Lee Szuba About

RF Power Meter

In testing the SDR filter, I figured an RF power meter would work well with my signal generator to make a poor-man’s network analyzer. The typical HP offerings are quite precise and resultantly very expensive. After stumbling across Analog Devices logarithmic amplifiers (I think in a QEX article), I figured building one would be not too difficult.

The design is based around an AD8307 logarithmic amplifier, alongside a simple linear power supply and an op-amp used for output amplification and a tiny bit of noise filtering. It is flat from DC - 500 MHz, then suffers roll off in the amplifier. The output is DC that can be easily converted into the equivalent input, in dBm. I tested it using an HP signal generator to check linearity and flatness over frequency. The only drawback of the AD part is it is really expensive, but for a one-off design like this, it’s perfect.

Everything is assembled in a case made of copper clad scraps, which does a fantastic job of preventing interference and noise. Power is provided by a 9 V battery to prevent noise from coupling in on the supply, and the output passes through a feedthrough capacitor for the same reason.

Overview

The circuitry is built on a piece of SMD prototype board that fits SOIC ICs very nicely. It also has a solid ground plane on the underside and an array of unplated vias. This makes for quite robust RF construction.

Closeup of the circuitry