Author Topic: Oscilloscope input signals have a noise.  (Read 2165 times)

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Offline XMATopic starter

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Oscilloscope input signals have a noise.
« on: March 21, 2018, 10:00:56 pm »
so as you can see that the scope shows the Sine wave as perfect but now on square wave it gets weird, how do i fix it?
 

Offline DC1MC

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Re: Oscilloscope input signals have a noise.
« Reply #1 on: March 21, 2018, 10:02:49 pm »
Did you calibrate your probe ?
 

Offline Benta

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Re: Oscilloscope input signals have a noise.
« Reply #2 on: March 21, 2018, 10:13:32 pm »
^ this.
 

Offline XMATopic starter

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Re: Oscilloscope input signals have a noise.
« Reply #3 on: March 21, 2018, 10:14:49 pm »
how do you do that? sry im new to scopes.
 

Offline Benta

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Offline TK

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Re: Oscilloscope input signals have a noise.
« Reply #5 on: March 21, 2018, 10:42:31 pm »
Did you calibrate your probe ?
I think it is known as probe compensation
 

Offline alsetalokin4017

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Re: Oscilloscope input signals have a noise.
« Reply #6 on: March 21, 2018, 10:57:20 pm »
Yep, compensation not calibration... but since we don't know where the "square" wave signal is coming from, we can't really tell from here if it is a compensation issue or a signal source issue, or both or neither.

XMA, please connect the probe to the scope's compensation signal (or "Probe Adjust") output on the front panel and show the signal from that.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline XMATopic starter

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Re: Oscilloscope input signals have a noise.
« Reply #7 on: March 21, 2018, 11:12:05 pm »
Yep, compensation not calibration... but since we don't know where the "square" wave signal is coming from, we can't really tell from here if it is a compensation issue or a signal source issue, or both or neither.

XMA, please connect the probe to the scope's compensation signal (or "Probe Adjust") output on the front panel and show the signal from that.

the source I was using was my phone using a tone generator app, and on each app, the square wave is always weird. and the other thing is I got a Tektronix 2221 and I don't see anything that is a probe adjust outputs.
 

Offline RobertPS

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Re: Oscilloscope input signals have a noise.
« Reply #8 on: March 22, 2018, 12:24:18 am »
I got a Tektronix 2221 and I don't see anything that is a probe adjust outputs.

Above the focus knob. The procedure is in the manual.
 

Offline XMATopic starter

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Re: Oscilloscope input signals have a noise.
« Reply #9 on: March 22, 2018, 12:43:26 am »
I got a Tektronix 2221 and I don't see anything that is a probe adjust outputs.

Above the focus knob. The procedure is in the manual.

it says i need to clip it, but how it's just like an input jack and where do i ground it to, theres a ground but its empty and cant clip to it.
 

Offline jh15

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Re: Oscilloscope input signals have a noise.
« Reply #10 on: March 22, 2018, 04:10:29 am »
ground your probe where it says ground nearby. I is a banana jack, but just hold your probe ground lead to it. You probably have an alligator clip lead on the probe.

Just stick the tip of your probe there. If you have the 'witch's cap' pull back to protrude the hook and touch it there. If you take it off, or likely possible, it is missing, just touch the pinpoint of your probe there.

Some may say this is a troll or a joke, I've been there, done that, got the tee-shirt, and explained to EE's that they did not have to return their new instrument batteries I installed  to the factory to be charged.

Likely it was  quality assurance engineer, forgot.

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Please don't blow it up look at dave's "how not to blow up your oscilloscope" video. You need to read and understand that.
If a bit technical, ask here.
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Offline BravoV

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Re: Oscilloscope input signals have a noise.
« Reply #11 on: March 22, 2018, 04:32:35 am »
You should download the manual and spend sometime there 1st, even you don't not understand it 100%, at least you will have an idea how to do basic scope preparation before using it.

Some T&M equipments do need some pampering when powered on for the 1st time, not like say a DMM.

Offline JS

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Re: Oscilloscope input signals have a noise.
« Reply #12 on: March 22, 2018, 05:31:36 am »
You are using your phone audio out (~20kHz bandwidth) to output a 4kHz square wave, so you would be just getting up to the 5th harmonic, that's only two useful harmonics as only odd ones matter here. To get the perfect square you need infinitly many harmonics, with 7 you might have a reasonable response. Try 1kHz square and come back!

Your probes should be compensated as someone already said...

JS
If I don't know how it works, I prefer not to turn it on.
 

Offline XMATopic starter

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Re: Oscilloscope input signals have a noise.
« Reply #13 on: March 22, 2018, 06:49:43 am »
thanks for all the replies, okay so when I do touch the scope probes to the probe adjust I get a perfect square wave, so then it's my phone that's putting out a bad square wave.
 

Offline JS

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Re: Oscilloscope input signals have a noise.
« Reply #14 on: March 22, 2018, 06:57:09 am »
thanks for all the replies, okay so when I do touch the scope probes to the probe adjust I get a perfect square wave, so then it's my phone that's putting out a bad square wave.

As I explained, a "bad" square wave is expected from the phone at that frequency, it isn't doing anything wrong or failing, it's just it's bandwidth, and the representation of a limited bandwidth square wave is quite similar as you are seeing.

JS
If I don't know how it works, I prefer not to turn it on.
 

Offline Performa01

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Re: Oscilloscope input signals have a noise.
« Reply #15 on: March 22, 2018, 02:09:35 pm »
Apart from probe compensation and source bandwidth, this looks like an AC-coupled signal (the slanted roof and bottom) - which makes sense, since the phone would sure have an output coupling capacitor, doesn't it?
 

Offline bson

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Re: Oscilloscope input signals have a noise.
« Reply #16 on: March 22, 2018, 07:57:01 pm »
Check that the scope inputs aren't set to AC coupling.  If so, switch over to DC.
 

Offline w2aew

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Re: Oscilloscope input signals have a noise.
« Reply #17 on: March 22, 2018, 09:15:27 pm »
thanks for all the replies, okay so when I do touch the scope probes to the probe adjust I get a perfect square wave, so then it's my phone that's putting out a bad square wave.

The smartphone based function generators are notoriously ugly for most waveforms other than a sine wave.
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