Distortion in Signal measured from a Signal generator through Oscilloscope - Page 1
Forum section: electronics / beginners

author: blackite
author_rank: Newbie
timestamp: October 06, 2021, 01:07:33 pm
content: 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.

author: CaptDon
author_rank: Super Contributor
timestamp: October 06, 2021, 01:26:36 pm
content: Was the second measurement using a 10:1 probe? If so the scope would have increased itsgain to display a trace of the same height as compared to the blue trace and you are seeinginternal scope noise, not external distortion.

author: blackite
author_rank: Newbie
timestamp: October 06, 2021, 01:58:22 pm
content: Yes it sems to be 10:1 probe. But usaually a 10:1 probe should reduce a 100v signal to 10v so in my case a 100mV signal should be shown on Oscilloscope display as 10mV but this is not true . I see a sinus signal which has peak amplitude of 100mv. If your statement is true the probes internal noise is attenuated but the main measured signal is not attenuated. why so?Also i detached the BNC to BNC cable and removed the blue measurement from display but the distortion seems to be still there. How can i get back the signal with unamplified noise?

author: HB9EVI
author_rank: Frequent Contributor
timestamp: October 06, 2021, 02:11:52 pm
content: the way you explain the setup, it's a bit tricky to follow, but if I see the scope probe with the ground line hooked up in this manner, the first thing popping up in my mind is 'common mode noise'

author: blackite
author_rank: Newbie
timestamp: October 06, 2021, 02:38:00 pm
content: https://www.reichelt.de/de/de/adapterleitung-bnc-al-0-64-mm-schwarz-bnc-al-0-64-p106209.html?PROVID=2788&gclid=CjwKCAjwkvWKBhB4EiwA-GHjFp9LSF_ycU6MMX8ruWdk8Yl67wUfdldpgmRJ47HNblMq1-_mMjUwPhoC0OoQAvD_BwE&&r=1wil such a cable help me to fix the noise or common mode noise if i want to use the sinus signal generated from SG???

author: RoGeorge
author_rank: Super Contributor
timestamp: October 06, 2021, 03:13:55 pm
content: The oscilloscope knows the dividing factor of your probe so it adjust the screen indication.Watch first these videos about probes and probing, in general, then you can redo the measurements.

author: Tom45
author_rank: Frequent Contributor
timestamp: October 06, 2021, 03:30:18 pm
content: Quote from: blackite on October 06, 2021, 01:58:22 pmCaptDon had the explanation.As you expect, the 10:1 probe sees 100mv at the probe tip and reduces it to 10mv at the scope end. But the scope and probe interface is smart enough to know about the 10:1 signal attenuation. So it increases the gain in the scope's input by 10x and displays the scale factor, taking that into account. So you can directly read the amplitude at the probe tip. The scope took care of that for you.So the displayed trace and amplitude correctly represent what is seen at the probe tip. But in one case, the vertical scale factor is 20 mv/div, while the other case it is only 2 mv/div. That isn't much.What about the noise?The higher gain in the scope's vertical input amplifiers will mean relatively more noise from the scope's input channel.Another possible source is the relatively long probe ground lead at the signal generator. It could be picking up some stray signal in your lab. You could try raising the sweep rate to see if you can trigger on the noise. If you can trigger on it at some much higher sweep rate, then the noise is most likely being picked up by the ground lead from something else in your lab. If you can't trigger on it, then it is more likely to be random noise in the scope's vertical amplifier circuitry.Another test would be to output 1 volt from the signal generator and see how much noise you see when using the 10:1 probe.Finally, if you have a 1:1 probe try using it. At audio frequencies, the limited bandwidth of a 1:1 probe probably won't be a problem. And the 10x increase in signal at the scope's input is better for low audio signal levels.

author: blackite
author_rank: Newbie
timestamp: October 07, 2021, 07:55:11 am
content: I found the solution while further testing for faulty probes(for my relative new OScilloscope). Thank god no faulty probes. The noise is measured by the Oscilloscope because my bandwidth was set to 1GHz. To test this i measured the voltage output of a 9v battery and observed that it was showing oscillating noisy signal between 8.9v and 9.3v which according to me should not be the case. it should be quite precise. Reducing the bandwidth to 20 Mhz did improve the battery signal shown on Oscilloscope and the SG's Sinus signal shown on Oscilloscope. I learned something new that depending on the frequency of the input signal from signal generator i should have a suitable bandwidth set in my oscilloscope otherwise the probe will measure unwanted noise signals.Thank you for the videos, recommendations and suggestions. I will look at those videos to have a better understanding of how 10:1 probes work.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 07, 2021, 08:50:07 am
content: 400mV noise when measuring a 9V battery is very big. How did you measure? 10x, 1x, AC, DC, V/Div, Bandwidth and most important, did you use directly the probe tip, or are you using the probe with the alligator clip? Are you measuring near a radio tower, or maybe you have LED lights somewhere in your lab?- How much noise do you measure when nothing is connected and all the probing cables are unplugged?- How much noise do you measure when the input BNC is short circuited to GND (without any cables)?On my Rigol DS1054 I measure about 1.7mVpp or 0.6mV RMS on a 9V NiMH battery probing with the spring GND contact, probe 1x, oscilloscope on AC, 20MHz bandwidth, 1mV/div, 5ms/div. With no band limit (which means 100MHz bandwidth for my oscilloscope) the noise is bigger, about 3.5mVpp or 1.2mV RMS. (see the 2 attached screen captures).With the same setup except the probe is set on 10x, I get about 5 more times the noise measured when probe was set to 1x. (no pics attached for 10x)