AD8307 Power Meter Update

In an earlier post (RF Power Meter), I discussed my power meter project which is based upon the amazing AD8307 logarithmic amplifier chip. This device takes a logarithmically scaled RF input and produces a linear DC output with a resolution of ~25mV/dB.

Well, I'm finally able to put this project to bed. After scratch building an initial board, I decided to go with a much cleaner solution by purchasing a pre-fabricated board from E&M Solutions. The finished board is shown below:

Completed board with RF shield covering log chip and input stage

The circuit board provides two ways to show the power level of a signal: an analog panel meter for an approximate reading plus a DC output jack for use with a DVM when I need more accuracy. In order to improve the overall resolution of the output, the board uses an op amp with a trimmer pot to produce a gain of ~2.1 to boost the AD8307 output to 5V when the maximum expected input signal is applied. The panel meter circuit has an additional trimmer pot to adjust the meter sensitivity. After adjusting both of these trimmers, whenever an input of 10dBm (10mW) is detected I should read 5.000V with my DVM and the panel meter should be at full scale.

Next, it was time to tackle the meter calibration. This required applying a known signal level, recording the output voltage, attenuating the signal by a specific amount, then recording the new output voltage. The response of the AD8307 could then be characterized and represented by a linear equation with an exact slope and y-intercept.

Previously I had built a neat little CMOS reference oscillator that puts out a square wave with a precise power level of -10dBm @ 10MHz. I used this tool to assist in the final calibration of the board. Applying this signal to the input of my meter produced 3.95V at the DVM output. Next, I removed the square wave reference and replaced it with a 10MHz sine wave oscillator I had also built earlier (Sine Wave Oscillator). I adjusted the amplitude of this signal source until it matched the power level output by my square wave reference. Now, with a sine wave of -10dBm as my signal source, I attenuated the signal by 20dB. The DVM output now read 2.87V. (Note - attenuating the signal from the square wave reference oscillator would have given inaccurate results due to design constraints with the AD8307). After a bit of math, I calculated the slope and y-intercept values. The final "response curve" for my power meter can now be represented by a simple, linear equation:

power (in dBm) = 18.5 x (voltage reading from DVM) - 83.15