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PBL38541 Datasheet(PDF) 5 Page - Ericsson |
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PBL38541 Datasheet(HTML) 5 Page - Ericsson |
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5 / 14 page  5 PBL 385 41 1 2 +Line R3 R6 PBL 385 41 + 3 C1 C2 -Line Rs ≈1Ω How to connect a complex network. 220 Ω+820Ω//Cx Example: a) b) c) 4 Cx 220 Ω 820 Ω Figure 7. Block connections. Figure 6. AC-impedance. 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 resistive (600 Ω) 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. 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/220 Ω because the 820 Ω 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/220 Ω+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. + 1 AM AT AR 2 Transmitter summing input Mute + Line - Line 3 4 |
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