iPhone app calculates Gamma / X-Ray Photon Attenuation
The AD8307 IC is advertised as being a “Low Cost, DC to 500 MHz, 92 dB Logarithmic Amplifier”. This is a block diagram:
From the AD8307 datasheet: The essential purpose
of a log amp is not to amplify, though amplification is utilized to
achieve the function. Rather, it is to compress a signal of wide
dynamic range to its decibel equivalent. It is thus a measurement
device. A better term may be logarithmic converter, because its
basic function is the conversion of a signal from one domain of
representation to another via a precise nonlinear transformation.
And here’s a basic AD8307 circuit, mine is similar:
In my case, I have an LC filter on the incoming DC power, as well as the outgoing DC signal level, to reduce noise pickup. My meter is built into a small paint can, on the underside of the lid, which works as an excellent ground plane:
And here is the top of the lid, mounted on the can:
The toggle switch isn’t being used. I was going to power the meter off of a 9 volt battery to further reduce noise, but comparison tests between the battery and DC power supply showed no difference.
The output of the meter is a voltage proportional to the input power, measured in dBm. Zero volts is output for −84 dBm,
corresponding to a sine amplitude of 20 μV. There is a noise floor, and the specified range of the AD8307 is −74 dBm to +16 dBm. The output voltage increases by 25 mV for each dBm increase in RF input.