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Old 26th May 2013, 12:00 PM   #1
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Default Oscilloscope results differ with volt/div settings

Hi

Don't think this has been covered in another post.

I am new to using scopes and have just got hold of one (Philips PM3217)

I have been testing a filter circuit, trying to get results for the corner frequency.

I found that I got different frequency results with differing volts/div settings.

With a higher v/div setting, I got c.4.4KHz which was what my SPICE result was. With a lower v/div setting I got 3.8KHz.

I know there's not enough info here for a full answer, but what I'm wondering is whether there is an generally accepted issue with results from different v/div settings.

My gut instinct is that the lower the v/div setting (and therefore the larger the signal image on the screen) the more accurate the reading.

Am I on the right lines with this? (If so, I need to alter some cap values!)

Also, I should say I am using a x10 probe and I found that the lower I set the v/div setting, the less clear the reading on the screen - it got a bit fuzzy... So I'm also wondering if this has some bearing on it too...

Any help would be great.

Thanks.
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Old 26th May 2013, 09:24 PM   #2
DF96 is offline DF96  England
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You may see a difference between 1:1 probe and 10:1 probe, as they have different capacitance and so load the circuit differently. You should not see any difference between different 'scope ranges.
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Old 26th May 2013, 10:01 PM   #3
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As your scope seems rather old I guess the initial calibration might have gone bad over the years. Not every setting or channel is affected equally.
On the input connector there should be adjustable capacitors that affect signal response on the corresponding setting on the v/div range switch. You may have to readjust them with reference to your scope's manual. There may also be inaccuracies regarding your timebase which affects all voltage ranges.
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Old 27th May 2013, 11:34 AM   #4
PChi is offline PChi  United Kingdom
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I agree that the waveform should almost fill the screen to be able to estimate the amplitude visually. If the x 10 amplifier is being switched in and out I can well believe that the gain changes by a couple of %. The vertical accuracy of the PM3217 is specified as +/- 3 % or +/- 5 % with the x10 amplifier (a long time ago).
I guess that you haven't got an AC voltmeter with reasonable accuracy to use instead.
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Old 27th May 2013, 01:31 PM   #5
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Quote:
Originally Posted by EpicFail View Post
Hi

Don't think this has been covered in another post.

I am new to using scopes and have just got hold of one (Philips PM3217)

I have been testing a filter circuit, trying to get results for the corner frequency.

I found that I got different frequency results with differing volts/div settings.

With a higher v/div setting, I got c.4.4KHz which was what my SPICE result was. With a lower v/div setting I got 3.8KHz.

I know there's not enough info here for a full answer, but what I'm wondering is whether there is an generally accepted issue with results from different v/div settings.

My gut instinct is that the lower the v/div setting (and therefore the larger the signal image on the screen) the more accurate the reading.

Am I on the right lines with this? (If so, I need to alter some cap values!)

Also, I should say I am using a x10 probe and I found that the lower I set the v/div setting, the less clear the reading on the screen - it got a bit fuzzy... So I'm also wondering if this has some bearing on it too...

Any help would be great.

Thanks.
Can I ask a question, how do you decide from the display what the filter cut-off frequency is?

jan
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Old 27th May 2013, 02:23 PM   #6
AndrewT is offline AndrewT  Scotland
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Default frequency flatness or response from a scope

I do this rather differently from most users.

I set the attenuation of a stepped attenuator to exactly equal the passband gain of the amplifier. Let's suppose the amp is +30.1dB, then the attenuator is also -30.1dB. The input to the attenuator will be exactly the same as the output from the amplifier. I can compare these voltages with a not very accurate DMM.

I set the input signal to the attenuator feeding amplifier under test that is appropriate for the output level I require. Say 2.83Vac. This will show as 8Vpp on the scope.
I can sweep the test signal frequency and see from the 8Vpp trace when the output signal starts to drop. I can go up a further two or three octaves and watch the scope trace fall to below 4Vpp (>-6dB)
I now decrease the attenuation of the stepped attenuator from -30.1dB to -27.1dB. I adjust the test frequency until the trace reads 8Vpp.

What I have now is the same output voltage as the input voltage. I can use a DMM set to 20Vac to measure the input and output signals. There will be a slight error. I fine tune the frequency until the input and the output read identically.
The amplifier is now amplifying by 27.1dB, i.e. -3dB relative to the passband.

I can now measure the frequency using a frequency counter.
I have the F-3dB frequency.

This method uses "compare" to get fairly accurate results even though the signal generator and the DMM are not accurate.
If I want a bit better resolution I reduce the input signal to 1.95Vac and use the 2Vac DMM scale. I might get a 0.3% difference in input to output voltage due to the smallest step in my attenuator being 0.05dB. I would aim for the same input to output difference when fine tuning the test frequency. The 2000count on the 1.95Vac signal allows very good repeatability, if that is needed.
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Old 27th May 2013, 02:54 PM   #7
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Thanks for all the responses.

PChi: Sadly I don't have a decent AC voltmeter...

jan: The way I measured it was by maintaining a constant signal from an oscillator and dialing the frequency down until the signal was attenuated by -3dB
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Old 27th May 2013, 02:59 PM   #8
AndrewT is offline AndrewT  Scotland
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Please put up my post on measuring frequency response.
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Old 27th May 2013, 03:05 PM   #9
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Sounds like a correct way to do.
But I suggest you measure the level on a multimeter that can handle that frequency. Much easier to watch it fall from, say, 1V to 0.7V than on a scope. Hard to read those graticule lines anyway.
And AC voltmeters are very useful and cheaper than a scope ;-)

jan
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Old 27th May 2013, 03:21 PM   #10
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Yeah, I think I'm going to have to get am ac voltmeter. I have a DMM but it's very basic and it certainly doesn't have a level of sensitivity I would need for measuring filters.

Can anyone suggest a good ac voltmeter? (I live in UK)

Thanks
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