Radio Frequency (RF) Interference occurs when a conductor is in the RF field of a transmitted radio source. Increasingly, wireless devices infiltrate our environment making the occurrence of RF interference more common. Cell phones operate at high frequencies (1 to 5 GHz which is hard to shield) and transmit up to 1 watt of signal. Often a cell phone call can be detected on microwire recordings before the phone rings! Radio Frequency energy, or electromagnetic radiation, consists of coupled electric and magnetic fields. Therefore both the electrostatic and magnetic noise abatement techniques from the previous two Noise Debug articles both apply. For RF coupling to induce a significant amount of energy, the distance between the source (transmitter) and the desired signal (receiver) is usually greater than 4 to 10 wavelengths. Less than this distance, the coupling type will be electrostatic or electromagnetic and can be treated as such. Wavelength is the speed of light divided by the frequency, (3*108 / F). For example, a 1 MHz AM radio station has a wavelength of 300 meters, while a 4G phone has a wavelength of only 7.5cm.
When debugging RF noise, always use the audio output feature of Neuralynx’s Cheetah and the Digital Lynx. We have heard classical music, talk radio, and campus security audio from the recording systems that are very hard to debug without using your ears. Cell phones may sound like “periodic noise patterns.”
Solutions to RF Interference
Question: How is high frequency RF energy coupled into my low frequency signals?
Answer: All active electronic parts contain diodes and transistors. If the coupled RF signal is strong enough, it will be present at the first amplifier chip and be “rectified” by a diode or transistor inside the amplifier IC. (This is the principle of a “crystal radio” receiver.) Good amplifiers employ high frequency filters before the inputs to keep out RF signals.
Tests for RF Coupling
The first step is to listen to the noisy channel through Cheetah and the Digital Lynx. If you hear music or speech, listen for the radio station’s “call letters” on the hour and half hour. If cell phones are suspected, make a call and position it close to the subject’s headstage to maximize the noise.
RF Coupling Solutions
Many times the same solutions for electrostatic conduction will work for RF interference because RF is coupled electrical and magnetic fields.
The easiest solution is to remove the RF noise source (i.e., ban cell phones in the recording room). If this can’t be done, as in the case of a radio station, try electrostatic and magnetic solutions. Try the “aluminum foil test” around the subject. We have seen a grounded copper-foil on the exterior of a microdrive work wonders.
Try a Faraday Cage around the animal although this may not be possible for large maze experiments.
Ground unused inputs on a headstage. This effectively removes the “open antenna” from picking up the RF signal.
If you have unshielded tethers, such as on the Neuralynx HS-8 or HS-18 Cooner, switch to a shielded tether cable model.