App note: Detecting and locating RF bugs

in app notes by DP | 3 comments

We wrote about a phone bug in a previous post. Here’s an app note about a circuit for detecting and locating radio frequency transmitters.

This design idea showcases a circuit that detects RF “bugs,” such as hidden wireless cameras, eavesdropping microphones, and other spying devices that emit RF frequencies in the 100MHz to 3000MHz range. A modification to this circuit not only detects RF bugs, but also locates their hidden positions.

The circuit is based around the MAX2015 RF detector which outputs a voltage proportional to the strength of a received RF signal in the 100 MHz to 3 GHz range. This voltage can be detected with a comparator that activates a buzzer at a predefined signal strength.

If the comparator is replaced with a op-amp the circuit could be used to locate RF transmitters by increasing the buzzer volume as one comes closer the source.

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Comments

  1. mike says:

    Unless I’m misreading the datasheet, this thing will output voltage based on signal strength for any frequency in that range. It’s not especially useful if the RF bug in the room is placed near another RF source like a cordless phone, a wifi hotspot, etc.

  2. hak8or says:

    Ah, I was staring at the image going “how on earth does a comparator do that when having feedback like that. Then I realized that the picture has a comparator in it instead of a op amp. May I suggest switching the picture to the op amp one instead, as the description seems to be op amp focused instead of comparator focused. Though, I may be the only one that has experienced the confusion. :P

    Also, can’t you put a band pass filter of sorts in the RF in so you get RF only from a specific spectrum? Not sure if that is something as easy as just sticking in three passives and calling it a day.

    • rsdio says:

      A passive bandpass would only have 6 dB/octave cutoff with low parts count. For steeper rejection outside the band, you’ll want a higher-order bandpass, and that could take up several op-amp channels. But it’s probably worth the PCB space for the enhanced performance. A really handy feature would be tunable bandpass, which would add even more circuitry.

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