Email   Home Page


Google  x
 
Web www.bcae1.com
www.bcot1.com
Use F11 to go to full screen
viewing if using Google
Translate.


Oscilloscope

images/jpegs/IMG_2792c.jpg
Click on the photo to view a larger version of it. Use the back button to return to this page.


The most common oscilloscopes use a cathode ray tube (CRT) similar to the picture tube in a television. The CRT 'shoots' an electron beam at a charged, phosphorous coated screen. When the electrons hit the screen they excite the phosphorous atoms and cause them to give off light. The beam is constantly scanning across the screen. When there is no input signal, the beam just scans in a horizontal line.

Some of the newer scopes use LCD displays but, in my opinion, the resolution isn't as good as the older, CRT based scopes. When I'm looking at a sine wave, I want to see a smooth line. Not a bunch of jaggies. Until they get the resolution up to that of a high definition computer monitor, I'll stick with the older CRT based scopes.

USB Scopes:
If you're interested in using your computer as an oscilloscope (few - I don't know any - working techs do this). A dedicated scope is almost always going to be a better option. Any scope that uses a computer as the processor has to use menus to change settings. Using a standard scope, if you need to change a setting, you reach up and twist a knob.

Sound Card Scopes:
Sound card scopes are useless for amplifiers troubleshooting because they don't have enough frequency response (audio bandwidth) and don't allow DC to pass through the card. These are OK if you just want to practice using a scope but that's about all they're good for. You also have to realize that if you touch the wrong point in an amp, you could blow the sound card. If it's on the motherboard, it could cause the motherboard to fail.


Features and Common Settings

There are a number of switches on a scope which have to be set correctly for the information on the display to be of any use.

Inputs:
Most scopes have 2 input 'channels'. They are labeled channel A and channel B. They can be used individually or together. They can even be combined to produce a single trace from two inputs.

Focus:
The focus control simply allows you to keep the beam in focus. (big surprise!)

Vertical and horizontal position controls:
These controls allow you to position the beam on the display. You will virtually always set the vertical position of the trace centered on the horizontal reference line.

Timebase:
The timebase determines the time it takes to scan 1 division (from side to side). For audio, I generally use 2 milliseconds; for switching power supplies, 5-10 microseconds (depending on the frequency at which the power supply is oscillating). To view the class D carrier waveform (rail to rail switching waveform on the output transistors), you generally set it to 10 microseconds.

Cal/Var:
For most oscilloscopes, the timebase and vertical amplifier selectors have a cal (calibrate) or var (variable) potentiometer. These allow you to alter the display from the preset selections. For example, if you have a waveform that's swinging to just beyond the top/bottom of the display and you want it to fit on the display, you can use the cal/var control on the vertical amplifier for the channel of the scope that you're using. This can also be done by choosing the next selection on the timebase but that may reduce the size of the waveform a bit too much. The cal/var control should be set to the 'cal' position (generally fully clockwise). This will allow the scope to display the waveform properly and allow you to use the graticule to get rough estimates of the waveform's amplitude and frequency.

Trace intensity:
The trace intensity allows you to adjust the beam to a suitable brightness level. When the timebase is set for very short times (very fast scanning speed), the display may appear dim. If the scanning is slow, the display may be uncomfortably bright. If an intense/bright display is used often, it will also reduce the life of the CRT display by burning a line on the screen.

Volts/division:
Determines the sensitivity of the scope's vertical amplifiers. It allows you to adjust for the best resolution. For car audio 10v/div is the most common, lower settings (more sensitive input) are used for preamp level troubleshooting. Higher voltages are used when checking rail voltage or viewing the output signal of the amp when the amp is driven to a high output level.

Trigger source:
The scope must be 'triggered' to display a stable waveform. There are several options for the trigger source. The most common trigger source is the signal on the input being used. If you are using the channel A input, the trigger source would be set to 'channel A'. This is the configuration which I use most. If you are using both inputs, you can select either channel as the trigger source.

Trigger level:
For the waveform to be 'locked' on the screen, the signal has to be of a sufficient level. If you want the scope to be triggered (locked onto the signal) when the voltage of the waveform reaches a certain point, you can set the 'trigger level' so that it will trigger properly. For car audio work, this control is usually set to its center '0' position. It will cause the scope to trigger on the weakest of signals.

Trigger mode:
The trigger mode allows the scope to lock onto different types of signals. My advice... use the trigger mode which gives you the best results for the waveform being monitored.

AC/DC input coupling:
You should remember that we talked about high pass crossovers and the fact that a high pass crossover blocks low frequencies. You should know that a crossover is actually a filter. The input to the scope can be switched to go through a high pass filter or to bypass the filter. When switched to pass through the filter, the scope is A.C. coupled and the D.C. component of the signal is removed. When the scope is D.C. coupled, the signal is not passed through the filter.

