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)
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
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.