Testing radiated immunity on a workbench, without an anechoic chamber or shielded tent, is typically illegal in most parts of the world. In the UK, obtaining a site license from Ofcom(The Office of Communications), which is unlikely to be granted for a city location, is required. Additionally, concerns arise about safe human exposure limits to RF fields at high levels.

Engineers are left with three options to test their units on a workbench:

•Option 1: Use a powerful handheld transceiver, such as a walkie-talkie.

•Option 2: Employ the localized field injection method, where a near-field probe connects to an RF amplifier.

•Option 3: Use a TEM cell within an EMC shield, but monitoring the DUT (camera, optical communication link, etc.) can be impractical for most design engineers.

Walkie-talkies

A walkie-talkie in the UK typically operates within the following frequency ranges:

•VHF: 136-174MHz

•VHF: 200-259.975MHz

•UHF: 400-480MHz

Most walkie-talkies have an RF power rating of 1W, although some can operate at up to 4W or higher. This makes a walkie-talkie an excellent tool for injecting noise. By simply pressing the send button and moving the antenna around a PCB, you can introduce exposed RF noise into the circuit under test. If the antenna runs parallel to a cable in close proximity, strong near-field coupling can occur, inducing RF current in the cable and affecting the unit. Another valuable application is placing the walkie-talkie antenna in parallel with seams or apertures of an enclosure. If the enclosure's shielding is not properly done, the field generated from the walkie-talkie may upset the unit.

The major disadvantages of walkie-talkie testing include:

a) The actual field strength that is being developed at the EUT is unknown, depending on the output power of the walkie-talkie, this could be very high

b) repeatable results are difficult to achieve,

c) spot frequency tests may well miss resonances in susceptibility that will appear in the compliance test.

This is not to say that a walkie-talkie test that does in fact create a susceptibility is useless, as it will prompt the designer to harden the design, but it can't be relied on to predict compliance results. The same considerations apply to "testing" with a mobile phone.

Localised field injection

An RF amplifier is a device that takes an input signal from a signal generator (in EMC testing, this could be an RF signal generator or the tracking generator of a spectrum analyser) and amplifies the signal to a much higher level at the output. By connecting a small near-field probe or a small antenna to the output of the RF amplifier and pointing it towards the device under test, you can create a very strong near-field (see the picture above). Many RF amplifiers also have amplitude modulation (AM) and pulse modulation (PM) capabilities, making them suitable for testing the immunity of a product. These modulations can simulate real-world scenarios where the device is exposed to varying RF conditions.

See a technical discussion on this subject in detail.