RF Detection

Published:2011/8/21 22:01:00 Author:Li xiao na From:SeekIC

By David Daamen

WiFi detectors are small gadgets that claim to detect wireless access points for computer networks. The effectiveness of these devices is already covered in many Internet articles, but here you will find out exactly how they work.

To begin with, we opened the first device we had to hand; see Figure 1 (Kensington ’WiFi Finder’). It was immediately apparent that it used a fairly large number of components. Apart from a large section of passive components (resistors, capacitors and inductors) we counted five transistors and throe ICs. All components, apart from three LEDs, were surface mounted. On the other side of the double-sided PCB were two watch batteries.



Principle

It soon became clear that the heart of the circuit was an RF detector IC made by Maxim, the MAX4003. This logarithmic amplifier is preceded by on input stage consisting of an aerial printed on the PCB followed by a few filter and amplifier stages. The T.1AX4O03 converts the RF signal into a DC voltage with an amplitude that corresponds logarithmically to the amplitude of the RF signal.

The DC output signal is amplified further by an opamp (MC33202. ON Semiconductor) and then fed to an A/D converter input of a microcontroller (MSP430F1101A, Texas Instruments).

Microcontroller

The primary function of the controller is to periodically measure and evaluate the signal strength of the input. Three LEDs are used for the display: a bicolor LED indicates when a measurement takes place (a short red Dash) and together with two green LEDs it forms a mini bar graph meter, which shows the signal strength as one of three levels.

It is clear that the controller doesn’t look at the contents of the received signal: there is nothing left of the digital information after the detector IC; just the RF energy is measured, this became even more evident by the fact that the detector also reacted to a Bluetooth signal (very) near by. In practice you’ll hardly notice this when searching for wireless networks, since Bluetooth signals are much weaker than WiFi signals.

Selectivity

The only selectivity offered by t circuit when it has to differentiate between wireless networks and other RF signals is achieved by the filters in the input stage. These are tuned to the band used by 802.11b and g wireless networks (2412 MHz to 2482 MHz). It is clear that such a passive filter is not perfect. When a GSM phone (900 or 1800 MHz) is nearby: this also causes a few LEDs to light up.

RF detection

You may well wonder if there isn’t an easier way. If you only need to measure the RF energy there should be some simpler alternatives. Although this is the case, there are still very good reasons for using a logarithmic amplifier.

There are two other methods for measuring high frequency power, but both of these have significant disadvantages. We’ve put the three methods in a table along with six important design aspects to show you how they compare.

Thermal detector

The thermal detector is the most conventional. The RF power is determined from the rise in temperature of a resistor that is Ted with the RF signal. From the table you can see that this option scores badly on all fronts, apart from cost. The main disadvantage is that the thermal detector is too slow for use as a WiFi detector.

Diode detector

Although the diode detector may appear to be a better alternative at first sight, the advantages it offers over the log amp are not important in this application. You may argue about the power consumption, but a good temperature stability is definitely not important m this case,

At about the same cost, the solution provided by the WiFi detector that we opened offers the smallest PCB with the best dynamic range and a fast reaction time. Furthermore, the microcontroller is used to perform multiple functions. Not only does it take care of the actual measurement, but it also drives the LEDs and implements an auto power-off function.

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