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PBL3852 Schematic ( PDF Datasheet ) - Ericsson

Teilenummer PBL3852
Beschreibung Universal Transmission Circuit
Hersteller Ericsson
Logo Ericsson Logo 




Gesamt 24 Seiten
PBL3852 Datasheet, Funktion
PABpLril31895926
PBL 3852
Universal Transmission Circuit
Description
The PBL 3852 is an universal transmission circuit in bipolar technology that performs
all the speech and line interface functions required to implement an electronic
telephone set suitable for the majority of existing telephone network requirements.
Easy adaptation of the DC-mask to different line feed systems. A summing point
for auxiliary signals to be transmitted like DTMF and hands-free audio signal. The
PBL 3852 has a low current consumption that enables the circuit to work with
reduced performance down to 2.1 volts (4.8 mA) across the circuit. The low current
consumption for a speech circuit is essential in telephone line powered handsfree
designs required to work at long line lengths. The PBL 3852 is especially suitable to
be used with Ericsson handsfree circuits like PBL 3786, PBL 3786/2, PBL 3881 and
PBL 3880 thanks to a specific interfacing arrangement.
The transmitting and receiving gains can be regulated in order to compensate for
the attenuation of the signals due to increasing attenuation with increasing line length.
It is also possible to limit high transmitting signal levels (soft clipping) thus preventing
excessive distortion caused by signal clipping. The gain regulation is set with discrete
external components.
The circuit is easily adapted to different markets by setting the application depend-
ent parameters individually in certain order, this preventing the interaction between
the same. PBL 3852 has up to four different power supplies to feed microphones,
auxiliary circuits and functions.
All pin numbers in this paper refer to DIP package.
(+Line)
6
13
+
12
5
Ref. DC
supply
DC
supply
18 9 8 11
3
1
+
PBL 3852
Ref.
27
10 15
17
-
+
16
14 4 +
Telephone
line
Key Features
• Generates its own supply from the
telephone line
• Adaptive to all types of telephone line
feeding systems (i.e. 48V 2x200,
60V 2x 600, 48V 2x800)
• Operates down to 2.1V (excl. polarity
bridge)
• Adjustable DC-characteristic to the line
• Few inexpensive external components
to function
• Easy adaptation for various market
needs
• Dialler interface with DC-supply, mute,
power -down and DTMF-input
• Confidence tone in the receiver at
DTMF-dialling
• ”Soft clipping” that prevents distortion
at high transmit signal levels
• Balanced microphone input for dyna-
mic, and electret microphones
• Balanced receiver output for dynamic
and magnetic receiver elements
• Transmitter and receiver gain regula-
tion for automatic loop loss compen-
sation (disabled in mute mode)
• Four separate DC supplies for different
requirements
• High gain of the receiver facilitates
volume control function
• Microphone cut-off function possible
by a switch
• All gain and frequency setting networks
in Rx, Tx and side tone are referred to
ground
• Excellent RFI performance
Power
down
DC1 DC2
A.
++
B.
+
+
DTMF
C.
(-Line)
A. Dynamic limiter
B. Sidetone network
C. Gain regulation with line length
Figure 1. Functional diagram.
18-pin plastic DIP 20-pin plastic SO
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PBL3852 Datasheet, Funktion
PBL 3852
V
16 V telephone line
14
V line
V pin 4
12
10
8 V pin 2
6
4
V pin 8
2 V pin 9
IL
20 40 60 80 100 120 mA
Figure 8. DC-Characteristics. (R7 = 75 )
Vcirc. = IPIN4 R19 + k1Vref + R7 Iline + k 2 Vpin2 + a
( )a = Ipin5 5. 5 103 if function DC control at pin5 is used
k1Vref = 1.1V
k 2 Vpin2 = 0. 5 R7 Iline
IPIN4 1mA
The R7 will set the slope of the DC-char.
and the rest of the level is set by some
constants in the circuits as shown in the
equation. The slope of the DC-char. will
also influence the line length regulation
(when used) and thus the gain of both
transmitter and receiver. R7 acts also as
current protection for the circuit, must be
considered when low values are to be
