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Voltage measurement on RF cable - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

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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 
 
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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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