1
PBL 385 71
June 1999
PBL 385 71
High current Speech and
Line interface Circuit
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
Figure 1. Functional diagram.
16-pin plastic BW SO
16-pin plastic BW DIP
REC
7
9
14
6
3
2
16
15
1
8
AR
AT
PBL 385 71
+
11
10
AM
+
Mute
(active low)
1
3
2
4
Telephone
line
4,5,
12,13.
Description.
PBL 38571 is a monolithic integrated speech transmission circuit for use in
electronic telephones and as DECT line interface with balanced in - and output. It is
designed to accomodate either a low impedance dynamic microphone or an electret
microphone that can be supplied from the circuits own DC - supply. A signal summing
point at the transmitter input is available. An internally preset line length
compensation can be adjusted with external resistors to fit into different current feed
systems as for ex. 48 V, 2 x 200 ohms, 48 V, 2 x 400 ohms and 48 V, 2 x 800
ohms.The line regulation can be shut off in low mode. Application dependent para-
meters such as line balance, side tone level, transmitter and receiver gains and
frequency responces are set independently by external components which means an
easy adaption to various market needs. The setting of the parameters, if carried out in
certain order will counteract the interaction between the settings. It features high line
current capability on a small footprint. Function compatible with the PBL 3781 family
of speech circuits.
Key features.
Minimum number of external
components, 5 capacitors and 10
resistors.
Easy adaption to various market
needs.
Mute control input for operation with
DTMF - generator.
Transmitter and receiver gain
regulation for automatic loop loss
compensation. Disconnectable.
Extended current and voltage range
5 - 130 mA, down to 2 V.
Differential microphone input for
good balance to ground.
Balanced receiver output stage.
In 16 - pin DIP and SO BW batwing
packages.
Short start up time.
Excellent RFI performance.
PBL 38571
PBL38571
2
PBL 385 71
Figure 4. Circuit with external compon-
ents for test set up.
Maximum Ratings
Parameter
Symbol
Min
Max
Unit
Line voltage, t
p
= 2 s
V
L
0
18
V
Line current, continuous, DIP (batwing) package
I
L
0
130
mA
Line current, continuous, SO (batwing) package (dependent on mounting)
I
L
0
120
mA
Operating temperature range
T
Amb
-40
+70
C
Storage temperature range
T
Stg
-55
+125
C
No input should be set on higher level than pin 6 (+C).
Figure 2. Test set up without rectifier
bridge.
Figure 3. Test set up with rectifier
bridge.
+
= 350
+ LINE
- LINE
ARTIFICIAL
LINE
I L
V
2
V1
V
L
R = 0 - 4k
L
0 ohm when artificial
line is used
MUTE
PBL 38571
with external
components
See fig. 4
Z
Mic = 350
Z
Rec
MIC
REC
R
feed = 400
+ 400
600
C
E = 48.5V
V 3
V4
C = 1
F when artificial line is used
470
F when no artificial line
V
M
5H + 5H
+
I
M
+
+ LINE
- LINE
I L
V
2
V1
V
L
R = 0 - 4k
L
MUTE
PBL 38571
with external
components
See fig. 4
Z
Mic
Z
Rec
MIC
REC
R
feed = 400
+ 400
600
+
E = 50.0V
V 3
V4
1
F
V
M
Uz= 15-16V
5H + 5H
= 350
= 350
I
M
REC
350
7
9
14
6
3
2
16
15
1
8
AR
AT
PBL 385 71
+
11
10
AM
4,5,
12,13.
310
47nF
2.7k
Mic.
350
R16
Mute
(active low)
910
62k
6.2k
910
100nF
18k
R4
C3
R7
R8
R10
R11
R3
R14
+Line
+
15nF
47
F
11k
100nF
11k
560
75
22k
R5
R6
R9
C5
R12
C1
C2
-Line
C6
Rg
3
PBL 385 71
Electrical Characterisics
At T
Amb
= + 25
C. No cable and line rectifier unless otherwise specified.
Ref.
Parameter
fig.
