How Does an EMF Meter Work?

In a previous post, we reviewed the best EMF meters. In this post we provide an overview of the main electronic building blocks in an EMF meter and how they work to detect RF energy.

An EMF detector consists of an antenna, at least one amplifier and a detector circuit. Here is an example of an EMF detector from a patent 4,631,473 granted in 1986 to Nippon Univac Kaisha, Ltd. The first element on the left is the antenna.

EMF Detector

The RF signal received by the antenna is amplified twice by amplifiers A1 and A2. The output of the second amplifier is compared with a reference voltage in a comparator, C1. If the amplifier output exceeds this level, the LED is lit or the buzzer makes a sound. This is simple threshold detector circuit that responds to the level of the RF energy received by the antenna.

A more sophisticated EMF detector can be built with a different type of amplifier called a logarithmic amplifier – also known as a log amp or log detector. The picture below shows a log amp from Analog Devices. The antenna is connected to the input pins INHI and INLO.

Log Amp Detector

In this type of design, the output voltage (VOUT) is proportional to the logarithm of the voltage or current at the input. This log output results from the use of cascaded amplifier stages.

EMF Detector Output Voltage as a Function of RF Input Level

The plot above indicated with a yellow arrow shows how VOUT varies with the input power as a function of temperature (Black: +25C, Blue: -40C and Red: +85C). In this case, as the input power decreases, the output voltage increases. However there is a unique relationship between VOUT and PIN. The log detector enables measurement of RF signals over a very large dynamic range of 65 dB. Log detectors can have a dynamic range as high as 160 dB!

Okay so what happens next with this DC voltage?  It is input to an analog-to-digital converter. We’ve discussed ADCs in an article previously. The ADC converts the analog DC voltage to a digital value. The values in the plot above are stored in a lookup table in the microcontroller. Each digital value corresponds to one (and only one) PIN value in dBm. For example, referring to the plot above, if the DC output is 1V, the ADC output will be a digital value that would be mapped to -20 dBm as per the lookup table. The microcontroller then passes -20 dBm to the display. Here is a block diagram of how this all works together.

EMF detector
Example EMF Meter Architecture

This is fundamentally how an EMF detector measures RF energy.

An EMF detector presents a view of all the signal energy that it is able to receive and process. The antenna for instance operates across a certain frequency range. The log detector is operational across a certain frequency range as well. The EMF detector is then rated across the smaller of the two frequency ranges and presents the total power received across that range. For a more granular view on signals and their frequency occupancy, individual amplitudes, total energy and other characteristics it becomes necessary to use a spectrum analyzer. In many applications, both an EMF detector and a spectrum analyzer are required to analyze a signal.

In part 2 of this article we will show you how to build your own EMF meter.

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