Vector input:
The vector input is used when you need to compare two signals. When the scope is in the vector mode, a voltage applied to one of the inputs will cause the beam to move in the vertical plane. Input to the other input will cause the beam to move in the horizontal plane. There is no scanning in the vector mode.

Best Initial Settings:
Unless you have a very good reason not to do so, start with the trace centered vertically on the display and set the coupling to DC. Only use AC coupling when it's absolutely necessary. Set the cal/var to the cal position.

If you're viewing an audio waveform, use a timebase setting of 2ms/div as a starting point. If you're viewing a power supply waveform, start with the timebase at 10us/div. If you're viewing the carrier wave for a class D amplifier, 5us/div is a good initial setting. For most signals in a car audio amp, the 10v/div vertical amplifier setting is a good starting point.

The suggestions above are good initial settings but you may need to make slight adjustments (generally only one click up or down) to get the waveform displayed properly. In general, you want to have 3-4 full cycles of any repetitive waveform (like sine waves or power supply drive signals -- audio is a bit different) displayed and the signal should deflect to plus/minus 3 major divisions for AC waveforms. For DC waveforms, you want the signal to be displayed from the center, reference line up about 3 major divisions. In some instances (when a relatively small signal is riding on a significant DC voltage), you may need to switch to AC coupling but this isn't generally necessary. For DC signals that only swing positive (like the gate signals for power supply FETs, in most amps), set it the same but leave it DC coupled and leave the trace centered on the display. It's OK if it doesn't deflect to below the reference line.

Some people insist on using AC coupling because they're too lazy to switch to the proper type of coupling. If, for example, you're looking at the drive waveform for the power supply FETs in an amplifier, the coupling and reference is very important. In the next photo, you can see that the trace is aligned to the reference line on the scope's display. This is how it should be aligned every time you use the scope unless you have a reason to move it from the reference. You can see, however, that the scope is set to AC coupling (wavy line over the 'v' in the lower-left corner) and to 0.5v/div. These are wrong for this application. You will see that they've been changed in the next three photos.

This photo shows the waveform properly displayed. It's important that the drive voltage drops to below the threshold voltage (turn-on/off voltage) for the FET. For most FETs, that's about 3v DC. It's VERY important to know the voltage with reference to ground. That's why you have to align the trace with the reference line (which represents ground). As you can see, the falling edge of the waveform drops down to about 1.2v above ground before it begins to taper off. This is a good drive waveform.

Below, you can see that the waveform straddles the reference line. The coupling is improperly set to AC instead of DC. Here, you cannot possibly know how close the drive voltage is going (with reference to ground). This is essentially useless as a troubleshooting tool. The waveform could be riding on 3v or more of DC. If you assumed that the waveform was OK but it was riding on 3v of DC (that you can't see due to the AC coupling), and you powered up the amp without using the various safety precautions, the FETs would immediately fail.

The next waveform is just as useless as the previous one. The trace was not aligned to the reference line. Again, it shows nothing except that there is oscillation.

If you're asking for help on the forum or via email and you state that you've read this page, I will expect you to know and use all of the settings mentioned above (best initial settings section). Can you answer these questions? You should be able to do so without looking at the previous paragraph.

  1. Coupling, AC or DC?
     
  2. To what point should the trace be aligned before making any measurements?
     
  3. For audio waveforms, what's the best initial setting for the timebase?
     
  4. For power supply waveforms, what's the best initial settings for the timebase?
     
  5. What's the best initial setting for the vertical amplifier for most initial car audio troubleshooting?
     
  6. After you've set the timebase as described in the previous paragraph and begin to make fine adjustments to the timebase, approximately how many complete cycles do you want displayed on the scope (for repetitive signals like sine waves or power supply waveforms)?
     
  7. When making fine adjustments to the vertical amplifier on the scope to get the correct amplitude on the display, how many major divisions should the waveform deflect (from the reference line to the top of the waveform)?
     


Scope Display Markings:
The display on the scope will have a graticule (grid lines). These allow you to make rudimentary measurements. The Major divisions are the ones that correspond to the vertical amplifier and timebase controls. The minor divisions are simply there to make it possible to be a bit more accurate. The center line is the reference line.


Click HERE to open this in a new window if you can't see the fine details clearly.

Voltage Measurement:
If the input of the scope has a positive or negative DC voltage applied to it, the trace will still scan across but will be deflected up or down from the reference line in proportion to the voltage applied. The volts/division selector allows you to keep the beam from being deflected off of the top or bottom of the display. The v/d selector also lets you compensate for a small voltage so that you may view the signal with better detail.

