PBL38573 データシートの表示（PDF） - Ericsson

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PBL38573

Speech Circuit for constant current feeding systems

Ericsson  

PBL 385 73

Battery feed

R15

R16

R6

Regulation:

{ 48V, 2 • 200Ω

450k

∞

75Ω

∞

∞

47Ω

48V, 2 • 400Ω

∞

∞

75Ω

48V, 2 • 800Ω Sweden apply for spec. application

dB

c.

b.

a.

High limit

No regulation:

All feedings:

Low limit

Set for low gain

∞

22k

47 - 75Ω

Set for high gain

75k

∞

47 - 75Ω

IL

Figure 13. Gain regulation principle.

What is balancing the side

tone?

To understand that side tone balancing

is to counteract the signal, that is

transmitted via the microphone and trans-

mitter to the line, returning to the earphone

via the receiver.

That presence of a strong side tone

signal is disturbing in a way that one quite

instictively lowers ones own voice level

thus lowering the signal level for the other

party. But again, if the balance is too good

(seldom the case) the earphone will feel

”dead”. In practical terms what is expected

is the same amplitude of ones own voice

in the ear as when not talking in a telephone.

The need to lower the side tone level

where no balancing has been done is in

the order of 6 - 12 dB.

To understand that the side tone is

influenced by other factors like, the

impedance of the line and the signal that

enters the ear acoustically directly from the

mouth and from the mouth through the

material in the handset. The signal that

enters the microphone from the earphone

acoustically will also influence the return

loss factor to the telephone line.

To understand that the side tone network

can be trimmed to form a veritable

”distortion analyser”, so that the distortion

that is present from the microphone, will be

the only signal entering the earphone and

this signal even being small will sound very

bad. It is better to induce some of the

fundamental frequency back by making

the balance less perfect at that frequency.

This is valid for a network that is trimmed to

only one frequency. It is to strive to trim the

network such that it will attenuate the fun-

damental and the harmonic frequencies

alike throughout the different line

combinations.

To understand that if one of the two

signals entering the balancing system

from either direction, direct from

microphone or via the line, is clipped, will

result in a very distorted signal entering the

receiver amplifier and thus the earphone.

Further , to remember that side tone is a

small signal that is the difference of two

large signals and that the amplitude of the

distortion can be up to ten times the

amplitude of the fundamental frequency.

a.)

1

Tx

PBL 38 573

13

Rx

2

12

14

b.)

R7 c.) R10

C6

R11

R8

R6

Zbal R12

C5

R9

Figure 14. The side tone suppression principle.

Telephone

set side

Line side

Z2

Z1

A short guidance for under-

standing the side tone

principle. (See fig.14.)

Assuming the line impedance to be 600Ω.

( theorethical value )

Z1 = Line impedance

Z2 = The telephone set impedance 600Ω

Z1//Z2 = 300Ω

R6 will have a certain value 39 - 100Ω to

give the telephone a specified DC-

characteristic and overcurrent protection.

Assuming that this DC-characteristic

requires R6=60Ω, hence it will be 1/5 of

the Z1//Z2. This will in transmitting mode

result that 1/5 of the ac-signal that is on the

line appear across R6.

9

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