PBL38541 Ericsson, PBL38541 Datasheet - Page 13

• Manufacturer Part Number: PBL38541
• Description: Universal Speech Circuit
• Manufacturer: Ericsson

Radio interference originating
from mobile phones
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
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 sensitive points like at the microphone
input.
Following aspects ought to be considered:
1).
2).
3).
4).
5).
6).
Do not make any points in the
circuitry more high impedive than
necessary.
Keep all cables, wires and tracks
on PC-board as short as possible.
Decouple all sensitive points to an
internal ground with capacitors
especially the microphone amplifier
input.
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.
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).
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
7).
8).
9).
10). Shields that are connected to the
11). Control that no already detected
12). Try to reach a high packing density
Figure 23. RFI elimination in the wiring.
Figure 21. How to measure the RFI pickup.
Figure 22. RFI elimination at PCB layout level.
via tracks on the board (See fig. 22).
Balanced points like a differential
microphone input may have to be
decoupled differentially between
the inputs and ”common mode” to
common ground each input
separately.
A virtual ground may have to be
created into which all outgoing
cables are decoupled in order to
bypass the RF- signal. See fig. 23.
Think that even overvoltage and
overcurrent protectors can be
internal ground can be of help.
signals from for ex. dialler enter the
speech circuit via the mute
function.
on the PC-board.
acting as HF detectors.
Line
in
Line
RF radiating measuring sheet.
Earphone
Microphone
Not like
this
SIOV
Virtual ground
Metallic shield, sprayed or foil
inductors
Resistor
5 - 10
or
C10
Like this
13). Connect components as close to
Components are manufactured in IC
processes with large internal capacitors
on the chip to counteract RFI disturbanses
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
and move it transmitting acros the phone,
cables and handset. Measure the signal
at earphone output aswell as on the line.
telephone to fullfil todays requirements is
not a low frequency but a high frequency
task.
RF-gen.
Finally; to design an ordinary analog
The terminal circuits from Ericsson
the IC as possible. Connect
especially decoupling capacitors
close to the ground pin of the IC.
V and I protection
Common gnd. of the
telephone
electronic
circuitry
The
PBL 385 41
Plastic
enclosure
13