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Old 28th August 2004, 09:27 AM   #1
sss is offline sss  Israel
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Default how to make a clean square wave?

i wonder how to make a square wave with no overshoots like the frequency generator .

plus
what type of input buffer/op amp used in scopes that can show a clean square wave (or any other)?
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Old 28th August 2004, 06:37 PM   #2
Pabo is offline Pabo  Sweden
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Build an oscillator with a schmittrigger inverter, a resistor and a capacitor. Set the frequency by altering the resistance. Put a flip-flop after for generating 50% duty cycle.
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Old 28th August 2004, 06:54 PM   #3
sss is offline sss  Israel
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will that be as good as the freq generator?
i got many op amps and when the input signal is a square wave theres always an overshoot , i guess thats because they are too slow right?



what type of amps/buffers used in scopes that shows the wave as it is?
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Old 28th August 2004, 07:08 PM   #4
sajti is offline sajti  Hungary
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Audio amplifiers/buffers are not the best solution for square wave amplification. So for this, use non feedback swithcing type output stage. Use low impedance and high current..

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Old 28th August 2004, 07:12 PM   #5
sss is offline sss  Israel
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does feedback causes the overshoots?
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Old 28th August 2004, 07:18 PM   #6
sajti is offline sajti  Hungary
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Quote:
Originally posted by sss
does feedback causes the overshoots?
Yes. I had a schematic for one HP scope. It contains 10kHz squarewave generator. In this generator there was no any feedback, and it produced extremly perfect squarewave

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Old 28th August 2004, 07:25 PM   #7
sss is offline sss  Israel
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so what kind of amp do they use at scopes input?
it should amplify / attenuate all types of signals
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Old 28th August 2004, 07:38 PM   #8
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Have you seen the MAX038? It is a very good chip that needs few external components to give a frequency generator, with nice even square waves.
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Old 28th August 2004, 07:48 PM   #9
OliverD is offline OliverD  Germany
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Quote:
Originally posted by sss
so what kind of amp do they use at scopes input?
it should amplify / attenuate all types of signals
The trick is bandwidth.

Remember a perfect square wave of frequency f is a sum of several sine waves: 1 x f + 1/3 x 3f + 1/5 x 5f + 1/7 x 7f.... up to infinity. To correctly amplify a mixture of sinewaves with frequency up to infinity, you need of course an infinite bandwidth.

There's nothing special about the input amplifier of a scope, except that it is a very sophisticated and very fast one.

In your case, where you limit yourself to squarewaves only, you could use a fast comparator at the final stage to square things up.
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Old 28th August 2004, 08:01 PM   #10
sss is offline sss  Israel
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Quote:
Originally posted by OliverD
There's nothing special about the input amplifier of a scope, except that it is a very sophisticated and very fast one.
yeah but those fast amplifiers also can amplify "slow" signals with no distortion - thats special

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