[Elecraft] OT -RFI question

[Jim Brown]

On 5/26/2011 1:30 PM, Alan Bloom wrote:
> TDMA (time division multiple access) cell phones share a frequency
> channel by transmitting only in certain time slots.  The power is pulsed
> on and off even though the modulation mode may be constant-power.

Yes. For at least one of the commonly used standards, the repetition 
rate is about 218 Hz, which puts the harmonics of the detected square 
waves right in the middle of the audio spectrum, and thus highly 
audible. Once you know what it sounds like, you will sometimes hear 
bursts of digital hash at public events, when the cell phone of a public 
figure gets close to a piece of audio gear with a pin one problem.

This used to be a LOT more common than it is now -- mfrs of mics and 
other audio gear started taking pin one problems and RFI a LOT more 
seriously when Blackberrys came on the scene bigtime (and reportedly 
gave a Blackberry to every member of congress and their staff). The 
senator would lean the Blackberry up against the tube holding the 
microphone in front of him, and the Blackberry would couple into the mic 
wiring.  It was so bad that they had to shut down hearings until they 
eventually found and purchased mics without pin one problems. I 
consulted on that problem, both with the system designer and two mic 
companies.

73, Jim Brown K9YC