Fundamental- distortion in Signal measured from a Signal generator through Oscilloscope

[blackite]

I have a doubt whcih is very fundamental. i wanted to generate a 100mv sinus signal from a signal generator and measure it on an oscilloscope. Actually to do this i attached a two sided BNC cable. The frequency was 100Khz.
1) First measurement- One side to signal generator and the other side to oscilloscope.
2) A second measurements i did was used one of the oscilloscope probe and directly attached the probe to BNC input port in the signal generator. Please see the picture attached to this thread.
As shown in picture the the oscilloscope signal - blue signal is from the first measurement BNC to BNC and the second measurement is the yellow signal. My question why are there distortions in the second measurement. I assume they are reflections whne i measure directly form the probe and in 2nd measurement the oscilloscope is filtering? But according to my knowledge the oscilloscope should show me the complete bandwidth also when a bnc cable is attached. I want to understand what exactly the oscilloscope is doing here because i want to use the sine wave generated form the SG for one of my projects.

 

[KlausST]

Hi,

Selecting just the sampling rate won't reduce noise.
Still the energy of the signal power is proportional to the the integral of each sample_voltage ^2.
Means there is no reduction in noise. Just some noise frequencies get aliassed to lower frequencies.

If one wants to improve signal to noise ratio one has to use a low pass filter.
An anti_aliasing_filter does this job.
It attenuates high frequencies, does not shift them down.
Then the signal looks more "noise free". SNR gets improved.

So
* just reducing sampling rate does not improve signal "quality" in the scope picture.
* reducing sampling rate plus using a suitable anti aliasing filter will make the signal look more "clean".
... and (like FvM mentiones) this usually don't do scopes, they just reduce sampling frequency.

Klaus

 

[barry]

No, scopes do not automatically adjust sample rate based on input frequency. As FVM stated, they will adjust it based on time base. On some scopes you can adjust the sample rate, but you are still limited by the selected time base (and memory).

 

[danadakk]

No, scopes do not automatically adjust sample rate based on input frequency. As FVM stated, they will adjust it based on time base. On some scopes you can adjust the sample rate, but you are still limited by the selected time base (and memory).

Of course we have the auto setting on modern DSOs that based on signal properties, like frequency and amplitude, will select the time base and vertical parameters to acquire and display waveform.



Regards, Dana.

 

[FvM]

The DSO simply can't know if you want to see the input with maximal available bandwidth or reduce noise and interferences as far possible. This is up to the operators decision.

 

[danadakk]

The DSO simply can't know if you want to see the input with maximal available bandwidth or reduce noise and interferences as far possible. This is up to the operators decision.

I would agree except I do not know what is offered these days in the high end Tek, LeCroy,
Rigol, Marconi in terms of settings, and methods.

In all my scopes its simple state machine on Auto, get a few cycles displayed with appropriate
vertical sensitivity.

But I could see a scope in the future that autos to minimize noise under some user preset
inputs. In fact I can see one where you talk to the scope and dialog with it to search for
best settings for desired results. One step further even using telepathy. just think it and
its done. I wont be around for that flavor however .


Regards, Dana.