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Old 6th September 2009, 11:32 PM   #1031
gerhard is offline gerhard  Germany
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Zoom from 1 to 6 KHz
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Old 7th September 2009, 12:13 AM   #1032
gerhard is offline gerhard  Germany
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Now I have increased the frequency of the square wave by 30 Hz to 3210 Hz.
The old spectrum is in the memory and overlaid with the new.


These are the peaks now:

1.120 KHz (was 1.370, delta = -250 Hz) ==> 8.33 * df, probably 8*
2.150 KHz (was 2.270, delta = -120 Hz) ==> 4 * df
3.210 KHz (was 3.180, delta = + 30 Hz) ==> 1 * df
4.260 KHz (was 4.080, delta = +180 Hz) ==> 6 * df
5.320 KHz (was 4.990, delta = +330 Hz) ==> 11 * df

I'll zoom in even more to get more precise frequencies, but not tonight.
It's 2:15 am local time.

Gerhard
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Last edited by gerhard; 7th September 2009 at 12:15 AM.
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Old 7th September 2009, 12:49 AM   #1033
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Thank you, Gerhard.
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Old 7th September 2009, 01:29 AM   #1034
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Quote:
Originally Posted by Wavebourn View Post
It reminds me of an old story about one professor: "They are dumb! I've explained them, but they did not understand! I explained them once more, but they did not understand! I explained the third time; I even myself understood, but they still did not!!! How dumb they are?!"
Actually this could interpreted quite differently, in my experience.
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Old 7th September 2009, 01:56 AM   #1035
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Please everyone, don't stray too far.
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Old 7th September 2009, 11:06 AM   #1036
gerhard is offline gerhard  Germany
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The questionable frequency pair in high resolution:

Squarewave = 3180 Hz ==> 1380 Hz
Squarewave = 3210 Hz ==> 1110 Hz

IMD product wanders 270 Hz down for input step of 30 Hz

=====> This is IMD with 9 * square wave frequency.

Methinks this argument is settled.

The not so funny thing is that everybody with a tone generator and a soundcard
could have done this in one evening, but nobody did it.
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Last edited by gerhard; 7th September 2009 at 11:15 AM.
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Old 7th September 2009, 12:02 PM   #1037
gerhard is offline gerhard  Germany
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Most IMD products are 10 dB better if the the LM741
is not sandbagged with +-5V supplies but +-15V
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Old 7th September 2009, 12:06 PM   #1038
gerhard is offline gerhard  Germany
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Even mature OpAmps like the AD797 are not so easy to impress by this abuse.
I just replaced the 741, no games with compensation etc.
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Old 7th September 2009, 12:10 PM   #1039
gerhard is offline gerhard  Germany
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This is a LT1028 made by Maxim, no comp cap in stage1:
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Old 7th September 2009, 12:20 PM   #1040
gerhard is offline gerhard  Germany
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The test board

... and now back to paid development work
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