PBL38541/1SO データシートの表示（PDF） - Ericsson

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PBL38541/1SO

Universal Speech Circuit

Ericsson  

PBL 385 41

Functional description

Design procedure; ref. to fig.4.

+Line

The design is made easier through that all

settable parameters are returned to gro-

und (-line), this feature differs it from bridge

type solutions.To set the parameters in the

following order will result in that the

interaction between the same is minimized.

1. Set the circuit impedance to the line,

either resistive (600Ω) or complex. (R3

and C1). C1 should be big enough to give

low impedance compared with R3 in the

telephone speech frequency band.Too

large C1 will make the start-up slow. See

fig. 6.

2. Set the DC-characteristic that is

required in the PTT specification or in case

of a system telephone,in the PBX

specification(R6). Observe the power

dissipated.There are also internal circuit

dependent requirements like supply volta-

ges etc.

3. Set the attac point where the line

length regulation is supposed to cut in

(R1 and R2). Note that in some countries

the line length regulation is not allowed. In

most cases the end result is better and

more readily achieved by using the line

length regulation (line loss compensation)

than without. See fig. 13.

4. Set the transmitter gain and

frequency response.

5. Set the receiver gain and frequency

response. See text how to limit the max.

swing to the earphone.

6. Adjust the side tone balancing

network.

7. Set the RFI suppression

components in case necessary. In two

piece telephones the often ”helically”

wound cord acts as an aerial. The

microphone input with its high gain is

especially sensitive.

8. Circuit protection. Apart from any

other protection devices used in the de-

sign a good practice is to connect a 15V

1W zener diode across the circuit , from

pin 1 to -Line.

PBL 385 41

1

4

3

C2

2

R6

Figure 6. AC-impedance.

Impedance to the line

The AC- impedance to the line is

set by R3, C1 and C2. Fig.6. The circuits

relatively high parallel impedance will not

influence it to any noticeable extent. At low

frequencies the influence of C1 can not be

neglected. Series resistance of C1 that is

dependent on the temperature and the

quality of the component will cause some

of the line signal to enter pin 4. This

generates a closed loop in the transmitter

amplifier that in it´s turn will create an

active impedance thus lowering the

impedance to the line. The impedance at

high frequencies is set by C2 that also

acts as a RFI suppressor.

In many specifications the

impedance towards the line is specified as

a complex network. See fig. 6. In case a).

the error signal entering pin 4 is set by the

ratio ≈Rs/R3 (910Ω), where in case b). the

ratio at high frequencies will be Rs/220Ω

because the 820Ω resistor is bypassed by

a capacitor. To help up this situation the

1

+

3

AM

2

AT

4

Transmitter summing

input

Mute

Figure 7. Block connections.

+ Line

AR

- Line

a)

b)

R3

c)

220Ω

Cx

820Ω

Rs

≈1Ω

+

C1

Example:

How to connect a

complex network.

220Ω+820Ω//Cx

-Line

complex network capacitor is connected

directly to ground, case c). making the ratio

Rs/220Ω+820Ω and thus lessening the

error signal. Conclusion: Connect like in

case c) when complex impedance is

specified.

DC - characteristic

The DC - characteristic that a

telephone set has to fulfill is mainly given

by the network administrator. Following

parameters are useful to know when the

DC behaviour of the telephone is to be set:

• The voltage of the feeding system

• The line feeding resistance 2 x.......

ohms.

• The maximum current from the line at

zero line length.

• The min. current at which the

telephone has to work (basic

function).

• The lowest and highest voltage

permissible across the telephone set.

• The highest voltage that the telephone

may have at different line currents.

Normally set by the network owners

specification.The lowest voltage for the

telephone is normally set by the volta-

ges that are needed for the different

parts of the telephone to function. For

ex. for transmitter output amplifier,

receiver output amplifier, dialler,

speech switching and loudspeaker

amplifier in a handsfree telephone etc.

5

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