On the scope circuit board there is a cal output test point in the little rectangle to the left of the power connector. Solder a short piece of stiff wire through the hole on the right. Leave it sticking up about 1/4" and trim it flush at the back. You can then use it as a quick input reference source because it is a 1Khz square wave with an amplitude from 0 to 3.3V

Is this (the green square i marked) the cal? output that you are refering? What is "cal"? calibration? It says J2 there 1kHz/3.3V and a square wave. Where should I add the wire? On A or B?
And 1inch = 2.54cm and 2.54/4 = 0.635cm or 6mm. Which is like a pin as short. Its very short.
And that can be a source of oscillation for something to be tested. I think I understand you.
Please confirm if is good or bad everything I said so far.
And thank you ! Very interesting ! I love it already.

 

The keyword for 10X is attenuation, both the signal and noise.

That is a gold information for me ! Super awesome. Love it. I hope I will remember it in the future.
And the 1x keyword? probably just "simple" probe.

 

See the square around the hole, that is the clue to pos or gnd. Cal is usually only needed for 10X. The idea is to adjust the variable cap in the probe to "straighten" out the square wave shown on the scope. Start with an active device such as a cap or coil and see what happens when you apply voltage. Then move on to oscillators.

 

Anything you attach to the circuit becomes part of the circuit. Even @ 1X there will be some attenuation.

 

Ok, let's look at some things we skipped.

The input channel has an INPUT COUPLING option switch, with settings GND - AC - DC

GND is for checking the 0V reference on the screen. The input signal does not reach the display and the trace on the screen should be horizontal and matched to the 0V arrow on the left side of the screen. If you see noise on the lowest range that is normal for this oscilloscope. The DSO138 suffers from a defect in that the trace does not match up with the 0V arrow. I have given a fix for this here:
https://forum.allaboutcircuits.com/threads/dc-offset-in-dso138-oscilloscope.151575/

AC and DC are not what we normally think of as being Alternating Current and Direct Current.
You can set the INPUT COUPLING to AC or DC depending on the nature of the waveform and what feature in the waveform you are interested in observing.

For example if you have an audio signal shifted by 1V, it would be difficult to observe what is happening on a DMM.
What we have here is a signal that is described as DC + AC. If you want to see the true nature of this signal then set the INPUT COUPLING to DC.

For the opposite scenario, suppose you have a PSU giving out 12VDC. Now you would like to measure the ripple voltage on the output. You cannot do this with most DMM. You may think that since this is a DC signal then you want to set the INPUT COUPLING to DC. Wrong. If the ripple voltage is low, say 100mV for example, you will not be able to observe the ripple on this DC + AC signal. You want to set the INPUT COUPLING to AC which will remove the DC voltage from the signal. In other words, AC setting introduces a HIGH-PASS FILTER and removes DC and low frequencies. The trace will now be centered about the 0V reference level and you will be able to observe the AC ripple when you adjust the voltage scale to an appropriate setting.

In summary, DC means observe all frequencies from 0Hz and up (DC + AC).
AC means remove DC and low frequencies using a high pass filter (AC only).

 

If you have something that can output sound or music such as an old radio or mp3 player, try looking at some sound waveforms.
If you have a microphone, build a mic preamp circuit and try singing into the mic.

 

If you have something that can output sound or music such as an old radio or mp3 player, try looking at some sound waveforms.
If you have a microphone, build a mic preamp circuit and try singing into the mic.

I had a spare audio jack and put 2 wires on it and inserted it into my headphones plug from my PC.
This osciloscope is very noisy. When nothing is connected to its probes, its just noisy and trembling (refresh rate most certainly).
I connected it to the audio jack I improvised, and I got the audio signal, by switching back and forth until I got an acceptable wave form.
But this osciloscope is having horrible delay. I paused and play from my shortcut key on my PC and the osciloscope is updating after 1s to 5s sometimes. Wow. Im discovering its limitations now. Hmmm....
I also have a mic to my PC and when I tap in it, I can see the sound waveform as a big spike, but more like a hill.
Excelent test so far. @Audioguru again will be happy.

 

Don't blame the scope yet. It could be the settings you have selected.

 

I use a couple of these CROs. Be aware of the rubbish on the horizontal line as its internal noise on all of these units.

 

Just remember how much you paid for this scope compared to others that cost 50 times more.

 

Is this (the green square i marked) the cal? output that you are refering? What is "cal"? calibration? It says J2 there 1kHz/3.3V and a square wave. Where should I add the wire? On A or B?
And 1inch = 2.54cm and 2.54/4 = 0.635cm or 6mm. Which is like a pin as short. Its very short.
And that can be a source of oscillation for something to be tested. I think I understand you.
Please confirm if is good or bad everything I said so far.
And thank you ! Very interesting ! I love it already. View attachment 229287

The wire should be attached to "B" in the rectangle. "A" is ground.

 

Just remember how much you paid for this scope compared to others that cost 50 times more.

