# Why coupling clamp dBuV to dBpW conversion factor is -17

Discussion in 'Physics' started by Basket_Case, Jan 15, 2009.

1. ### Basket_Case Thread Starter New Member

Jan 15, 2009
1
0
Hi all,

I am refering to the earlier query about the disturbance power measurement coupling clamp used for CISPR14 and other standards. As already pointed out, the ferrite coupling clamp is really a 1 turn 1 to 1 transformer. The receiver in use is usually designed to show dBuV and not dBpW and would have an impedence of 50R. If we consider a fundamental 1 volt applied across 50R, then the power would be 1/50 W (V squared over R). We need to divide our fundament 1 Volt measuement by 50. In power terms, dividing by 2 is easy (-3dB) and dividing by 100 is easy (-20dB). To divide by 50 so the measurement is "fixed", divide by 100 and then double the result. In other words a correction factor of -20dB but then +3dB, i.e, -17dB factor. I hope this easilly clears up the correction factor. It has nothing to do with the ferrite coupling clamp really, its just dBuV to dBpW conversion in a 50 ohm system