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William K.
User Rank
Platinum
Faulty oscillator? or not?
William K.   3/1/2012 10:30:30 PM
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IT is easy to get results that lead to incorrect conclusions when measuring with a scope. The challenge with digital scopes is that they can be misused in many more ways. It is not a fault of the scope, except for the ones that don't allow an adequate amount of bandwidth reduction.Sometimes it is very handy to nt have the 20MHZ bandwidth displaying all of the ambient noise, when what I want to see is the ten KHZ noise from a system. It is really hard to read the value of a signal that has an eighth-inch thick fuzzy trace of 17MHz noise from the ambient, when I know that the noise I want to read accurately is in the hundresds of Hz.

But with all of the features, it can be challenging to even understand how to use some digital scopes. That certainly does lead to problems and incorrect readings.

anatech
User Rank
Iron
Digital oscilloscopes are okay ...
anatech   3/1/2012 6:54:02 PM
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As long as you really understand how to use them.  Knowing their weaknesses, an analog 'scope will always have a place on my bench.

Any instrument can "lie" to you if you're not careful about how you set it up for the measurement.  It also pays to ensure the instrument is appropriate for the measurement you are trying to make.  This point was made earlier on but bears repeating.

Jim S
User Rank
Gold
Digital Scope Trickery
Jim S   3/1/2012 3:24:55 PM
NO RATINGS
I too have been burned by digital scopes and aliused signals. A new analog scope is hard to come by but I got a Tektronix 2466 used. It usually spots the real problems when the digital scope doesn't.

didymus7
User Rank
Platinum
Re: Why didn't they....?
didymus7   3/1/2012 9:07:53 AM
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Then that doesn't say much for that very expensive digital scope.  It was definitely mis-leading them.  For the life of me, I can't figure out how you can have 2V pp of noise and not see it.  How can you have that amount of noise and see, on your very expensive digital scope, a dead flat line???  Case in point, we had one customer complaining about a 25 hz oscillation coming out of our system.  It's an eddy-current displacement system and the only frequency normally on this system is the 500khz to the sensor.  As it turned out, their (also very expensive) digital scope was aliasing the residual 500khz carrier.  As they changed the time base, the frequency of the 'noise' changed until they got to 500khz.

Tmachell
User Rank
Iron
Re: Why didn't they....?
Tmachell   3/1/2012 8:59:20 AM
NO RATINGS
Yes, a digital scope was used in this instance...a very expensive one.

didymus7
User Rank
Platinum
Why didn't they....?
didymus7   3/1/2012 6:29:54 AM
NO RATINGS
The first thing that came to mind is, if there was a lower frequency waveform on the power supply line, why couldn't they see it even at a short time base?  One end or the other of the wave would have wandered from baseline.  Is it because they were using a digital scope?  One of the persistent problems I see is that digital scopes do not represent noise correctly.  You either don't see it or it looks horrendous but isn't really there.  They need a good combo scope, oh, wait, you mean almost nobody makes them anymore???

jmiller
User Rank
Platinum
Re: This example is why digital scopes are BANNED in my lab!
jmiller   2/29/2012 8:33:58 PM
NO RATINGS
The more posts I read I am reminded of an old supervisor of mine that whenever he see any data first asks what am I looking at, and how are you measuring it. 

Tmachell
User Rank
Iron
Re: Instruments readings != word of God
Tmachell   2/29/2012 8:10:22 AM
NO RATINGS
Ha...! Yeah...I know what you mean. I had one where a techie was testing an inductor using the ringing test on a Sencore RCL meter and found that all the devices were failing. Turns out that the manual stated that the ringing test was not appropriate to test inductors of that value.

Another one was where someone had spec'd an intrinsically safe milliohm meter to be used in fuel tanks. Turns out that the accuracy wasn't good enough to make the application measurement. He commented on the fact that he hadn't considered the accuracy specs prior to recommending the meter. Wha?? It all worked out in the end. The nota bona here is that no one is perfect, including me. This is why all aircraft maintenance requires that there be an independent inspection post repair.

Reminds me of another coworker who was trying to get an inverter chip to work and became really annoyed that he couldn't get any output. He fought with it most of that day only to realize that he had grounded the chip...boy did he feel foolish...to this day, he and I still use "did you ground the chip?" question to poke fun when either of us does something stupid or misses the obvious. It happens. 

Now, where's my pencil...?? Over my ear...duh!! :-)

Tmachell
User Rank
Iron
Re: This example is why digital scopes are BANNED in my lab!
Tmachell   2/29/2012 7:47:03 AM
NO RATINGS
How true! Turns out the main reason why I didn't look closer the first time was because I was working with techies who already assured me that the DC was fine. I had originally suspected the power supply but assumed that they had checked it ok. Never assume anything.

Tmachell
User Rank
Iron
Re: Following the trail
Tmachell   2/29/2012 7:43:35 AM
NO RATINGS
In this regard, "being in tolerance" means passing the automated test. The test can't move forward if the UL or LL are exceeded.

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