RF power meter (part 1)

Lately I’ve been interested in some RF stuff.

The “goal” is to build an entire RF spectrum analyzer with a range of about 50-100MHz.

For this I have to learn basic stuff like filters, oscillators, amplifiers, etc.
This implies having some basic tools like an oscilloscope, RF power meter, signal generator, etc.

The oscilloscope was already present so I’ve build a simple RF power meter using an Analog Devices log amplifier: AD8307
AD8307 datasheet

The device uses the basic connection – figure 32 from the data sheet. The INP signal is connected to a 50 ohm terminated BNC input via 100nF capacitor.
The INM pin is connected to ground via another 100nF capacitor.

The output signal is fed to a LM358 op-amp connected as a buffer. The op-amp output goes via a RC filter to the output.
The entire RF section is build dead-bug style in a small enclosure made from FR4 pieces. I’ve run out of space on the base plate, so I had to build it in a “3d” fashion.
Here are some pictures:


The read-out is based on an 4×20 lines LCD display driven by a STM32F0 board. The detector’s output is connected to one of the ADC’s input.
The microcontroller code samples the ADC, creates a buffer containing 256 values from witch it extracts the Min / Max and medium values. The displayed values are updated around 2 times / second.

I’m still working on the code. For now i’ve created a simple calibration table in 50Mhz steps, from 50 to 500MHz.

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