Re: Radio Frequency Interference ("Richard F. Lyon" )

Subject: Re: Radio Frequency Interference
From:    "Richard F. Lyon"  <dicklyon(at)ACM.ORG>
Date:    Wed, 8 Mar 2000 12:28:44 -0800

At 10:33 AM 3/8/00 , you wrote: >An interesting phenomenon has been happening on occasion when we run our >experiments: Radio Frequency Interference (RFI). This is interference in >which when running an experiment you can hear intermittent faint sounds >from radio stations. We use the Rane Headphone HC-6 when running, and the >engineers suspect it may be something about the headphones picking up the >frequencies. Does anyone know of any headphones that can definitely avoid >this problem? Or any other advice in avoiding RFI? AM radio stations can have very large signals. They won't hurt you except when they get into a nonlinear circuit element that demodulates them back into the audio baseband. So it's not the headphones themselves, since they're pretty linear, but the driver amps connected at the other end of the "antenna" wires, most likely. Assuming it's not pickup elsewhere, upstream in your equipement, but is related to the headphone wire as antenna, you can suppress the problem enormously by putting a passive attenuator (made from a pair of resistors or a variable R) close to the amplifier output. This cuts down the amount of RF signal getting into the amp, and causes you to run the amp at higher output, so the interference would be attentuated again going back out. The nonlinearity at least squares the interference, so a 6 dB attenuator would give at least 18 dB improvement (12 dB less demodulated audio, attenuated by 6 dB going back out), which may be enough. Or you can try shielding the headphone cables. Dick

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