Electronic Design and Family Site

How I derived the numers contained in the "digit" tables

Math behind the Digital Power Meter

-Choose desired range of coverage:

-10dBm to +70 dBm.

-The voltage divider provides a 1000:1 voltage division ratio (Power ratio X 2). Attenuate desired range by 60dB to account for for divider:

-70dBm to +10dBm

-Convert Range to dBv (-dBm is dBv +13. (0dBm is -13dBv….0dBv is +13 dBm)):

-83dBv to -3dBv

-Voltage out is derived by the formula:

X = |108 – Y|*.025,

Where X is the voltage out of the Log Amp & Y is the Measured signal in dBv

(The intercept (0V output) of the AD8310 is -108 dBv, and the slope is .025V/dB)

See chart below:

Setpoint

AD8310 Output Voltage

dBv dBm +60dB from Divider Watts
  0 -108 -95 -35  
Low .625 -83 -70 -10 .1mW
  1.625 -43 -30 +30 1W
  1.875 -33 -20 +40 10W
  2.125 -23 -10 +50 100W
  2.375 -13 0 +60 1kW
High 2.625 -3 +10 70 10kW

I then set the low- and high-reference inputs of the ADCs to the points noted low and high on the chart above.

The digit charts in the firmware were determined by taking the corresponding log reading of each voltage represented by the "step" of each ADC count and splitting each of the three resultant digits into tables of their own.

ADC Calculations

ADC     Approx    Value     Range

Count dBm__________________________

0          -10             100    Under Range

1         -9.921875   102    Under Range

2         -9.84375     104    Under Range

3          -9.765625  105    Under Range

4          -9.6875       107   Under Range

5          -9.609375   109   Under Range

6          -9.53125     111   Under Range

7          -9.453125   113   Under Range

...becomes a MSD table of:

1

9

9

9

9

9

9

...Next digit table of:

0

9

8

7

6

6

5

4

...And a LSD table of:

0

2

4

6

9

1

3

5