PBL 385 73
Radiointerferenceoriginating
from mobile phones
V and I protection
SIOV
5 - 10Ω
Line
in
The problem with direct radiated RFI has
accentuated nowadays because of the
growing numbers of mobile and especially
pocket telephones. Thus it is today rather
common that a RF transmitter with output
power of several watts in form of a mobile
telephone is placed quite close to an ana-
log telephone. There is a simultaneous
even bigger problem coming from these
portable phones of digital time-multiplex
typeliketheGSM.TheGSMsignalconsists
Plastic
enclosure
The
electronic
circuitry
C10
Metallic shield, sprayed or foil
RF radiating measuring sheet.
RF-gen.
Figure 24. How to measure the RFI pickup.
via tracks on the board (See fig. 31).
13). Connect components as close to
the IC as possible. Connect
of 900 MHz carrier that is transmitted in 7). Balanced points like a differential
especially decoupling capacitors
close to the ground pin of the IC.
short signal bursts 1/8 of time and with a
repetition frequency of slightly higher than
200 Hz. This signal will be directly radiated
to all parts in a conventional telephone set.
All unlinear elements as most of the
microphone input may have to be
decoupled differentially between
the inputs and ”common mode” to
common ground each input
separately.
The terminal circuits from Ericsson
Components are manufactured in IC
processes with large internal capacitors
onthechiptocounteract RFIdisturbanses
in every possible way.
The simplest method to test the
susceptibility of an apparatus to RFI is to
take a portable phone of an actual type
andmoveittransmittingacross thephone,
cables and handset. Measure the signal
at earphone output as well as on the line.
Finally; to design an ordinary analog
telephone to fullfil todays requirements is
not a low frequency but a high frequency
task.
semiconductors will envelope detect this 8). A virtual ground may have to be
signal and thus feed the 200 Hz signal with
harmonics into all points of the telephone.
The methode to counteract this problem is
the same as before with a difference that it 9). Think that even overvoltage and
has to be done with much more precision.
The principle is to attenuate the HF signal
toalevelwherethedetected200Hzsignal 10). Shields that are connected to the
is below a disturbing level especially at
internal ground can be of help.
high sensitivepointslikeatthe microphone 11). Control that no already detected
created into which all outgoing
cables are decoupled in order to
bypass the RF- signal. See fig. 31.
overcurrent protectors can be
acting as HF detectors.
input.
signals from for ex. dialler enter the
speech circuit via the mute
function.
Following aspects ought to be considered:
12). Try to reach a high packing density
on the PC-board.
1). Do not make any points in the
circuitry more high impedive than
necessary.
2). Keep all cables, wires and tracks
on PC-board as short as possible.
3). Decouple all sensitive points to an
internal ground with capacitors
especially the microphone amplifier
input.
Not like
this
Like this
4). To include series elements like
resistors and inductors in all long
wires or cables that could act as
aerials. For ex. microphone cable,
earphone cable, cable to the
telephone network, mute wire and
cable to the keypad.
Figure 25. RFI elimination at PCB layout level.
Microphone
5). Comprehend that it is a question of
a HF- design,so that all used
decoupling components are well
suited to the frequencies at hand.
(up to several GHz).
Earphone
Line
6). HF- design includes also that
tracks on the PC-board act as
inductors and therefore it is the
more important that the decoupling
capacitors are placed directly
between the actual points and not
Resistors
or
inductors
Virtual ground
Common ground of the telephone
Figure 26. RFI elimination in the wiring.
14
ERICSSON [ ERICSSON ]
