PBL 385 82
Note that the signals at points a. and b. are
180 degrees off phase.
lowsignaltobalancethesidetonewithand
make the balancing difficult. Making any of
the impedances unnecessary high will
makethecircuitsensitivetoRFI. Allvalues
given here are approximate and serve as
starting entities only. The finaltrimming
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).Caremustbetakenwhen 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.Thiscircuitcannotretriggerbefore
the voltage level at C1 drops below 2V or
the line voltage is below 1V. See fig. 10.
10 x R6 ≈ R7 + Zbal
R7 ≈ Zbal
Note #1
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).
Hence10xR1≈R2 tosatisfythebalancing
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
influenceis1dB, andwithratio1/20it´s0.5
dB.
+Line
1
PBL 38 5 82
Start up circuit
Tx
DC supply
R3
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
thyristorfunctionbetweenpins1and4that
will stop conducting when the current
drainat pin 4 is lower than ≈ 700 µA + the
2
4
R6
Note #2 If the R7 is made low ohmic
compared with Zbal, it will load the latter
C1
-Line
and result in
a bad side tone
perfomannce, again if the R7 is made high
ohmic compared with Zbal will result in a
Figure 11. Fast startup function.
3
1
PBL 38582
C7
1
mF
Fast start - up
1 8
1 7
RECEIVE
DC
Reference
D1
D2
supply
Limiter
D5
D3
D4
2
3
+
4
14,16
5,
1 5
6
Telephone
line
C4
47nF
2
R1 6.2k
R2 62k
1
R3
910 W
C3
0.1mF
R6
75W
>0.5W
R14
10k
TRANSMIT
C2 15nF
C1
68 mF
C6
Figure 12. Typical insulated DECT-set line interface. DIP package.
8
ERICSSON [ ERICSSON ]
