PBL38570 Datasheet, PDF (8/12 Pages) Ericsson – Universal Speech Circuit

PBL 385 70
amplitude that can be transmitted to the
line undistorted is dependent of R6.
(amplitude limiting).
The transmitter gain and frequency re-
sponse are set by the RC-network between
the pins 11 and 3. See fig.10. The capacitor
for cutting the high end of frequency band
is best to be placed directly at the
microphone where it also will act as a RFI
suppressor. The input signal source
impedance to the transmitter amplifier in-
put TI should be reasonably low in order to
keep the gain spread down, saying that
R4//R5 (see fig. 4) must be at least a factor
5 lower than the ZTin. Observe that the
capacitor C1 should have a reasonably
good temperature behaviour in order to
keep the impedance rather constant. The
V+C´s influence on the transmitter DC-
characteristic is shown in the fig.7 (DC-
characteristic), therefore the transmitter
gain would change if the transmitted
signal gives reason to an ac-voltage leak
signal across C1 since this is a feedback
point. If the transmitter has an unacceptable
low sving to the line at low line currents
<≈10mA the first step should be to examine
if the circuits DC characteristic can be
adjusted upwards.
Receiver amplifier
The receiver amplifier consists of three
stages, the first stage being an input buffer
that renders the input a high impedance.
The second stage is a gain regulated diffe-
rential amplifier and the third stage a
balanced power amplifier. The power
amplifier has a differential output with low
DC- offset voltage, therefore a series
capacitor with the load is normally not
necessary. The receiver amplifier uses at
max. swing 4-6 mA peak. This current is
drawn from the +Line. The gain and
frequency response is set at the input RI
with a RC-network. The receiver gain can
be regulated. The range of regulation
from the input to the output is 5 ± 2 dB (19
to 24dB). The driving capacity of the power
stage can be optimized by a resistor on
pin16, an other method is to connect a
resistor series in with the earphone itself.
The balanced earphone amplifier can not
be loaded to full (both current and signal
level ) single ended.The signal would be
distorded when returned to ground. A
methode is shown in fig.11d. how to connect
a light load (5k ac. or DC wise) to the
output. It is preferred that both outputs are
loaded the same. The receiver has, as a
principal protection, two series diodes anti
(a)
PBL 385 70
17
+
+ _Rx
18
(b)
(c)
PBL 385 70
+
+
Rx
Figure 11. Receiver arrangements.
parallel across its output to limit the signal to
the earphone and thus preventing an
acoustical shock. A resistor in series with
the output can very well be used to increase
the protection level. Note, that the noise in
the receiver is allways transmitter noise
that has been more or less well balanced
out by the side tone network.
The RC - network (optional) at the output
is to stabilize against the inductive load that
an earphone represents.
Gain regulation.
Both the receiver and transmitter are gain
regulated (line loss compensated).
There is a fixed default compensation on
the chip that can be adjusted or or set to
constant high or low gain mode. The input
impedance at the gain regulation pin 6 is
5.5k ± 20%. The default regulation pattern
is valid when the input is left open. Fig. 12
shows a typical transmitter or receiver gain
pattern versus line length. The following will
show, what to alter, to change the look of
the curve.
Battery feed
R1
Regulation:
48V,2 ⋅ 200Ω
∞
48V,2 ⋅ 400Ω
∞
48V,2 ⋅ 800Ω
∞
No regulation:
Set for low gain
All feedings
∞
Set for high gain
18k
Set for high gain
22k
(d)
PBL 385 70
17
17
+
+
Rx
-
(C)
Z
Z
18
18
(C) Z > 5k
The capacitor C is optional
a). Adjustable with the divider R4,R5
for the transmitter and with R12 for the
receiver.
b). The attack point of the regulator is
adjusted with the divider R1,R2 to either
direction , up or down ,on the line current
axis.
c). The angle of elevation of the curve is
mainly set by the value of R6. If the DC-
characteristics is set according to the line
parameters and a correct value for R6 is
chosen the angle is mostly correct but it
can be adjusted with R6. The adjustement
will affect the DC-characteristics aswell
as most of the other parameters. This is
why the DC-characteristic is set early in
the design phase.
dB
c.
b.
a.
High limit
Low limit
IL
Figure 12. Gain regulation principle.
R2
∞
∞
180k
R6
47Ω
75Ω
100Ω
22k
47 - 100Ω
∞
47Ω
∞
75 - 100Ω
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