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Número de pieza PBL385411SO
Descripción Universal Speech Circuit
Fabricantes Ericsson 
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PBL 385 41
Universal Speech Circuit
Description.
Key features.
PBL 38541 is a monolithic integrated speech transmission circuit for use in • Minimum number of external
electronic telephones or in any other line interface application. High settable supply
components, with two filtered DC-
current for auxiliary functions, up to 6.0 mA (at high line currents). The circuit is designed
supplies, 7 capacitors and 11
to accomodate either a low impedance dynamic or an electret microphone. Microphone
resistors.
can be muted separately. Payphone signaling and DTMF dialling tones have a separate • Easy adaption to various market
input that is controlled by a mute signal. A signal summing point is available at the
needs.
transmitter input. An internally preset line length compensation can be adjusted with • Mute control input for operation with
external resistors to fit into different current feed systems as for ex. 48 V, 2 x 200 ohms,
DTMF - generator.
48 V, 2 x 400 ohms and 48 V, 2 x 800 ohms. The line length compensation can be shut • A separate signaling input for
off in either high or low gain mode. Application dependent parameters such as line
payphone and DTMF tones controlled
balance, side tone level, transmitter and receiver gains and frequency responces are set
by mute.
independently by external components which means an easy adaption to various market • Transmitter and receiver gain
needs. The setting of the parameters if carried out in certain order will counteract the
regulation for automatic loop loss
interaction between the settings. The circuit provides four different DC - supplies to feed
compensation.
microphones,diallers and other more current consuming functions like handsfree systems. • Extended current and voltage range
4 - 130 mA, down to 2 .2V.
Pin numbers in this datasheet refer to 18-pin DIP package unless otherwhise noted. • Differential microphone input for good
balance to ground.
• Balanced receiver output stage.
• One stabilized DC - supply for low
current CMOS diallers and or electret
DTMF
input
Mic.
Mute
(active low)
DC1- output for
external devices
10
AD
12
13 AM
DC-supply
89 7
6
5
1
PBL 385 41
AT AR
17
18
5 11 3
2
15 16 14 + 4
43
1
Telephone
line
microphones. One settable current
limited supply with 6 mA max. current.
• Short start up time.
• Excellent RFI performance.
• 18 - pin DIP and 20 - pin SO packages.
DC2 - output for
external devices
+
Gain
+ regulation
2
+
1. Impedance to the line and radio interference suppression
2. Transmitter gain and frequency responce network
3. Receiver gain and frequency responce network
4. Sidetone balance network
5. DC supply components
Figure 1. Functional diagram DIP package.
20-pin plastic SO
18-pin plastic DIP
1

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PBL385411SO pdf
PBL 385 41
Functional description
Design procedure; ref. to fig.4.
+Line
The design is made easier through that all
settable parameters are returned to gro-
und (-line), this feature differs it from bridge
type solutions.To set the parameters in the
following order will result in that the
interaction between the same is minimized.
1. Set the circuit impedance to the line,
either resistive (600) or complex. (R3
and C1). C1 should be big enough to give
low impedance compared with R3 in the
telephone speech frequency band.Too
large C1 will make the start-up slow. See
fig. 6.
2. Set the DC-characteristic that is
required in the PTT specification or in case
of a system telephone,in the PBX
specification(R6). Observe the power
dissipated.There are also internal circuit
dependent requirements like supply volta-
ges etc.
3. Set the attac point where the line
length regulation is supposed to cut in
(R1 and R2). Note that in some countries
the line length regulation is not allowed. In
most cases the end result is better and
more readily achieved by using the line
length regulation (line loss compensation)
than without. See fig. 13.
4. Set the transmitter gain and
frequency response.
5. Set the receiver gain and frequency
response. See text how to limit the max.
swing to the earphone.
6. Adjust the side tone balancing
network.
7. Set the RFI suppression
components in case necessary. In two
piece telephones the often ”helically”
wound cord acts as an aerial. The
microphone input with its high gain is
especially sensitive.
8. Circuit protection. Apart from any
other protection devices used in the de-
sign a good practice is to connect a 15V
1W zener diode across the circuit , from
pin 1 to -Line.
PBL 385 41
1
4
3
C2
2
R6
Figure 6. AC-impedance.
Impedance to the line
The AC- impedance to the line is
set by R3, C1 and C2. Fig.6. The circuits
relatively high parallel impedance will not
influence it to any noticeable extent. At low
frequencies the influence of C1 can not be
neglected. Series resistance of C1 that is
dependent on the temperature and the
quality of the component will cause some
of the line signal to enter pin 4. This
generates a closed loop in the transmitter
amplifier that in it´s turn will create an
active impedance thus lowering the
impedance to the line. The impedance at
high frequencies is set by C2 that also
acts as a RFI suppressor.
In many specifications the
impedance towards the line is specified as
a complex network. See fig. 6. In case a).
the error signal entering pin 4 is set by the
ratio Rs/R3 (910), where in case b). the
ratio at high frequencies will be Rs/220
because the 820resistor is bypassed by
a capacitor. To help up this situation the
1
+
AM 2 AT
Transmitter summing
input
Mute
Figure 7. Block connections.
