PBL 385 70
a)
c)
b)
d)
PBL
385 70
PBL
385 70
PBL
385 70
PBL
385 70
Mute
Mute
I
Mute
I
Mute
17
18
5
5
5
14
-L
Rx
V
Mute
14
-L
14
Mute
V
Mute
15
-L
Muting
points
Receiver mute only.
The diode has to be low
voltage drop type.
Figure 15. Mute principles.
Mute function
Power supplies DC1, DC2, V
and
+C
V
(See fig.17)
PA
The circuit has a mute function at pin 5.
Sinking current from this pin will cut off the
gain in the microphone amplifier
(attenuation min. 60dB) and decrease the
gain in the receiver amplifier to reach the
confidence tone level at DTMF-dialling.
Thereceivermuteis≈ 40dBdownfromthe
unmuted value to satisfy those who keep
the handset close to the ear at dialling.
Optional conditions.
For users who keep the handset from
theeartheconfidencetonelevelistoolow.
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
thesignalconditioninactivespeechmode.
The microphone amplifier only, can be
muted by sinking current from the output
pin 11. See fig.4 or 9.
ned 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.
PBL38570 generatesitsownDCsupply
dependent of line current with an
internal shunt regulator. This regulator
V
+C
senses the line voltage V via R and line
L
3
current via R in order to set the correct
6
Start up circuit
V
sothecircuitcangeneratetherequired
+C
DCcharacteristicforagivenlineresistance
The circuit contains a start up device
which function is to fast charge the
capacitor C1 when the circuit goes into
hook- off condition. The fast charge circuit
is a thyristor function between pins 1 and
4thatwillstopconductingwhenthecurrent
drain at pin 4 is lower than ≈ 700 µA + the
internal current consumption. ( about 1
mA) This circuit can not retrigger before
the voltage level at C1 drops below 2V or
the line voltage below 1V. See fig 16.
R
and the line feeding data of the
Line
exchange. A decoupling capacitor is
needed betweenpins+Cand -L. TheV
+C
supplychangesitsvoltagelinearlywiththe
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 deciding 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
dependencyisoftenfalselytakenasasign
ofthatthecircuitdoesnotworkdowntothe
lowlinecurrentspecified, butinfactitisthe
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. 7.
Figure 15 b.) If the system mute signal
is used to other tasks than muting the
speech circuit it has to be isolated. If a
diodeisusedithastobealowvoltagedrop
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 e.) In case it is required to
mute the receiver only, d.) it can be done
by shorting the receiver input to ground
beforeoraftertheinputcapacitor.Shorting
theinputpintoground(doesnothavetobe
absoluteground)actuatesamutebydriving
the amplifier into saturation thus blocking
the signal path and rendering 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 softe-
+Line
1
PBL 385 70
Tx
DC supply
R3
2
4
C1
-Line
Figure 16. Fast start up principle.
10
ERICSSON [ ERICSSON ]