When the scope is set so that the reference line and the trace are aligned (when there is no input to the scope) and you touch the probe to a ground connection in the amp, the trace will not (should not) deflect up or down. If it does, there is a problem with the ground connection between the amp and the scope. The following image initially shows the trace aligned with the reference line.

This is VERY important... Too many people have the wrong probe for their scope or don't pay attention to the position of the switch on the probe (for switchable 1x/10x probes). Many times, this means that the scope markings for the vertical amp will be off by a factor of 10. As was stated before, a vertical amp setting of 10v is a good initial setting. To confirm that the scope is working properly, set the vertical amp to 10v and touch the probe to the positive terminal of the 12v power supply. The trace should deflect 1.2-1.5 major divisions. If it does not, you MUST determine why it's not deflecting properly (moving to the proper/expected position on the display). In the following demo, the scope has an input voltage of 12.5v (the voltage on the positive terminal of the imaginary DC power supply). You can see that the scope is set to AC voltage. In the AC coupling mode, it cannot 'see' the DC on the input so the trace does not deflect. If you had the scope set like this (and were not aware of it), you may be checking to see if there was DC on a point and may (erroneously) believe that there was no DC. This can lead to errors and can even be dangerous. Now, click the DC coupling button and see how far the trace deflects. With the scope set to 10v/div, it deflects 1.25 divisions. If you set the vertical amp to 5v, you can see that it deflects 2.5 divisions. If you set it lower than 5v, it will deflect off of the top of the display. At 2v/div, the scope would have to have 6+ vertical divisions (to remain on the display) and it only has 4.

Time Measurement:
The time/division control tells the scope to scan at a predetermined rate of speed. If it is set at .2 microseconds/division (as it is in the picture). The time that it takes the beam to scan one horizontal division will be .2 microseconds. When using the scope for viewing audio waveforms, it is usually adjusted so that the scanning beam looks like a straight steady line. For viewing extremely low frequencies, it may be necessary to adjust the timebase (volts/division) control to a point that you may see the beam scanning across. If you set the timebase control to 100 milliseconds/division, it will take 1 second to scan across the whole display (10 divisions*.1 seconds/division). I haven't mentioned it yet but it is easy to determine the frequency of a sine wave if you know how long it takes to complete a full cycle. The frequency is the reciprocal of the time it takes to complete one cycle. If it takes 1/1000 of a second to complete one full cycle, the frequency of the signal is 1000 hertz. 1/500 of a second (or .002 seconds) for 1 cycle would be 500 hertz.


The following demo shows how changes in the time base and voltage selector will change the way the waveform is displayed. The signal is a 1kHz 1vpeak (2vp-p) sine wave. Depending on the speed of your processor, there may be a second or more delay between the time you push a button and the time the display is updated.


Click HERE to make this applet fill this window.


This scope has a few added features. The signal is the same 1vpeak (2vp-p) 1kHz signal except for the fact that it has 6 volts of DC offset. This is the type of signal that you'd find on the speaker output wires of a typical high power head unit. Since the volts/division includes DC, selecting a voltage lower than 2 volts (while DC coupling is selected) will result in the sine wave disappearing past the top of the screen.


Click HERE to make this applet fill this window.

Calibrating the Scope Probe:
The capacitance of the input circuit varies slightly from one scope to another. To calibrate the probe to the scope it's being used with (so that it displays square waves as proper square waves), you need to adjust the probe's internal capacitor. This is generally accessible through a hole in either the probe itself or the part of the probe that attaches to the scope (the part with the BNC connector). The following shows the calibrator signal with an unadjusted probe (first image) and then with a properly adjusted probe (second image).

For this scope, you simply insert the tip of the probe into the port on the front of the scope labeled 'probe adj'. While it's in the port, adjust the trimmer capacitor on the probe. This is best done with an insulated screwdriver. If you use an uninsulated screwdriver, you may have to make slight adjustments then remove the screwdriver to see if the waveform is square. Other scopes have various types of connectors for the calibration signal. The scope at the top if the page has a loop. The loop also allows you to check the function of a current probe (not very common). You'll also see BNC connectors and small rings. The calibration signal port isn't always labeled as such. Some labels you'll see are cal, calibrate, probe cal, probe adjust, 1kc cal...

The calibration test signal can also be used to test the function of a scope if a probe isn't included. Simply use a piece of 20g solid wire to connect the calibration signal to the center conductor of the input of the scope. That will allow you to test the scope to see if it's triggering properly and to see if it has a clean trace. If you're going to buy a scope online and the owner says that they cannot test it because they don't have a probe, tell them to test it as described above. If they will not do it, don't buy it unless they expressly offer a warranty.



 

Click HERE to visit a friend's new car audio tech site.

backward

forward
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
eXTReMe Tracker