08 March 2014

Power Meter calibration using the Weinschel step & fixed attenuators

Now that I had established a few calibration points I wanted to establish further reference points. These would assist in determining the variances in linearity over the range -10dBm through -70dBm.

On hand I had a step attenuator with the following markings:

MANUFACTURER: Weinschel Engineering, Gaithersburg, MD.
MODEL: 9332
SERIAL: 441 0955-0085.
STEPS: 0dB, 10dB, 20dB, 30dB, 40dB, 50dB

I compared the Weinschel with my fixed attenuators using the power meter as the comparator.

The following results were obtained @11MHz.

dB(marked)         Weinschel DVM (mV)         Fixed Attenuator (mV)        Difference (mV)
0                             1565                                      1566                                  1
10                           1361                                      1368                                  7
20                           1167                                      1166                                  1
30                           962                                         959                                   3

The above shows excellent fit between the fixed and step attenuators with a maximum difference between the 10dB pads of 0.5%.

I now derived a result for the 40dB & 50dB attenuators based on a nominal slope of 20mV/dB.

dB          Weinschel DVM (mV)     Slope     dB derived              
0                1565                                -             -
40              761                              20            40.2
50              565                              20            50

This gave me confidence that the Weinschel was indeed an accurate attenuator .

By combining my fixed attenuators with the Weinschel settings I was able to measure reference points from -10dBm through -70dBm in steps of 0dB, 3dB, 6dB, 10dB. Refer to the table as shown below.

Referring to the linear equations on W7ZOI's essential notes. http://w7zoi.net/Power%20meter%20updates.pdf 

For two reference points P1(dBm) & P2(dBm)  having respective DVM voltage measurements V1(mV) and V2(mV)

The linear equation has the following form  P(dBm) = A + B*V(DVM)

Where B = (P1-P2)/(V1-V2), A= P1-((P1-P2)/(V1-V2))*V1

Using an excel spreadsheet I developed an equation for each decade of the slope. This can be used to obtain the most accurate possible power meter reading should it be so desired. It is interesting to examine the table below. The dB/mV slopes are indeed 'slightly' different for each decade.

The equations below are tedious to plug into a calculator, however they do yield the most accurate result. Alternatively the last column slope numbers can be used.


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