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I would like to know is it possible to induce RF emissions (incidental waves (re-radiation) from a RF device that is not transmitting ? The purpose of this testing is discover whether there is a hidden undocumented and undisclosed embedded RF transmission device that is likely in fact powered on but not transmitting and its transmission frequency range is unknown.

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  • \$\begingroup\$ Practically speaking, I don't think so, unless you are talking about large devices in the VHF/UHF band where the antenna is conspicuous. Seems that it would be more practical to scan over a long time base and try to catch it when it does transmit. \$\endgroup\$ – mkeith Jun 29 '16 at 3:57
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There is no foolproof way but indeed it is possible in some cases with non-transmitting devices. In fact that is how some "bug sweeping" devices called NLJD (Non-Linear Junction Detectors) work. The key is to sweep a frequency and look for faint return harmonics. Any non-linear device will clip the waveform and cause a "retransmission" As long as you have a way or getting some energy into the device then you should get a response back. The non-linear device may be protection diode on the IC pads, transistor on power supplies and even voltage regulators.

It is very hard to do in some cases as the signal can get swamped/absorbed by capacitance.

If you have a perfectly shielded device then it becomes increasingly difficult. But shielding a device that has openings for sound is also hard to do.

Looking for faint frequencies from on board clocks is also possible, but there is usually a very large ambient level of electronic noise in the environment.

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    \$\begingroup\$ And note that a NLJD will detect any nonlinear junction, not just ones that are part of a radio transmitter. Even certain types of metal-to-metal junctions may be detected! \$\endgroup\$ – duskwuff Jun 29 '16 at 7:00
  • \$\begingroup\$ @placeholer and everyone thanks for the helpful replies. Placeholder in terms of the NLJD the RF device is embedded in an IC a CPU to be exact and is not a standalone device. Having said that would a PC CPU be considered a linear or non-linear device ? \$\endgroup\$ – David J Jul 1 '16 at 9:23
  • \$\begingroup\$ @placeholer and everyone thanks for the helpful replies. Placeholder in terms of the NLJD the RF device is embedded in an IC a CPU to be exact and is not a standalone device. Having said that would a PC CPU be considered a linear or non-linear device ? I don't believe its either since the signal is neither analog or digital. \$\endgroup\$ – David J Jul 1 '16 at 9:30
  • \$\begingroup\$ @DavidJ the point is that it doesn't have to be a transmitter or an RF device, it just needs to a junction. As long as some of the probe RF energy can get into the junction then it's possible that there will be return signal at a harmonic. The non-linearity refers not to the device (like a CPU) but to the exposed wire, pin or other sub-element. \$\endgroup\$ – placeholder Jul 1 '16 at 14:15
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Although it is possible to detect electronic circuits by the fact that they will generate low level harmonics of an incident RF signal (when powered or not), in this case you are asking to detect an RF circuit adjacent to a known 'benign' circuit. Typically the benign circuit will generate as much or more re-radiated harmonic energy so this approach won't be reliable.

The only real possibility is to perform a frequency scan, and detect any band-specific absorbtion of energy - this would indicate the presence of a resonant circuit (oscillator, filter, antenna), but it is perfectly possible to build a transmitting device without any particularly frequency-sensitive analogue parts. A DDS transmitter without a tuned antenna seems hard to detect.

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  • \$\begingroup\$ could you (@Sean Houlihane) give the meaning of "bsnd" could not find the meaning of this acronym ? If you could provide some links to sme resources that would be great Sean Thank you \$\endgroup\$ – David J Jul 1 '16 at 9:35
  • \$\begingroup\$ thanks @Sean. One question when you say tuned antenna do you mean one that is directional and does not radiate intentional RF waves back at the RF front end and transceiver or baseband IC ? \$\endgroup\$ – David J Jul 2 '16 at 5:29
  • \$\begingroup\$ All antennas are symmetric, transmit and receive will be the same. No, a tuned antenna is resonant due to its matching components and length, as opposed to a random length of wire. A wire will have less of a frequency-dependent coupling characteristic. \$\endgroup\$ – Sean Houlihane Jul 2 '16 at 8:31

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