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Old 24th March 2008, 11:04 AM   #1
spooney is offline spooney  United States
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Join Date: Dec 2006
Default oscilloscope test procedures?

Now that I finally have a scope I was just wondering if there was maybe a few tests that you guys generally use to gauge an amps overall health.Also wondering if anybody knew of a good online tutorial about using a scope or maybe a book I could pickup.
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Old 25th March 2008, 12:09 AM   #2
sdoom is offline sdoom  Germany
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Join Date: Mar 2004
Location: Heidelberg
hi spooney,

when fixing "regular" amps you wont have to fear the scope. Basically you check the SMPS (Most SMPS will run with a square wave form and a frequency around 20khZ to aprox 40 kHz. ) with a scope or if there is a bad fault in an analog amp you will check the frequency response of a channel. that┤s about it.

And to go further a scope acts more like a "schaetzeisen" (now you guys wanna look up that ) than a good meter, but a scope will "show" whats going on , thats the real big advantage. You can use a small signal sine wave generator for testing purposes. i built my own with an ICL8038 . this IC generates sine and triangle wave output with almost no parts. it works for checking the pre-amp and the power stage of an analag car amp.

When it comes to a digi amps you will probalbly go further into detail with your scope. To start with your scope you actually only need the basic functions.


those would be

Channel selection,voltage range, DC-offset, triggering and time base.

the most common scopes have to input channels. You can select the screen to show channel A , Channel B or both channels (on some scopes you can read this switch called "ADD"). For normal measurements with only one channel you want to see "everything" , that means you want the complete voltage, like a "true RMS - digital meter". That menas your input selector will be set to "DC". The DC setting allows the scope to measure both, AC and DC voltages. Set to "AC" will only show real AC voltages (you can use this setting to only measure a ripple voltage , all DC voltages will be filtered away). the GND - setting of an input will allow you to adjust where the "Null" - Level will be on the screen.


Triggering: The trigger function of a scope is used for telling the scope about what time the input voltage should be "analizyed" and put on the screen. Vor DC voltages you set the trigger to "DC" , for AC voltages you set the trigger to "AC". Most scopes allow external triggering. For basic measurements you wont need this setting, for digital measurements where time plays a role external triggering allows you to "see" time shifts. While checking car amps no need for external triggering.


DC - Offset. With DC offset you can "add" voltage to the voltage beeing measured. Usually scopes have a fixed setting . Leave that in the tested position.

Time / base: With that setting you can choose the "amount of time" that will be displayed on the screen. Most scopes have 8 digits (squares) in the vertikal axes and 10 squares in the horizontal axes.
the time / base sets the visible frequency that can be displayed.

e.g. you want to check a 1kHz signal. That would be 1000Hz or 1000 changes per second, or one change each millisecond. So if you set your time base to 1ms you will see one change per square unit on your screen.

Basic setting on a scope :

When you start, put your input to Dhannel A , set the input voltage type to "GND" and use the offset pod to center the line on the screen to the middle. Knowing about what voltage you are about to measure set the voltage switch to it┤s position (eg you want to check the driving voltage of the mosfets in a SMPS you would expect the signal to go from 0V (=GND) to about 12V). When you set GND to the center position of ypur scope then there are 4 squares to the top and 4 squares to the bottom, you would set the voltage switch to 5V/dig , eahc square would show you 5V. the peak voltage of the SMPS output would thaen read 2 and a half squares oin your scope ) Then switch the input voltage switch to DC. Expecting to check a SMPS running with 20 kHz you would calculate as follows:

20 kHz = 20000Hz or 20000 changes per second. That means 1 change every 50 Á seconds. So set the time base to the closest as 50Ásec you can. Set the triggering to DC (because a car amp SMPS switches DC voltages !)


this picture shows a SMPS signal running at about 50 kHz , the time base is set to 5Ásec. As you can see the complete cycle uses about 4 squares, 4x5Ásec = 20Ásec --> 1/ 20Ásec = 50000Hz




http://www.audiohobbyist.com/images/smps/squarewave.jpg


like I said, a scope will never be that accurate as a digital meter, but it makes things visible for us and thats what makes a scope real worthy !!!
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