08 March 2014

Calibration of the AD8307 Power Meter using fixed attenuators

I was now in a position to calibrate some additional points on the Power meter having completed the calibration of the 11MHz sine wave generator to the -10dBm reference point.

On hand I have the following attenuators which were purchased new from a commercial source. These attenuators do not have a specified tolerance, however they are marked to be used upto 1GHz. For this purpose I assume they are accurate and will form the baseline for calibrating the system. The attenuator values are:
3dB, 6dB, 10dB, 20dB,

The attenuators now allowed me to derive the slope in mV/dB at each measurement point. This was an encouraging result shown in the following table.

Pad(dB)           dBm        DVM(mV)      mV/dB
0                      -10            1565               ----
3                      -13            1507              19.33
6                      -16            1449              19.33
10                    -20            1367              19.80
20                    -30            1166              19.95
30                    -40              958              20.23
39                    -49              775              20.26

This shows a slope variation over the measured range of 20.26-19.33 = 0.93mV  which equates to 0.93/20 = 0.05dB. Not sure this is a meaningful calculation?

Looking at it a different way.  Midpoint over the range -10dBm to -49dBm
= (20.26-19.33)/2 + 19.33 = 19.80

Thus at the bottom of the range @ -49dBm and using 19.8 we derive (1565-775)/19.8 = 39.9dB
ie. at the reference point of -49.9dBm the power meter shows an error of 0.9dB. Acceptable?

Herewith Lab notes showing the calculations.

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