Oh, Im very much aware of it. Now I am going a bit deep into it. But you guys have the same delays?
On the surface, it is doing its job. I mean, it reads the signal from that 555, and I am sure it will be a fantastic debugger for my circuits to come, (if i remember to take out the 220 from them - haha). My goal for it, what I think I will MOST use it as, first as a circuit tester, repair diagnose tool, then as a prototype tester. Especially for this Last part, I put my money in it.

 

he keyword for 10X is attenuation, both the signal and noise.

But, it increases bandwidth. Why? You have a filter with the 22 pf of capacitance to ground. With a compensated probe, ideally you have no capacitance to ground.

Note: the Fourier series for a square wave is the sum of n infinate number of odd harmonic sine waves. See https://www.mathsisfun.com/calculus/fourier-series.html

So basically, in the limit, you need infinite bandwith for ~0 second rise time edge of a square wave. That's not achieveable, nor is it practical.

Bandwidth is specified for Sine waves. It's defined as the frequency where the signal has been attenuated by the sqrt(2) ~ 0.707 or 70.7%.

 

Oh, Im very much aware of it. Now I am going a bit deep into it. But you guys have the same delays?

I don't understand what you mean by the delay. What is the TIME/div setting on your scope?
I have a DSO138 and I don't observe any delay.

 

There is a clip on lead on the scope probe. Generally, you try not to use it. Most scopes would come with a ground binding post or the scope would be connected to ground. When the scope is connected to ground sparks are possible.

Where it would "bite you" is that in a typical line switching power supply, the common of the primary side of the switcher is about 160 VDC off of earth ground. A 160VDC to Ground makes a spark. I have a blackened alligator clip to prove it when i wasn;t paying attention.

Dual channel scopes have the ability to sum channels and invert one of them, so you can measure A-B. Well, not quite. A has to be connected to ground and B has to be connected to ground and the scope ground has to be connected to ground.

 

I don't understand what you mean by the delay. What is the TIME/div setting on your scope?
I have a DSO138 and I don't observe any delay.

I figure it out.
It was this scroll bar. Originally it is in the middle. But by moving it to the right as I draw it with blue here, Maximum to the right, there is no more delay between my probe and screen rendering. When it was in the middle I had to wait 1s to 5s in some cases, for the signal I was probing to finally appear on the screen. Now is instantaneous as it should. You write it also in the picture you draw the meaning for each thing. I just look on it now. Something in my passive memory was working right.

So that is resolved.
Now, a new circuit to test, or new functionalities.

The wire should be attached to "B" in the rectangle. "A" is ground.

Actually, after a close visual inspection, I see that the 2 pads are connected each other and then go to a resistor.
Look closely at the red tracks. I marked with a green line their trajectory. So it didnt matter where I should have put the wire pin.
But I did exactly as you said, 6mm high (1/4") with a solid wire, on the B round pad. Also on the back is flushed/horizontal.
I also check for continuity on all 3 pads A,B and the resistor pad that is in line and yes all 3 are connected.
Now what? I probe it and I can receive a signal from it.


Very interesting thing... should I probe with both probes every time? Like with a DMM? Or is fine with only the red one? I still get some results by using only one probe (the red probe). Or those are not really results?

 

Common is to the scope itself, so only one wire. The common is implicit.

 

Big difference between the handheld and bench scope is ground. On the bench scope connecting the ground clip is a path to earth ground which CAN short a circuit. Not so on the handheld where ground is not earthed. On the bench scope, the GND clip MUST be connected to the system common ground. After a few smoked wires and hopefully not the scope it starts to sink in. Ground on the scope is NOT like the DMM GND probe which connects the circuit across the meter. Another good reason to start with low voltage and current circuits until you get the hang of it.

 

Actually, after a close visual inspection, I see that the 2 pads are connected each other and then go to a resistor.
Look closely at the red tracks. I marked with a green line their trajectory. So it didnt matter where I should have put the wire pin.
But I did exactly as you said, 6mm high (1/4") with a solid wire, on the B round pad. Also on the back is flushed/horizontal.
I also check for continuity on all 3 pads A,B and the resistor pad that is in line and yes all 3 are connected.
Now what? I probe it and I can receive a signal from it.
View attachment 229296

Very interesting thing... should I probe with both probes every time? Like with a DMM? Or is fine with only the red one? I still get some results by using only one probe (the red probe). Or those are not really results?

The two pads are not connected together. If you look on the reverse side of the circuit board you will see that A is connected to the ground plane. B is not.
On the clip lead, the black lead is connected to the ground, The red one is connected to the input. When you check the cal signal, you only need to connect the red clip to it. You will find that if you hold a short, solid piece of wire in the red clip jaws and probe A with it, you will get no signal. See the probe calibration instructions on you "How to use" instruction sheet.

 

But you guys have the same delays?

My very old analog 'scope has absolutely NO delay.
Many digital circuits produce delay, especially cheeep ones.

On the surface, it is doing its job. I mean, it reads the signal from that 555.

Have you used your new 'scope to show the triangle-wave from the 555 timing capacitor?