PBL 3786
level. This prevents the continuous noise
at transmit channel input from
influencing the switching balance.
The input signal to the background
noise detector is taken from the output of
the transmit detector, a signal repre-
senting the sound pressure envelope at
the microphone input. This signal is
phase shifted and amplified by a factor
of 2 during the transmit period. The
capacitor C4 at the output is decharged
(relative to the reference) each time
when in transmit mode. The time
Tip
LS2 18
LS1 15
17
16
50 ohm
50 ohm
+
+
+
+
PBL 3786
+L 20
–C 22
+
Ring
Ring
HFµPS
µPC
LFµPA
12
PWM
14
13
constant for decharging C4 is formed by
an internal resistor of 100k + R6. C4 is
charged each time when the absolute
voltage level across C3 at the Txdet
output reaches half of the value across
C4 during one transmit period. The
charge time constant is C4 x R6. R6 is
there to limit the charging current of C4
that otherways could cause signals in
the ground leads and thus disturbing the
swithing performance. A sample and
hold function will retain the voltage level
across C4 ower periods when the
receive channel is active.
The voltage across C4 is connected to
the CMP input via R5. The extent to
which the Ndet will influence the switching
level at CMP input is set by the gain of
the detectors (balance level), the
maximum signal swing and R5.
The function in detail: When a
constant input signal, without any longer
breaks, is received from the microphone,
it is treated as noise. The level across
C4 is pulled negative (relative
(Volume)
Figure 26. Toneringer basic application.
Tip
LS2 18
17
50 ohm
50 ohm
LS1 15
+
+
+
+
PBL 3786
+L 20
–C 22
+
16
Ring
Ring
HFµPS
13
µPC
LFµPA
12
PWM
14
Regulated Dc-supply for
µP or melody generator
at toneringing.
+
TTL
Signal
Input
Figure 27. Toneringer application with external microprocessor.
reference) with a time constant 100k x
R4. This time constant should be round
5-10 sec. Since the output of Ndet is
going negative it counteracts the Txdet
signal (or helps the receive channel to
break through by having the same
V
out
(Vp)
I
(mA)
µP
2.4
2.0
1.6
3.0
2.0
1.2
0.8
1.0
0.4
ILine
(mA)
RPin12
(kohm)
10
20
30 40
50
60
70
80
90 10
18
2
4
6
8
10 12 14 16
Figure 28. Output current on pin 14, toneringer µ-processor
Figure 29. Typical loudspeaker output swing for application 2.
11
ERICSSON [ ERICSSON ]
