Carrier current transmitters are devices that transmit sound over powered lines (usually electrical mains) by modulating an independant carrier frequency (usually AM, though it presents a noisy signal. FM is more expensive but clearer) with an audio signal and impressing it onto an electrical line. The signal is sent down the line to be picked up directly off the line by a specially designed receiver, or by a receiver very close to the line.
Carrier current surveillance devices can manifest themselves as re-tasked consumer goods such as telephone jacks and baby monitors. If you leaf through enough gadget catalogs, you're bound to see a carrier current transmitter marketed as some magic product that "lets you turn any power outlet in your home into a phone jack!". Covert carrier current transmitters will almost exclusively be hidden in plug-in devices, such as lamps or surge protectors.
The devices also present a dangerous native threat. What the catalog fails to tell you is that everyone else served by the same pole transformer (usually everyone else on the block) can turn their power outlet into your phone jack too. Some carrier current products (usually wireless intercoms) have several variations on the same model operating at different frequencies in order to combat the 'crossed wires' effect.
Carrier current devices suffer from a major flaw, in that they are usually stopped dead by powerline transformers. Bridging the hot leads with a 0.1 microfarad 240VAC capacitor will allow carrier-current transmissions to bridge the transformer. Alternatively this can sometimes be overcome through the use of a passive coupler (also called a signal bridge), though a repeater would be a better option.
Carrier-current devices can be remotely controlled via an X-10 link, frustrating detection.
Carrier Current Frequencies
Carrier current devices can be detected as standard wire line devices with a TDR, oscilloscope, or with a carrier current receiver (which can be purchased from <http://www.martykaiser.com>).
Connecting a wet line TDR to the powerline with fused leads will show any buildouts or undocumented loads on the line. Take care to isolate straight sections of the circuit, or T-joints will present an overly complex waveform.
Using an oscilloscope to test for carrier current devices calls for connecting the scope to the line with fused leads and checking for any RF on the line other than the 60 hz sine wave thats present on all power lines.
While there's been some discussion of using commercially available carrier current receivers (from carrier current telephone extensions or intercoms) as fill-ins for a good receiver this isn't the best of ideas. Carrier current devices can transmit on any frequency below 500 kHz (150-300 kHz being VERY popular) and in one of several modulation formats, while commercial receivers are generally tuned to only a small fraction of the available band and using one modulation scheme. An electrical tester designed to test for EMR noise on power lines would work as a detector, but could easily be triggered by RF on the line. A tunable carrier-curent receiver makes the best detection method.
Physical searches for carrier current devices should include removing the covers of all power outlets, and the disassembly of anything that runs on AC power.
The most obvious way to prevent native carrier current threats is to prevent friendly threats. Don't use carrier current telephones or intercoms. Audio jamming will prevent the microphones on CC surveillance devices from picking up conversations. A good surge protector or power line conditioner will incorporate a filter that will block carrier current emissions. Make sure that the power line conditioner or surge protector you select blocks RFI down to at least 1000 Hz. Its possible to use blocking capacitors or low-pass filters (blocking above 60 Hz) on AC power lines in order to negate carrier current devices, but certainly not recomended for anyone lacking in electrical knowlege.