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Digital RF Level & Power Meter

Need to measure small signals at radio frequencies? Here is a low-cost digital level and power meter which will allow you to measure RF signals from below 50kHz to above 500MHz. As well as indicating the signal level in volts and dBV, it also shows the corresponding power level (into 50 ohms) in both milliwatts and dBm.

By Jim Rowe

Specifications

  • Input impedance: 50Ω (can be changed to 75Ω or 1.1kΩ)
  • Measuring frequency range: from below 50kHz to above 500MHz
  • Maximum input signal level: 2.238V RMS (+7.0dBV)
  • Minimum input signal level: 22μV RMS (-93dBV)
  • Maximum input power level: 100mW into 50Ω (+20dBm)
  • Minimum input power level: 1nW (0.001μW/-60dBm)
  • Measurement linearity: approximately ±0.3dB
  • Measurement accuracy: approximately 0.2%
  • Power requirements: 9V DC at 35mA (no backlight) or 120mA with backlight
  • TRADITIONALLY, RF level/power meters have been quite expensive beasts costing many hundreds of dollars, even secondhand. Small wonder that many of us have simply had to do without them. Such RF level/power meters have always been expensive because of the measurement technique they used: converting the RF energy into heat and then measuring the temperature rise using a sensitive thermocouple system.

    Luckily for us, advancing semiconductor technology now provides an easier way: the wideband logarithmic amplifier/detector IC. Its DC output is closely proportional to the logarithm of the RF input voltage. We can achieve the desired result by combining one of these chips with an "intelligent" metering circuit, capable of processing this logarithmic DC voltage to indicate both signal level and the corresponding power level.

    In a nutshell, our circuit uses an Analog Devices AD8307AN logarithmic amplifier/detector to convert RF signals into DC which is processed by a PIC microcontroller. The micro uses some fairly fancy maths routines to work out the signal level and power, which is then displayed on a standard 2-line LCD panel. The whole set-up works from a 9V battery or DC plugpack and draws less than 35mA.

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