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The desktop computer doesn't have any Wi-Fi accessories. The keyboard and the mouse are wired. But even when the computer and the LCD monitor are turned off (while the power plugs are still connected), I get a high frequency signal from several accessories.

When I am moving a RF detector (which suppose to detect RF signals at frequencies between 100Mhz-2500Mhz), I get high frequency signals from the wired keyboard, the wired mouse, in front and behind the wired monitor (even when it turns off) and around the desktop case (whenever it turns on or off).

Why do I get those high frequency results?

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    \$\begingroup\$ Please tell us exactly which device are you using and describe measurement procedure in detail. \$\endgroup\$
    – AndrejaKo
    Commented Dec 5, 2016 at 17:50
  • \$\begingroup\$ Disconnect PC and monitor from power mains, and measure again. \$\endgroup\$
    – Anonymous
    Commented Dec 5, 2016 at 17:52
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    \$\begingroup\$ Parts of the computer can be powered from the +5VSB rail even when it is powered down. \$\endgroup\$
    – Andrew Morton
    Commented Dec 5, 2016 at 17:52
  • \$\begingroup\$ Digital systems are often noisy and computers are notoriously noisy. How much RF are you detecting? You should probably expect the about the FCC limit. \$\endgroup\$
    – user117772
    Commented Dec 5, 2016 at 17:56
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    \$\begingroup\$ Why do you think that your detector only detects between 100MHZ and 2.5GHz? What kind of detector is this? Especially the beeping part... \$\endgroup\$
    – Ale..chenski
    Commented Dec 5, 2016 at 19:10

1 Answer 1

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As long as your desktop PC is plugged into AC outlet, the internal power supply (PSU) is in standby mode providing +5VSB standby power to mainboard, so the computer can wake up from power buttons and/or keyboard and mouse resume signals. This +5VSB voltage is always on.

To produce this voltage, the PSU must be operating, and therefore continuously switching at primary amplitudes of 200V-300V (DC rectified from AC mains). PSU switching generates noise on all rails and return grounds, switching amplitudes of this scale are difficult to contain. More, when in standby mode, the consumed power is much smaller than the nominal power, so at low load the PSU switchers and inductors/transformers usually operate under very unfavorable conditions, and ripples and ringing from PSU switchers can be even higher than under normal operations. As result, this "noise" propagates along every conductor, including shields of cables that connect mice and keyboards, which acts as antennas.

The same happens in LCD monitor, its PSU is still in standby mode, and still switching and emitting noise.

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  • \$\begingroup\$ As far as i know the PSU is switching at the Khz-1Mhz frequency range,while the frequencies that my detector detected are over the 100Mhz.So,did you mean that the PSU creates a "noise" at frequencies over the 100Mhz which i detect on the mouse,keyboard etc.? \$\endgroup\$
    – xchcui
    Commented Dec 5, 2016 at 18:51
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    \$\begingroup\$ @xchcui, yes, the edges of voltage switch are pretty fast, and many PSU designs may have substantial ringing, see example here, electronics.stackexchange.com/questions/270406/… . The signal amplitude is huge (200V !!!), signals are nonlinear, so it should be of no surprise that some higher harmonics penetrate >100MHz area with substantial (mV-level) amplitude. \$\endgroup\$
    – Ale..chenski
    Commented Dec 5, 2016 at 18:55
  • \$\begingroup\$ What is,more all less,the strength of the radiation that should i excpected from the keyboard,mouse etc. regards to that high frequencies signals? \$\endgroup\$
    – xchcui
    Commented Dec 5, 2016 at 20:57
  • \$\begingroup\$ If you are not familiar with the amount of the signal strength,may you,at least,compare it vs. the signal strength of the wireless mouse,keyboard etc?What is the ratio signal strength between them?Since,what is the point of connecting a wired computer accessories in order to reduce radiation exposure,while they,also,radiate very high frequencies as the wireless accessories? \$\endgroup\$
    – xchcui
    Commented Dec 6, 2016 at 7:56
  • \$\begingroup\$ How can such devices pass regular EMC tests? Is there some special, lax standard for PC computers? \$\endgroup\$
    – Lundin
    Commented Dec 6, 2016 at 14:53

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