PBL385701N データシートの表示（PDF） - Ericsson

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PBL385701N

Universal Speech Circuit

Ericsson  

PBL 385 70

Functional description

Design procedure; ref. to fig.4.

The design is made easier through that all settable parameters are returned to ground (-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 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). There are also internal circuit dependent requirements like supply voltages 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 endresult is better and more readily achieved by using

the line length regulation (line loss compensation) than without. See fig. 12.

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 design a good practice is to connect a 15V 1W

zener diode across the circuit , from pin 1 to -Line.

Impedance to the line

The AC- impedance to the line is set by

R3, C1 and C2. Fig.4. 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 temperature and quality will

cause some of the line signal to enter pin

4. This generates a closed loop in the

transmitter amplifier that 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/

R19 (909Ω), 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

complex network capacitor is connected

directly to ground (-line), case c). making

the ratio Rs/220Ω+820Ω and thus less-

ening the error signal. Conclusion: Connect

like in case c) when complex impedance

is specified.

PBL 385 70

3

C2

Figure 6. AC-impedence

1

4

2

R6

+Line

a)

b)

c)

R3

Rs

≈1Ω

+

C1

Example:

How to connect a

complex network.

-Line

5

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