PBL 3852
Radio interference origina-
ting from mobile phones
V and I protection
SIOV
5 - 10Ω
Line
in
The problem with direct radiated RFI has
accentuated lately because of the
growing numbers of mobile and
Plastic
enclosure
The
electronic
circuitry
C10
Metallic shield, sprayed or foil
especially pocket telephones. Thus it is
today rather common that a RF transmit-
ter with output power of several watts in
form of a mobile telephone is placed quite
close to an analog telephone. There is a
simultaneous even bigger problem
coming from these portable phones of
digital time-multiplex type like the GSM.
The GSM signal consists of 900 MHz
carrier that is transmitted in 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
semiconductors will envelope detect this
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 has to be done with
much more precision. The principle is to
attenuate the HF signal to a level where
the detected 200 Hz signal is below a
disturbing level especially at high
RF radiating measuring sheet.
RF-gen.
Figure 29. Measuring RFI.
12). Try to reach a high packing density
on the PC-board.
13). Connect components as close to
the IC as possible. Connect
capacitors are placed directly
between the actual points and not
via tracks on the board (See fig. 31).
7). Balanced points like a differential
microphone input may have to be
decoupled differentially between
the inputs and ”common mode” to
common ground each input
especially decoupling capacitors
close to the ground pin of the IC.
The terminal circuits from Ericsson
Components are manufactured in IC
processes with large internal capacitors
on the chip to counteract RFI disturban-
ses in every possible way.
separately.
8). A virtual ground may have to be
created into which all outgoing
cables are decoupled in order to
bypass the RF- signal. See fig. 31.
9). Think that even overvoltage and
overcurrent protectors can be
acting as HF detectors.
10). Shields that are connected to the
internal ground can be of help.
11). Control that no already detected
signals from for ex. dialler enter the
speech circuit via the mute
The simplest method to test the
susceptibility of an apparatus to RFI is to
take a portable phone of an actual type
and move it transmitting acros the phone,
cables and handset. Measure the signal
at earphone output aswell as on the line.
Finally; to design an ordinary analog
telephone is not a low frequency but a
high frequency task.
sensitive points like at the microphone
input.
function.
Following aspects ought to be thought of:
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
Figure 30. RFI elimination at PCB layout level.
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.
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).
Microphone
Earphone
Line
Resistor
or
Virtual ground inductors
6). HF- design includes also that tracks
on the PC-board act as inductors
and therefore it is the more
Common gnd. of the
telephone
important that the decoupling
Figure 31. RFI elimination in the wiring.
19
ERICSSON [ ERICSSON ]