4
3
+ Line
AR
- Line
a) b)
R3
c)
220
Cx
820
Rs
1
+
C1
Example:
How to connect a
complex network.
220+820//Cx
-Line
complex network capacitor is connected
directly to ground, case c). making the ratio
Rs/220+820and thus lessening the
error signal. Conclusion: Connect like in
case c) when complex impedance is
specified.
DC - characteristic
The DC - characteristic that a
telephone set has to fulfill is mainly given
by the network administrator. Following
parameters are useful to know when the
DC behaviour of the telephone is to be set:
• The voltage of the feeding system
• The line feeding resistance 2 x.......
ohms.
• The maximum current from the line at
zero line length.
• The min. current at which the
telephone has to work (basic
function).
• The lowest and highest voltage
permissible across the telephone set.
• The highest voltage that the telephone
may have at different line currents.
Normally set by the network owners
specification.The lowest voltage for the
telephone is normally set by the volta-
ges that are needed for the different
parts of the telephone to function. For
ex. for transmitter output amplifier,
receiver output amplifier, dialler,
speech switching and loudspeaker
amplifier in a handsfree telephone etc.
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PBL385411SO arduino
PBL 385 41
DTMF input.
The circuit provides a specific input at pin
10 for DTMF dialling signals. The output of
the amplifier is controlled by the mute
signal provided by the dialler. This input
makes it easy to time the DTMF signals
entering the line. Most of the diallers wake
up at pressing of a key and the output will
not be stable imidiately. This unstable
state will be of some length and is limited
in some telephone specifications.
Power supplies DC1, DC2,
V+C and VPA (See fig.18)
PBL 38541 generates its own DC
supply V+C dependent of line current with
an internal shunt regulator. This regulator
senses the line voltage VL via R3 and line
current via R6 in order to set the correct
V+C so the circuit can generate the required
DC characteristic for a given line resistance
RLine and the line feeding data of the
exchange. A decoupling capacitor is
needed between pins +C and -L. The V+C
supply changes its voltage linearly with the
line current. It can be used to feed an
electret microphone. Caution must be ta-
ken though not to drain too much current
out of this output because it will affect the
internal quick start circuit by locking itself
into active state. (max. permissible current
drain 700µA)
Care has to be taken when deci-
ding the resistance value of R3. All
resistances that are applied from +Line to
ground (-Line) will be in parallel, forming
the real impedance towards the line. This
will sometimes result in, that the ohmic
value of R3 is increased in order to comply
to the impedance specification towards
the line. The speech circuit sinks 1mA
into pin 4, which means that the working
voltage for the speech function V+ will
decrease with increasing R3, thus starving
in the end the circuit of its working voltage
. This dependency is often falsely taken as
a sign of that the circuit does not work
down to the low line current specified, but
in fact it is the working voltage at pin 4 that
has become too low. It is obvious that this
problem is also connected into what kind
of DC-characteristic is set. See fig. 8.
The circuit has further two tempera-
ture and line current compensated DC
supplies DC1 and DC2. DC2 is a voltage
supply for supplying diallers, can be used
for memory back up because it does not
leak any current back into the circuit. Typical
voltage 2.4V down to line voltage of 4.1V,
in case the line voltage is lower than 4.1V
calculate ; actual line voltage minus 1.9V.
In order to prevent noise entering the line,
a series resistor and a reservoir capaciotor
is recommended in for this output.The
output current is given to be 2 mA in the
specification.
DC1 is a 3.7 V (typ.) supply whose,
for the design required maximum output
current can be set at the control input pin
7. The set current will flow constant
(necessary to keep the line current cons-
tant) used or not used by a function, so
care has to be taken when setting the
current limit so it won´t be unnecessary
high. The maximum current that can be set
and drawn by maintaining the voltage level
is 2.0 mA (up to 6 mA can be set if the line
current so allows). This supply is ment to
be used to supply microphones, Ericsson´s
handsfree IC’s switching parts (see
applications), opto couplers etc. and in
payphones its auxiliary functions.
The fourth DC-supply VPA has an
advantage that it does not influence the
circuits DC characteristics even at high
current drain. The supply has a floating
ground reference in the +line in order to
minimize RFI problems and is used to
supply the power amplifier of a handsfree
telephone ( PBL3881, 38813 ). These
circuits have a current controlled charging
of the supply capacitor and the control
signal is taken across the resistor R6. In
case a monitor amplifier is required where
the ground reference is hardly necessary,
it can be supplied from VPA or like in alt. b
in fig. 18.
IL
+Line
RLine
VL
RFeed
+ Vexh.
-Line
Hook switch
+
VPA
b.
VMon.
a. +
R3
V+C 4
+
Tx
-
ƒ
2
+
C1 R6
1
PBL 385 41
V Ref.
1.2V
++
-
15k
Lim
15k
3 14
1-10M
7
R17
9
3.7V
8 0-470
6V 6V
VDC1
VDC2
+ 4.7-47
µF
+ 4.7-47
µF
a. Supply arrangement for a handsfree system power amplifier. For ex. PBL 388 13
b. Supply arrangement for a call monitor cicuit.
Figure 18. DC - supply system for external loads.
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