Quick question on lower voltage reading on oscilloscope

Thread Starter

RichW

Joined Nov 8, 2019
40
Hi, I'm learning to use some of my equipment so these may be dumb questions, but if anyone could spare a minute to answer it would really help me out. Thanks in advance.

If I connect my signal generator directly to my oscilloscope (at 500 mV) I get a reading of 512 mV (for 1us) and 505 mV (for 109 ns). Why does changing the period affect the voltage reading?

Also, if I connect the signal generator to my amplifier and oscilloscope using a tee piece, the oscilloscope reads 340 mV, for the 500 mV signal. I'm not sure why the voltage reduces.

Lastly, on my signal generator, there is something called HiLevel, which is set at +250 mV, when I adjust it, it change the mV reading, I am not sure what this offset is used for? Or why it is set agt +250 mv.
 

drjohsmith

Joined Dec 13, 2021
378
Can you share a picture of your setup please,

Any measurement has an effect on what is being measured.

So if your scope is 50 Ohms, then it is loading the signal generator,
the output impedance of the generator and the load impedance of the scope are in series,
and your measuring the mid point of this resistor divider.

As to why is signal going up and down on change of period,
send us a picture

You should also have a probe with the scope,
which might have a x1 / x10 switch
this changes the input impedance of the scope probe,
on X10, the scope probe impedance is probably M ohm,
so has little effect on slow frequency signals
on X1, then the impedance is much lower, and can have an effect on a signal

https://learn.sparkfun.com/tutorial...'s ground clip,the waveform around the screen.
 

Thread Starter

RichW

Joined Nov 8, 2019
40
Thanks very much for the reply, yes I have made a couple of image below, hopefully they are clear enough. Thanks also for the link provided!

EXPLAN1.pngEXPLAN 2.png
 

drjohsmith

Joined Dec 13, 2021
378
You see a few things there
If it's meant to be a square wave , it's very round .
Its also very distorted , indicating your loading the source excessively.

You see in the longer trace , the amplitude is modulated which is most likely the generator , or mains pickup .
 

KeithWalker

Joined Jul 10, 2017
2,297
You did not mention the frequency of the signal that you are measuring.
If you check in the manual for your signal generator, you will find its output impedance. It has an internal resistance of that value built in series with its output and that is the value of external load required to give the specified output levels.
Your oscilloscope has a high input impedance, usually around 1Mohm (10Mohm with a x 10 probe). That is the load on the generator when you just have the scope connected. The input impedance of your amplifier is much lower, usually around 10Kohm. When you connect that to the output using the "T" connector, you are changing the load on the generator and therefore changing the voltage at its output.
When you change the horizontal deflection on your scope you are changing the sampling rate of the ADC that measures the amplitude of the signal. this will result in a very small change in the accuracy of the ADC. I am sure that you will find that the difference in measured amplitude is well within the specifications given in your scope manual.
The "HiLevel" setting on your signal generator is to give you a larger signal on the ocaisions when you need it.
My advice to you is to read the manuals and familiarize yourself with their features and limitations. That way you will be able to make much better use of your test equipment.
Sometimes, the technical specifications are a litte difficult to understand. Let us know if you need any help.
 

Thread Starter

RichW

Joined Nov 8, 2019
40
Apologies for the delay, I've been snowed under. I am using ~ 1MHz, and have another image.

Picture2.png

Your oscilloscope has a high input impedance, usually around 1Mohm (10Mohm with a x 10 probe). That is the load on the generator when you just have the scope connected. The input impedance of your amplifier is much lower, usually around 10Kohm. When you connect that to the output using the "T" connector, you are changing the load on the generator and therefore changing the voltage at its output.
Awesome, I understand, thanks.

When you change the horizontal deflection on your scope you are changing the sampling rate of the ADC that measures the amplitude of the signal. this will result in a very small change in the accuracy of the ADC. I am sure that you will find that the difference in measured amplitude is well within the specifications given in your scope manual.
The "HiLevel" setting on your signal generator is to give you a larger signal on the ocaisions when you need it.
My advice to you is to read the manuals and familiarize yourself with their features and limitations. That way you will be able to make much better use of your test equipment.
Sometimes, the technical specifications are a litte difficult to understand. Let us know if you need any help.
Again, very clear. Thank you very much. I'm testing the oscilloscope more and picking it up gradually. Will poast again if I have any more questions.

Thanks all, very much appreciated!
 
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