RF Power Meter
Measuring the RF power output of typical WiFi devices operating in the 2.4 and 5GHz frequency region has historically required expensive test equipment beyond the reach of many developers. Recently a number of economical devices have come onto the market and changed this situation.
Notable among these devices is the RF-Explorer range of instruments which offer robust hand held spectrum analysers covering a range of frequency bands:
These instruments provide a range of measurement functions, including RF power measurements.
There are also small, very low cost instruments available that just measure RF power over a range of frequencies up to 5GHz. A typical example is shown here:
These devices are based on a logarithmic power detector module such as the device shown here:
These modules in turn utilise detectors such as the D8318 device from Analog Devices shown here:
The detector essentially produces a DC output voltage which is proportional to the power of the RF signal present at the input. This voltage is then fed to the second part of the instrument which displays the measurenment as a voltage and power level.
This is fine for a continuous RF signal eg from a hand held two-way radio, but WiFi signals consist of a series of pulses of transmitted signal. The detector has a fast response time, and when presented with a WiFi signal, outputs a series of pulses whose amplitude represents the RF power of the signal.
In order to display the power measurement of the WiFi signal, it is necessary to provide a peak detector circuit that converts the pulse output of the detector into a steady voltage suitable for input to the display module.
The schematic diagram of the peak detector circuit connected between the RF power detector board and the display board is shown below.
The circuit uses a dual op-amp as a simple peak detector, with the capacitor C1 holding the peak voltage between pulses. The circuit can be assembled on a small prototyping board and fitted to the detector and display modules as shown in the photo below.
The RF power detector board and the display board run from a 5V supply, however the op-amps require a higher voltage in order to be able to handle the signal range from the detector. A 5V linear regulator is provided so that the whole device can be operated from 9 - 12V supply.
The RF detector module presents a 50 Ohm input impedance which matches the antenna impedance of typical WiFi devices. However the detectors are typically limited to a maximum input power of 0dBm, so it is usual to provide an attenuator (typically 20dB) at the input to the detector. A typical device is shown here:
The display module has a facility to define an Offset value equal to the attentuation, so that the display shows the correct power level.
In addition, the Frequency of the signal being measured can be set on the display module so that a calibration correction can be applied to correct any non-linearity in the detector module.
To set the Offset and Frequency values, use the Left/Right, Up/Down push buttons to select and adjust the digit to be changed. When the desired value has been set, push the centre button to set the value. These values will be retained when the power is cycled.