"chipsoars" wrote in
ps.com:

On Mar 7, 5:09 am, Tom Gardner wrote:
TDMA systems (e.g. GSM) have pulsed RF tx power at 217Hz
1/8 duty cycle and 8 times peak power. Put one of these
near to a phone or hifi or hearing aid and the results
are clearly audible. Such interference can completely
disrupt the operation of some equipment.

is that the reason why when from time to time my Motorola GSM phone
pulses (not necessarily for a call) I can hear the pulse through my
computer, car radio etc? If so, that could be an issue for
instruments and radio in the aircraft if I understand this correctly.

I expect so. The GSM interference during call-setup is
"di-di-di, di-di-di, di-di-di, brrr..." duration ~2s.

There can and will be different interference at other
times, e.g. location update, SMS rx/tx, cell handover.

Whether it is a problem for instruments depends on the
amount of power coupled into the instrument (filtering,
orientation, range) and whether is is susceptable to
217Hz changes in DC offset.

How loud it sounds is a strong function of distance,
falling off at somewhere between r^4 and r^8; test
it with you phone and hifi