PBL38582 查看數據表（PDF） - Ericsson

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PBL38582

Telephone Line interface circuit for DECT, DAM, CT Unisolated or Isolated

Ericsson  

PBL 385 82

Functional description

Design procedure; ref. to fig.4.

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 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. 10.

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 ( if used ) is supposed to

cut in. 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.

4. Set the transmitter gain and

frequency response.

5. Set the receiver gain and frequency

response. See text how to limit the max.

swing.

6. Adjust the side tone balancing

network if used.The network in most cases

is just a coarse resistive divider to take

care of the first order of balancing. The fine

balancing is done by the DSP in the sys-

tem.

7. Set the RFI suppression

components in case necessary.

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.

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 the line impedance to any

noticeable extent.At low frequencies the

influence of C1 can not be neglected.

Series resistance of C1 that

PBL 38 582

a)

1

4

+Line

b)

R3

820Ω

c)

220Ω

C

3

C2

2

R6

Rs

≈1Ω

+

C1

Example:

How to connect a

complex network.

220Ω+820Ω//C

-Line

Figure 6. AC-impedance.

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 (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, 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 voltages that are

needed for the different parts of the

telephone to function. For ex. for

transmitter output amplifier, recei-

ver output amplifier, dialler, speech

switching. R6 will set the slope of the

DC-char. and the rest of the level is

set by some constants in the circuit

as shown in the equation below. The

slope of the DC-char. will also

influence the line length regulation

(when used ) and thus the gain of

both transmitter and receiver. See

the table under gain regulation. R6

also acts as power protection for the

circuit, this must be kept in mind

when low values of R6 are conside-

red. See fig. 7.

VLine ≈ 2 + 1. 5 ⋅R 6 ⋅Iline

Vtelephoneline ≈ 1. 5V + Vline

5

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