Conditions
Min
Typ
Max
Unit
Line voltage, V
L
2
I
L
= 15 mA
3.3
3.7
4.1
V
2
I
L
= 100 mA
11
13
15
V
Transmitting gain, note 1
2
20
10
log (V
2
/ V
3
); 1 kHz
No gain regulation, low mode
R
g
= 20k
41
43
45
dB
Transmitting gain, note 1
2
20
10
log (V
2
/ V
3
); 1 kHz
With gain regulation
R
L
= 0, R
G
=
41
43
45
dB
R
L
= 400
43.5
45.5
47.5
dB
R
L
= 900
- 2.2 k
46
48
50
dB
Transmitting range of
2
1 kHz, R
L
= 0 to 900
3
5
7
dB
regulation
Transmitting frequency
2
200 Hz to 3.4 kHz
-1
1
dB
response
Microphone input impedance 2 1.7//(2.7) note 3 k
Transmitter input impedance
2
1 kHz
13.5
17
20.5
k
pin 3
Transmitter dynamic output 2 200 Hz - 3.4 kHz
1.5 V
p
2% distortion, I
L
= 20 - 100 mA
Transmitter max output 2 200 Hz - 3.4 kHz
3 V
p
I
L
= 0 - 100 mA, V
3
= 0 - 1 V
Transmitter output noise 2
Psoph-weighting, Rel 1 V
rms
, R
L
= 0
-75
dB
Psoph
Receiving gain, note 1
2
20
10
log (V
4
/ V
1
); 1 kHz
No gain regulation, low mode
R
g
TM = 20k
-18.5 -16.5 -14.5 dB
Receiving gain, note 1
2
20
10
log (V
4
/ V
1
); 1 kHz
With gain regulation
R
L
= 0, R
G
=
-18.5 -16.5 -14.5 dB
R
L
= 400
-16 -14 -12
dB
R
L
= 900
- 2.2 k
-13.5 -11.5 -9.5
dB
Receiving range of regulation
2
1 kHz, R
L
= 0 to 900
3
5
7
dB
Receiving frequency response
2
200 Hz to 3.4 kHz
-1
1
dB
Receiver input impedance
2
1 kHz,
38
k
Receiver output impedance
2
1 kHz,
3(+310)note 3
Receiver dynamic output
2
200 Hz - 3.4 kHz
0.5
V
p
note 2
2% distortion, I
L
= 20 - 100 mA
Receiver max. output
3
Measured with line rectifier
0.9
V
p
200 Hz - 3.4 kHz, I
L
= 0 - 100 mA,
V
1
= 0 - 50 V
Receiver output noise
2
A-weighting, Rel 1V
rms
, with cable
-85
dB
A
0 - 3 km, = 0.4 mm
0 - 5 km, = 0.5 mm,
Mute input voltage
2
0.3
V
at mute (active low) pin 7.
Notes
1. Adjustable to both higher and lower values with external components.
2. The dynamic output can be doubled, see applications information.
3. External resistor in the test set up.
4
PBL 385 71
16-pin BW DIP
16-pin BW SO
Figure 5. Pin configurations.
Pin Descriptions
Refer to figure 5.
Pin
Name
Function
1
+L
Output of the transmitter amplifier. Connected to the line through a polarity guard diode bridge.
2
TO
Output of the transmitter amplifier. Connected through a resistor of 47 to 100 ohm to -L,
sets the DC-resistance of the circuit. The output has a low ac. impedance and the signal is used
to drive a side tone balancing network.
3
TI
Input of transmitter amplifier. Input impedance 17 kohm
20 %.
4,12
- L
Negative power terminal, connected to the line through a polarity guard diode bridge.
5,13
6
+C
Positive power supply terminal for most of the circuitry inside the PBL 385 71 (about 1 mA current
consumption). The +C pin should be connected to a decoupling capacitor of 47
F to 150
F.
7
Mute
Mute input. Maximum voltage (to mute) is 0.3 V, current sink requirement of external driver is 100
A.
8
GR
Input for the gain regulation control circuitry.
9
MO
Output of the microphone amplifier.
10
MI 1
Inputs of the microphone amplifier. Input impedance 1.7 kohm
20%.
11
MI 2
14
RI
Input of receiver amplifier. Input impedance is 38 kohm
20 %.
15
RE 1
Receiver amplifier outputs. Output impedance approximately 3 ohms.
16
RE 2
}
}
+L
TO
TI
+C
GR
-L
1
2
3
4
5
6
7
8
14
13
12
11
9
10
RE 2
RE 1
-L
MO
-L
15
16
-L
Mute
M1
M2
RI
1
2
3
4
5
6
7
8
16
15
14
13
TO
+L
GR
-L
RE1
MI
TI
+C
RI
-L
MO
9
12
10
11
-L
RE2
Mute
MI
-L
}
5
PBL 385 71
Figure 7. Block connections.
Figure 6. AC-impedance.
Functional description
Design procedure; ref. to fig.4.
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).There are also internal
circuit dependent requirements like supply
voltages etc.
3. Set the attac point where the line
length regulation is supposed to cut in
(R1 and R2 in fig. 18). 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.
Impedance to the line
The AC- impedance to the line is
set by R3, C1 and C2. Fig.4. 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 6. This
generates a closed loop in the transmitter
amplifier that in its 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 6 is set by the
ratio
Rs/R19 (910
), where in case b).
the ratio at high frequencies will be Rs/
220
because the 820
resistor is
bypassed by a capacitor. To help up this
situation the complex network capacitor is
connected directly to ground, case c).
making the ratio Rs/220
+820
and 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 voltages that are
needed for the different parts of the
telephone to function. For ex. for
transmitter output amplifier, recei-
ver output amplifier, dialler, speech
switching.
1
2
+Line
R3
R6
PBL 38 571
+
3
C1
C2
-Line
Rs
1
How to connect a
complex network.
220
+820
//Cx
Example:
a)
b)
c)
6
220
820
Cx
+
1
AM
AT
AR
2
Transmitter summing
input
Mute
+ Line
- Line
3
4
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