PBL 385 82
Note that the signals at points a. and b. are
180 degrees off phase.
10 x R6
≈
R7 + Zbal
Note #1
R7
≈
Zbal
Note#2
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 R1
≈
R2 to satisfy the balancing
criteria. R12 is to set the receiver gain. (
can also be a volume control potentio-
meter).
Note #1
These values ensure that the
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.
Note #2
If the R7 is made low ohmic
compared with Zbal, it will load the latter
and result in a bad side tone
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 propo-
sition because a part of the balance lies
in the acoustical path between the
microphone and earphone.
internal current consumption ( about 1
mA). Care must be taken when connecting
external load to pin 4 in order not to exeed
the
≈
700
µA
limit. Should this happen, it
would result in an inoperative speech
funktion. This circuit can not retrigger before
the voltage level at C1 drops below 2V or
the line voltage is below 1V. See fig. 10.
+Line
1
Start up circuit
Tx
PBL 38 5 82
DC supply
The circuit contains a start up device
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 4 that
will stop conducting when the current
drainat pin 4 is lower than
≈
700
µA
+ the
R3
2
4
R6
C1
-Line
Figure 11. Fast startup function.
3
C7
1m
F
18
PBL 38582
1
Fast start - up
RECEIVE
Reference
17
Limiter
DC
supply
+ 4
D1
D5
D2
6
3
2
C4
15
5,14,16
47nF
D3
D4
Telephone
line
2
R1 6.2k
C3
0.1m
F
TRANSMIT
R14
10k
C6
R6
75W
>0.5W
R2 62k
R3
910 W
1
C1
68 m
F
C2 15nF
Figure 12. Typical insulated DECT-set line interface. DIP package.
8
ERICSSON [ ERICSSON ]
