pbl38571 ETC-unknow, pbl38571 Datasheet - Page 10

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

PBL 385 71
Note that the signals at points a. and b. are
180 degrees off phase.
10 x R6
R7
The ac-signal at point c. is now 1/10 of the
signal on the line because it is further
divided by two from point b. (R7 Zbal).
Hence 10 x R10
balancing criteria. R12 is to set the receiver
gain. ( can also be a volume control potentio-
meter).
frequency behaviour of the transmitter is
not influenced. With the ratio 1/10 the
influence is 1 dB, and with ratio 1/20 it´s 0.5
dB.
compared with Zbal, it will load the latter
and
perfomannce, again if the R7 is made high
ohmic compared with Zbal will result in a
low signal to balance the side tone with and
make the balancing difficult. Making any of
the impedances unnecessary high will
make the circuit sensitive to RFI. All values
given here are approximate and serve as
starting entities only. The final trimming of
side tone network is a cut and try proposi-
tion because a part of the
in the acoustical path between the
microphone and earphone.
Reverse side tone network.
help when for some reason there is a need
to make the R6 low < 47
signal for balancing gets small across R6.
By placing the balancing network like shown
in fig.15 the possible signal level is 6 dB
higher than in the first case and it will also
help in case when a volume control is
added to the receiver.
Figure 15. Reverse side tone network
with complex R11.
10
Note #1 These values ensure that the
Note #2
R6
This type of side tone balancing will
Zbal
2
result in a bad side tone
R10
PBL 38571
R7 + Zbal
R12
If the R7 is made low ohmic
C6
C
*
14
* To give receiver flat
frequency response
R11 to satisfy the
R11
Note#2
Note #1
balance lies
and thus the
Z bal.
+Line
Start up circuit
which function is to fast charge capacitor
C1 when the circuit goes into hook- off
condition. The fast charge circuit is a
thyristor function between pins 1 and 6
that will stop conducting when the current
drain at pin 6 is lower than 700 A + the
internal current consumption ( about 1
mA). Care must be taken when connecting
external load to pin 6 in order not to exeed
the
would result in an inoperative speech
funktion. This
before the voltage level at C1 drops below
2V or the line voltage is below 1V. See
fig. 17.
Figure 16. Mute input.
Mute function.
Sinking current from this pin will cut off the
gain in the
(attenuation min. 60dB) and decrease the
gain in the receiver amplifier to reach the
confidence tone level at DTMF-dialling.
The receiver mute is 40dB down from the
unmuted value to satisfy those who keep
the handset close to the ear at dialling.
Optional conditions:
the ear, the confidence tone level is too
low. To alter the level, a signal can be taken
from DTMF generator output to receiver
input before the capacitor C6. The added
impedance to this point will hardly disturbe
the signal condition in active speech mode.
The microphone amplifier only, can be
muted by sinking current from the output
pin 9. See fig. 4 or 9.
The circuit has a mute function at pin 7.
Mute
The circuit contains a start up device
For users who keep the handset from
700 A limit. Should this happen, it
V
Mute
I
Mute
a)
7
38571
PBL
-L
circuit can not retrigger
microphone amplifier
Mute
The diode has to be low
voltage drop type.
V
I
Mute
Mute
b)
7
38571
PBL
-L
signal is used to other tasks than muting
the speech circuit it has to be isolated. If a
diode is used it has to be a low voltage
drop type. The input at mute has to be
below 300mV. If the mute signal has
reverse polarity out of the system it can be
phase changed like in c.) In case it is
required to mute the receiver only, d.) it
can be done by shorting the receiver input
to ground before or after the input capacitor.
Shorting the input pin to ground (does not
have to be absolute ground) actuates a
mute by driving the amplifier into saturation
thus blocking the signal path and rende-
ring a mute with high attenuation but will
cause a DC-level shift at output which in
its turn will cause a ”click ” in the earphone.
This can be softened with a slower mute
signal flank. If the second approach,
grounding before the input capacitor is
chosen, the grounding has to be low ohmic
in order to render a high attenuating mute.
Figure 17. Fast startup circuit.
c)
-L
Figure 15 b.) If the system mute
Tx
38571
7
PBL
1
2
PBL 38 571
Mute
DC supply
6
Receiver mute only.
Muting
points
d)
38571
14
PBL
R3
C1
Rx
+Line
-Line
16
15