8

PBL 385 82

Figure 11. Fast startup function.

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.

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.

Start up circuit

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

3

1

4

+

2

Telephone

line

1

2

14,16

15

18

17

RECEIVE

TRANSMIT

C7

1mF

C3

0.1mF

R6

75W

>0.5W

R1 6.2k

R2 62k

C4

47nF

68 mF

910 W

C2 15nF

D1

D2

D3

D4

D5

PBL 38582

Reference

Limiter

DC

supply

Fast start - up

R14

10k

6

2

3

5,

R3

C1

C6

Figure 12. Typical insulated DECT-set line interface. DIP package.

PBL 38 5 82

Tx

1

2

DC supply

+Line

-Line

C1

R3

R6

4