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Will the real voltage please stand up.

Question asked by ScopeDope on May 29, 2018
Latest reply on May 31, 2018 by mychal

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Hello Forum,

I've keyword searched the scope forum here and still have not found exactly what I am looking for. As the topic says it all, I'm trying to determine what probe attenuation ratio will show the actual circuit voltage.

I understand that reading higher voltages in circuits is followed suit by greater probe attenuation at the scope probe setting. My question is this:

In reading the Measurements (Amplitude) on the scope with a attenuation rate set at 10:1 am I seeing the true circuit voltage or is this reading nulled by a factor of 10? Taking in to account the voltage being measured and applying the correct probe attenuation I am looking for a "best setting" that will truly represent the circuit voltage present at the probe tip. I hope I didn't confuse myself.

Any assistance is greatly appreciated from the community.

Cheers,

Chris

Fungible May 30, 2018 6:05 PM

Correct Answer

“It sounds as if I'm going to need to know the source voltage and then adjust the probe attenuation accordingly to get the actual probe tip value.”

I can see why that makes your head hurt. A scope is a voltage measurement device but you are thinking you have to know the voltage to measure the voltage. Ouch!

But, you don’t need to know the voltage. You only need to know the probe attenuation. Any probe you buy will list its attenuation value. Simply enter that specified value in the scope menu and the scope’s readings will be the voltage applied to the tip of the probe.

If you build a probe yourself, you will need to measure its attenuation so you can enter it into the scope.

Your scope does not automatically detect the probe attenuation. You will need to manually set it based on the probes specified attenuation.

So why didn’t your experiment work? Watch out for the following.

If you use a signal generator with 50ohm source impedance, set to 1Vp-p and applied to a scope channel with 50ohm input impedance, you should/will read 1Vp-p (attenuator set to 1:1 since no probe is attached). However, our scope does not have a 50ohm input impedance mode, only 1megohm. Unless you use an external feedthrough 50ohm termination, you will read 2Vp-p when the source is set to 1Vp-p. 2Vp-p is the voltage applied to the input of the scope under these conditions. Now if you probe the Tee with a 10:1 probe and the probe attenuation set to 10:1 (manually set), that reading will also be 2Vp-p.

Bottom line- if you use the probe that came with the scope, set the attenuation of the channel that has the probe attached to 10:1. Set the attenuation of the channel connected to the source to 1:1. Whatever the source is set too and whatever the scope input impedance is, the two reading should be the same.

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algoss

May 30, 2018 5:00 AM

If the scope can identify the probe, and the attenuation of the probe, then it will do the math so that the voltage seen on the screen is the actual voltage at the probe tip. Most scopes can identify most probes.

You can check this yourself. Attach some some of signal generator, through a cable and a tee and cable into one of the channels of your scope. Set up the generator and the channel at 50 ohms impedance. Put a known signal, like a 1V pk-pk sine wave into the cable, and see what it looks like on the scope. Now attach a probe to another channel, and probe the signal at the tee. You should see the same voltage value on both the probed and direct channels.

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ScopeDope @ algoss on May 30, 2018 6:11 AM

... If the scope can identify the probe, and the attenuation of the probe, then it will do the math so that the voltage seen on the screen is the actual voltage at the probe tip. Most scopes can identify most probes.

I think that answered my question right there, My scope (MSOX-2024A) does not automagically register the probe attenuation. In setting up the test as you mentioned I get different Vpp values at 10:1 attenuation on both Ch1 (Probe) and Ch4 (50 Ω WaveGen). If I change (manually) the attenuation factors of both channels then yes, the Vpp values can be made to read 1 Vpp. I guess I was looking for a way to read the true voltage on the scope without first knowing the voltage setting at the source if that makes sense.

It sounds as if I'm going to need to know the source voltage and then adjust the probe attenuation accordingly to get the actual probe tip value. Does that sound correct?

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Fungible @ ScopeDope on May 30, 2018 6:05 PM

“It sounds as if I'm going to need to know the source voltage and then adjust the probe attenuation accordingly to get the actual probe tip value.”

I can see why that makes your head hurt. A scope is a voltage measurement device but you are thinking you have to know the voltage to measure the voltage. Ouch!

If you build a probe yourself, you will need to measure its attenuation so you can enter it into the scope.

Your scope does not automatically detect the probe attenuation. You will need to manually set it based on the probes specified attenuation.

So why didn’t your experiment work? Watch out for the following.

1 person found this helpful

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ScopeDope @ Fungible on May 30, 2018 6:47 PM

Ding!...Ding!...Ding!...Ding!... We have a winner!

After I read your response (a couple of times) it became clear. No surprises as I followed through on your example and it worked as you described, to a "T".

As you can tell I'm trying to get my head wrapped around this scope thing and it is nothing short of fascinating. I'm fortunate, and thankful, that community members reach out to aspiring electronics enthusiasts.

