pbl38571 ETC-unknow, pbl38571 Datasheet - Page 12

• Manufacturer Part Number: pbl38571
• Description: High Current Speech Line Interface Circuit
• Manufacturer: ETC-unknow

Short about Radio Frequency
Interference RFI.
HF suppression at the microphone in-
put.
The HF-signal at the microphone input can
be seen composed as of two components.
One component being the differential
(between pins 10 and 11) and the second
related to ground at pin 4. Of these two, the
first is the most serious, entering the
PBL 385 71
Figure 19. RFI elimination at microphone amplifier input.
HF-suppression at the receiver output.
at the microphone amplifier input but will
be easier to solve because of the much
lower impedance and level of gain. The
solution is
capacitors should be connected directly
from pins17 or 18 to ground because of the
low output impedance, series resistance
of at least 10
tendency to self oscillation.
Other paths for the HF-signal to enter
the audible system.
DTMF-generator disconnect the genera-
tor and disconnect the mute input at pin 14.
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-
12
The problem here is of the same kind as
To find out if the problem originates in the
Mic.
100
100
10n
10n
10n
Dynamic microphone
shown in the fig. 20.
1n
1n
10n
10n
must be used if there is a
*
*
11
9
10
4,5,12,13
+
M
a)
PBL385 71
Line
No
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
amplitude that the amplifier can not balance
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
ning pulses (1 s) that will be loaded 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.
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 0.01-10
the shortest lenght of connector on the
PCB board or wire wil be in the same
region of resistance and thus of greatest of
importance.These actions described above
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
The cappacitor C10 shoud be connected
<20n
<20n
Dynamic microphone (simplified)
Mic.
1n
9
10
11
4,5,12,13
+
M
b)
PBL385 71
hence even
Line
10n
10n
1n
goes all the way to the PCB-layout. Small
unbalance signals can be corrected with
capacitors marked with*) this requiring high
precision components. See fig.19a. The
solution shown is rather expensive but
with precision components it renders good
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.
measure to this kind of RFI penetration 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.
Figure 20. RFI elimination at receiver
amplifier output
14
- Line
200-
470
Electret microphone
PBL385 71
Mic.
+
+
Rx
200-
470
4,5,12,13
+
1
1
+
15
16
-
+
9
11
10
4,5,12,13
10-100
+
10-100
M
PBL385 71
c)
Line
<47n
<47n