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
“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.