11 November 2013

RF Power Measurements with a scope 50ohm terminator and attenuators

I wanted to measure the power output of my homebrew rig using a commercial scope terminator. The diagram of the two configurations is shown below. Because the terminator can only dissipate 1Watt maximum and to get some variation in readings, I used a set of calibrated attenuators of the following values 3dB, 6dB, 10dB and 20dB to reduce the input signal level from the TX into the scope load by a known amount. I performed two sets of measurements 1) with the pad at the TX end of the coax and 2) with the pad at the terminator end of the coax. (see the diagram below). 

 Thus the signal level into the scope was varied from +30dBm down to +13dBm. By applying the pad multiplier to the recorded result I came up with a variation in power measurement of anywhere between 2205mW - 1823mW. This is a swing around the average of +13% and -6%. The average being 1946mW. This is a swing from 33.4dBm - 32.6dBm. This result showed a wider variation than I was expecting. 

Examination of the two sets of results with different pads shows the following variations in mW. 
  1. 3dB = 2205-2000 = 205mW
  2. 6dB = 1904-1850 = 54mW
  3. 10dB = 1936-2025 = -89mW
  4. 20dB = 1823-1823 = 0mW
It is difficult to draw any conclusions based on the above other than the observation that the measurement with the 3dB pad shows a variation of much greater magnitude than the others. 

Resolution in the reading must be dependent on a number of differences in parametsrs. V/div, Linearity of the Y amplifier, accuracy of the pads etc. 

The accuracy is exacerbated by the fact that Power is a Squared function of voltage. P = Vp-p^2/8R. Thus any error is significantly magnified. Hence the usefulness of the dB result..

If the result is examined in dB then the variation is only +1.5% and  - 0.9% above and below the average of 32.9dBm. Interesting!

Next I will make measurements based on the built RF Power meter (see my previous blog entry). This will provide a good comparison between the two methods of measurement. 

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