used. The level of DC-characteristic can
be adjusted up at input pin 5 (some
100mV´s). The R21 adjusts a fix amount
where R20 couples the adjusted value to
line current. See fig. 32.
Microphone amplifier
The microphone amplifier in the PBL
3852 is divided into two stages. The first
stage is a true differential amplifier
providing high CMRR (-55 to -65 dB
typical) with voltage gain of 19 dB. This via a capacitor to the - input at pin 13.
stage is followed by a gain regulated
The DC supply resistors for the
amplifier with a regulation range from 6.5 microphone should be round 200(in
dB to 14.5 dB, see fig. 15. The input of order not to overdrive the microphone
the microphone amplifier can be used for amplifier) and the feedback resistor (17k)
electret, magnetic or dynamic transducers is of that magnitude that it either
see fig. 9. The PBL 3852 has basically a influences the CMRR balance at the
higher gain regulation range (8 dB) than input or destroys the send mute by
the more or less standard 6 dB´s for gain bypassing signal round the microphone
regulation with line length, this in order to amplifier in mute state. For a dynamic
be able to be used in applications where microphone some more components are
”softclipping” is required. In case lower necessary, see fig. 10c. In order not to
regulation range is necessary, it is
influence the send mute the feedback
possible with some additional
signal is taken from transmitter output at
components.
pin 2 and because this signal is in
See reference figs. 4, 10c, 10f, 32 and opposite phase with the signal at pin 11,
33. For an electret microphone the
it is taken to the other input at pin 12. Also
circuitry will be simple, see fig. 10f. A
in order not to influence the DC-balance
resistor is added from the microphone of the microphone amplifier a capacitor
amplifier output, pin 11, to the positive has to be included in the feedback path
termination of the microphone and further and to maintain the CMRR of the
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PBL3852 pdf, datenblatt
PBL 3852
Dynamic limiter
The dynamic limiter consists of a full wave rectifier that senses the signal amplitude on the line and produces a control signal that
reduces the gain of the transmitter and the receiver when the signal on the line reaches a certain set level. The reason to this is to
reduce distortion at high signal levels (See fig. 15, 16, 17, 18 and 34).
The attac point for the dynamic limiter is set by the voltage divider R8, R10 and the internal resistor of 30k (peak signal) the lower
frequency limit is set by the input capacitor C5 to pin 7 (RTC). The diodes Da and Db that make the function logarihtmic have a 0.6V
voltage drop on this signal which is then added to the reference 0.855V. The signal will be further rectified with a ratio 1:1, attenuated
(can be neglected) in the filter at pin GR and forvarded to the gain regulation.
At a enough high signal in, the voltage at pin GR is set by:
VpinGR V 2peak R 8 / / 30k
0.6 + 0.855
R 10 + R 8 / / 30k
The time constant ”up” is set by the internal 2.2k and C7, where the time constant ”down” is R14 parallel with R15 and C7. The
DC-voltage at pin GR with no input signal is set by the resistor divider R14 and R15 at a level just below where regulation starts see
fig. 16. It is possible by adjusting this DC-level down to make the time constant ”up” longer. With no AC signal in, or a very small and
no resistors R14,R15 the rectifier output is at reference in level (0.855V). At no AC signal in the voltage at pin GR is set up by:
Vpin GR = VDC1
R15
R14 + R15
If gain regulation with line length is used together with softclipping the time constant ”down” will be influenced by the parallel value of
R14,R15, and R16. The DC-level at pin GR without AC signal will be set by these three resistors.
V2
Vp
2.25
2
1.75
1.5
1.25
1
0.75
0.5
0.25
V3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 mVp
Vin +10dB
+15dB
+20dB
Figure 17. Dynamic limiter.
12

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