PBL 385 41
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.19a. 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.19b.
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 12 and 13) and the second
related to ground at pin 14. Of these two,
the first is the most serious, entering the
a)
c)
b)
+
10n
10n
11
11
12
13
200-
470Ω
11
10n
1n
<20n
*
+
100Ω
100Ω
12
12
13
1µ
Mic.
1n
M
Mic.
Mic.
M
M
1n
13
+
+
+
+
PBL385 41
PBL385 41
1µ
<20n
*
PBL385 41
10n
200-
470Ω
14
14
14
10n
10n
Line
Line
Line
Dynamic microphone (simplified)
Electret microphone
Dynamic microphone
Figure 19. RFI elimination at microphone amplifier input.
HF-suppression at the receiver output.
ning pulses (1µs) that would be loaded
tration is to shield the telephone set, at
least the bottom of it, that is closest to the
main PCB board by metal foil or by
spraying the plastic casing with metallic
matter. See figure 21. This methode does
not necessarily count out the RFI
components that are recommended
earlier.
down. To shield the keyboard will some
times help. The polarity guard bridge can
also act as a rectifier and demodulator, of
the HF-signals. Connect 1nF capacitors
across 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 22. 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 used and effective
counter measure to this kind of RFI pene-
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. 20. No
capacitors should be connected directly
frompins17or18togroundbecauseofthe
low output impedance,series resistance
of at least 10Ω must be used if there is a
tendency to self oscillation.
<47n
Other paths for the HF-signal to enter
the audible system.
10-100Ω
17
+
+
Rx
18
To find out if the problem originates in the
DTMF-generator disconnect the genera-
tor and disconnect the mute input. If the
problem is small try to connect a capacitor
from mute input to -line pin 14. DTMF
circuits are sensitive to RFI because 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-
-
10-100Ω
<47n
PBL385 41
15
14
- Line
Figure 20. RFI elimination at receiver
amplifier output.
12
ERICSSON [ ERICSSON ]
