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Voltage measurement on RF cable - RF Cafe Forums

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 Post subject: Voltage measurement on RF cable
Posted: Thu Oct 20, 2005 4:03 pm 
I have a piece of RG58 going into an into an o-scope with a 100 MHz BW. The signal is a sinewave (almost) and I know the power out of my source (10 MHz) is +10 dBm. Problem is the o-scope is reading about exactly twice the voltage I expect to see (which is 2 Vpk-pk). I'm measuring 4 Vpk-pk. Any ideas what might be going on. Thanks.
--- Ted


 
  
 
 Post subject:
Posted: Thu Oct 20, 2005 4:08 pm 
 
Colonel
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Joined: Mon Jul 18, 2005 4:31 pm
Posts: 40
Location: Baltimore, MD
I'll bet your scope's input is set to the high impedance mode. Make sure it is set to be a 50 Ohm load. You are exactly right to be expecting about 2 Vpp.


 
   
 
 Post subject:
Posted: Fri Oct 21, 2005 7:17 am 
Agreed, your scope impedance is not 50 ohms. If you do not have an option for 50 ohms, place a 50 ohm resistor in parallel between clip leads.


 
  
 
 Post subject:
Posted: Fri Oct 21, 2005 10:24 am 
kanling wrote:
I'll bet your scope's input is set to the high impedance mode. Make sure it is set to be a 50 Ohm load. You are exactly right to be expecting about 2 Vpp.


Thanks kanling - you are exactly right :!:

I can't believe I was dumb enough to not think of that. :D


Ted




Posted  11/12/2012
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