time to cool down, The second driver completes the circuit through a resistor, reducing the current to only what is needed to hold the injector open for the remainder of its on time.
The fi rst voltage spike occurs when the peak driver circuit is opened and the second when the hold driver is opened.
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Analyzing Waveforms
You now know the basic settings and have seen them used in examples.
In the following pages we will look at a couple of captures and what they tell us about the events.
TPS Sweep Test:
Fig. 9 is a TPS sweep test. This test is used to look for glitches
(abnormalities) that may be present at any level of throttle opening.
These glitches may appear in the trace as upward lines (spikes), downward lines (dropouts) or erratic areas (areas where voltage either doesn’t change with throttle position change or the change doesn’t follow a normal trend). We may also fi nd a fl at line throughout the sweep at or near either 0 volts (open circuit or short to ground), 5 volts (shorted to VREF) or between 5 and 15 volts (usually shorted to system voltage).
The capture in Figure 1 shows a normal sweep. Voltage is about .7 volts with the throttle at rest and about 4.3 volts wide open with smooth transitions and no spikes or dropouts.
Fig. 17 is a typical port fuel injector at idle. As previously stated, we are connected to the control, or ground side, of the injector and a good ground. The power side of the injector has B+ to it whenever the key is on. Until the injector is pulsed we will also have B+ at our connection on the control side unless the injector winding is open circuit, as you can see by looking at the horizontal part of the trace on either side of the event.
When the driver in the computer completes the circuit on the control side, we see the signal pulled to ground (fi rst vertical line). As the falling voltage passes the trigger level the scope begins to draw the trace.
The voltage is held at ground for about 3 mS. This is the injector on time, or pulse width.
The second vertical line shows the driver opening the circuit, which causes the magnetic fi eld in the injector winding to collapse. This produces an inductive kick, or voltage spike, of about 44.3 volts (varies with application). If you look at the curved line as voltage returns to system voltage you should see a small hump. This is the injector pintle altering the magnetic fi eld as it slams shut.
The capture in Figure 2 shows a good injector and good circuitry. The fact that the control side can pull the voltage to ground and hold it there proves the driver and control side circuitry. The inductive kick shows the integrity of the winding and power side circuit. The pintle hump verifi es that the injector is opening. For further confi rmation this injector could be compared to others on the same vehicle
MAF 2 Second Snap Throttle Test:
Figure 11 is a voltage trace of a mass airfl ow (MAF) sensor when the throttle was snapped open for about 1.5 seconds. The positive scope lead is connected to the signal wire and the negative lead to signal return (signal ground). This is a hot wire type sensor that is used on most Ford applications.
This MAF sensor operates in a 5 volt range, as do most sensors. Increases in airfl ow will be seen as increases in voltage. Looking at this capture you can see that the voltage is 1.3 volts at cold idle and has a lot of hash in it.
The hash is airfl ow pulsation caused by manifold vacuum changes as intake valves open and close. The fi rst rapid increase and then drop in voltage occurs when the throttle is snapped open and a surge of air is drawn into the intake manifold. At idle the pressure was negative in the intake manifold and positive on the air intake side of the throttle plate, so this surge is a result of the pressure equalizing when the throttle is opened wide. You can see
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