Thank you again for the response,

Chris

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mychal @ ScopeDope on May 31, 2018 4:09 AM

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5 Replies

algoss May 30, 2018 5:00 AM
Mark Correct
Correct Answer
If the scope can identify the probe, and the attenuation of the probe, then it will do the math so that the voltage seen on the screen is the actual voltage at the probe tip. Most scopes can identify most probes.

You can check this yourself. Attach some some of signal generator, through a cable and a tee and cable into one of the channels of your scope. Set up the generator and the channel at 50 ohms impedance. Put a known signal, like a 1V pk-pk sine wave into the cable, and see what it looks like on the scope. Now attach a probe to another channel, and probe the signal at the tee. You should see the same voltage value on both the probed and direct channels.
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- ScopeDope @ algoss on May 30, 2018 6:11 AM
  Mark Correct
  Correct Answer
  ... If the scope can identify the probe, and the attenuation of the probe, then it will do the math so that the voltage seen on the screen is the actual voltage at the probe tip. Most scopes can identify most probes.
  
  I think that answered my question right there, My scope (MSOX-2024A) does not automagically register the probe attenuation. In setting up the test as you mentioned I get different Vpp values at 10:1 attenuation on both Ch1 (Probe) and Ch4 (50 Ω WaveGen). If I change (manually) the attenuation factors of both channels then yes, the Vpp values can be made to read 1 Vpp. I guess I was looking for a way to read the true voltage on the scope without first knowing the voltage setting at the source if that makes sense.
  
  It sounds as if I'm going to need to know the source voltage and then adjust the probe attenuation accordingly to get the actual probe tip value. Does that sound correct?
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  - Fungible @ ScopeDope on May 30, 2018 6:05 PM
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    Correct Answer
    “It sounds as if I'm going to need to know the source voltage and then adjust the probe attenuation accordingly to get the actual probe tip value.”
    
    I can see why that makes your head hurt. A scope is a voltage measurement device but you are thinking you have to know the voltage to measure the voltage. Ouch!
    
    But, you don’t need to know the voltage. You only need to know the probe attenuation. Any probe you buy will list its attenuation value. Simply enter that specified value in the scope menu and the scope’s readings will be the voltage applied to the tip of the probe.
    
    If you build a probe yourself, you will need to measure its attenuation so you can enter it into the scope.
    
    Your scope does not automatically detect the probe attenuation. You will need to manually set it based on the probes specified attenuation.
    
    So why didn’t your experiment work? Watch out for the following.
    If you use a signal generator with 50ohm source impedance, set to 1Vp-p and applied to a scope channel with 50ohm input impedance, you should/will read 1Vp-p (attenuator set to 1:1 since no probe is attached). However, our scope does not have a 50ohm input impedance mode, only 1megohm. Unless you use an external feedthrough 50ohm termination, you will read 2Vp-p when the source is set to 1Vp-p. 2Vp-p is the voltage applied to the input of the scope under these conditions. Now if you probe the Tee with a 10:1 probe and the probe attenuation set to 10:1 (manually set), that reading will also be 2Vp-p.
    
    Bottom line- if you use the probe that came with the scope, set the attenuation of the channel that has the probe attached to 10:1. Set the attenuation of the channel connected to the source to 1:1. Whatever the source is set too and whatever the scope input impedance is, the two reading should be the same.
    1 person found this helpful
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    - ScopeDope @ Fungible on May 30, 2018 6:47 PM
      Mark Correct
      Correct Answer
      Ding!...Ding!...Ding!...Ding!... We have a winner!
      
      After I read your response (a couple of times) it became clear. No surprises as I followed through on your example and it worked as you described, to a "T".
      
      As you can tell I'm trying to get my head wrapped around this scope thing and it is nothing short of fascinating. I'm fortunate, and thankful, that community members reach out to aspiring electronics enthusiasts.
      
      Thank you again for the response,
      
      Chris
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      - mychal @ ScopeDope on May 31, 2018 4:09 AM
        Mark Correct
        Correct Answer
        If you use a signal generator with 50ohm source impedance, set to 1Vp-p and applied to a scope channel with 50ohm input impedance, you should/will read 1Vp-p (attenuator set to 1:1 since no probe is attached). However, our scope does not have a 50ohm input impedance mode, only 1megohm. Unless you use an external feedthrough 50ohm termination, you will read 2Vp-p when the source is set to 1Vp-p. 2Vp-p is the voltage applied to the input of the scope under these conditions. Now if you probe the Tee with a 10:1 probe and the probe attenuation set to 10:1 (manually set), that reading will also be 2Vp-p.
        
        Bottom line- if you use the probe that came with the scope, set the attenuation of the channel that has the probe attached to 10:1. Set the attenuation of the channel connected to the source to 1:1. Whatever the source is set too and whatever the scope input impedance is, the two reading should be the same.
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Will the real voltage please stand up.

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