PBL 385 73
Short about Radio Frequency
Interference RFI.
amplifier directly being amplified and
detected. The second component is less
serious because it affecting both inputs
alike and most of it will be balanced out of
the amplifier. There might be the case
where the HF-signal will have such an
amplitudethattheamplifiercannotbalance
it out. Then components must be filtered
with capacitors and maybe resistors. It is
extremely important that everything that is
done at the input is in balance, otherways
the problem might get worse instead of
better. The extreme balance requirement
goes all the way to the PCB-layout. Small
unbalance signals can be corrected with
capacitorsmarkedwith*)thisrequiringhigh
precision components. See fig 22a. The
solution shown is rather expensive but
withprecisioncomponentsitrendersgood
filtering at the input. If the main problem is
the signal between the inputs, try to
increase the 1nF capacitor but make the
others procentually smaller in order to
maintain the frequency response. A more
simple solution, that is sufficient in most of
the cases is also shown in fig. 22b.
HF suppression at the microphone in-
put.
The HF-signal at the microphone input can
beseencomposedas oftwo components.
One component being the differential
(between pins 9 and 10) and the second
related to ground at pin 14. Of these two,
the first is the most serious, entering the
a)
b)
c)
+
10n
10n
8
8
200-
470Ω
8
10n
1n
<20n
*
+
9
100Ω
100Ω
9
9
1µ
Mic.
1n
M
Mic.
Mic.
M
M
1n
10
10
10
+
+
+
+
PBL385 73
PBL385 73
1µ
*
PBL385 73
<20n
10n
200-
470Ω
11
11
11
10n
10n
Line
Line
Line
Dynamic microphone
Dynamic microphone (simplified)
Electret microphone
Figure 22. RFI elimination at microphone amplifier input.
HF-suppression at the receiver output.
To shield the keyboard will some times
usedandeffectivecountermeasuretothis
kind of RFI penetration is to shield the
telephoneset,atleastthebottom of it,that
is closest to the main PCB board by metal
foil or by spraying the plastic casing with
metallic matter. See figure 24. This
methode does not necessarily count out
help. The polarity guard bridge can also
act as a rectifier and demodulator, of the
HF-signals.Connect1nFcapacitorsacross
each diode in the bridge. There is a
capacitor across the line C10, this is for
RFI suppression but also to stabilise the
whole system.
The cappacitor C10 shoud be connected
like in figure 24. The frequencies at which
the RFI comes through are in the region of
10-1000MHz. The resistance of the C10
will be somewhere 0.01-10Ω hence even
the shortest lenght of connector on the
PCB board or wire wil be in the same
regionofresistanceandthusofgreatestof
importance.Theseactionsdescribedabove
should, when applied correctly, take care
of the RFI coming in from the telephone
line. The second way for the RFI to enter
the system is to penetrate the PCB board
capacitively. The test methode is to place
a metal sheet under the telephone set to
be tested and inject the sheet with RF
signal. The most
The problem here is of the same kind as
at the microphone amplifier input but will
be easier to solve because of the much
lower impedance and level of gain. The
solution is shown in the fig. 23. No
capacitors should be connected directly
from pins 13 or 14 to ground because of
the low output impedance, series
resistance of at least 10Ω must be used if
there is a tendency to self oscillation.
the RFI components that
recommended earlier.
are
<47n
10-100Ω
Other paths for the HF-signal to enter
the audible system.
13
+
+
Rx
To find out if the problem originates in the
DTMF-generator disconnect the genera-
tor and disconnect the mute input at pin 5.
If the problem is small try to connect a
capacitor from mute input to -line pin 11.
DTMFcircuitsare sensitivetoRFIbecause
of their high impedance at the input pins,
especially the keyboard inputs. These
inputs are not possible to filter with large
capacitors because of the keyboard scan-
ning pulses (1µs) that will be loaded down.
14
-
10-100Ω
<47n
PBL385 73
12
11
- Line
Figure 23. RFI elimination at receiver
amplifier output
13
ERICSSON [ ERICSSON ]
