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PBL3853 Datasheet(PDF) 6 Page - Ericsson |
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PBL3853 Datasheet(HTML) 6 Page - Ericsson |
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6 / 16 page  PBL 3853 6 Functional Description Design Procedure The first decision to make is, how much current is needed at what VDC and how much line current is available at longest line length. 1. Set the circuit impedance to the line, either active or passive. C 3 should be big enough to give low impedance compared with R1 in the telephone speech frequency band. Too large C 3 will make the ”start up” slow. 2. Set the DC-characteristic that is required in the PTT specification, or in case of a system telephone design, in the PABX specification (R5). 3. If the line length regulation (line loss compensation) is used, set the attac point where it should start (RC and RD). Using the line length regulation makes it in most cases easier to achieve the gain/line length mask in both transmitter and receive function. Note, that in some countries the line length regulation is not allowed. 4. Set the transmitter gain and frequency response. See text for the clipping feature. 5. Set the receiver gain and frequency response. 6. Adjust the side tone balancing network. 7. Apply the RFI suppression components in case necessary. In two piece telephones the often ”helically” wound cord acts as an aerial where especially the microphone input with its high gain and input impedance is the more sensitive. Figure 6. AC-impedance, to the line. Figure 7. Adjusting voltage level across the circuits. PBL 3853 4 I=0.3mA Circuit supply VF +Line R1 C3 R1 The voltage across the circuit can be increased by method shown above without influencing the impedance towards the line. C2 Impedance to the Line The AC-impedance to the line is set by R1 (+ R2 if active impedance is used) and C 2. See figure 4. The circuits relatively high parallel impedance will influence it to some extent. At low frequencies the influence of the C 3 can not be neglected. Series resistance of the C 3 that is dependent on temperature and quality will cause that some of the line signal will enter pin 4 and generate 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 C 2 that also acts as a RFI supressor. In many specifications the R1 is specified as a complex network. See figure 6 b) in the example. In case a) the error signal entering pin 4 is set by the ratio ≈ RS/R1 (909 Ω Swedish spec.), where in case b) the ratio at high frequency will be RS/220 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+820) and thus lessening the influence of the error signal. To save current the circuit can be implemented to have an active impedance to the line, the level is set by resistors R1 and R2. When an active impedance is used the transmitter (see figure 16) amplifier does not feel its own active output-impedance thus using less current to create output swing to the line. Case c) above can not be used together with active impedance. Do not use the active impedance if not necessary, it complicates things greatly. A full mathematical expression is found under Detailed Description. DC - Characteristics The DC - characteristic that a telephone set has to fulfill is mainly given by the network administrator. Following para- meters 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 Ω • The maximum current from the line at zero line length • The minimum current at which the telephone has to work (basic function) • The lowest and highest voltage across the telephone The DC-characteristic of the circuit is a function of the voltage on pin 4. There is also a possibility to adjust the DC- characteristic with resistors (dc-voltage) at pin 5 (RA and RB in figure 4). Note that altering the DC-characteristic slope will also influence the line length regulation (when used) and thus the gain of both transmitter and receiver. A closer mathematical study is done under Detailed Description. Line Length Regulation The line length regulation is to compensate the gain in both transmitter and receiver with changing line length (impedance). The dynamic range of regulation is ≈ 6 dB. The starting point of the regulation can be set by RC and RD that take the information from the circuits supply voltage which actually mirrors the line current value in voltage. In case line length regulation is not required it can be omitted either in the high or in the low gain mode (6 dB range of regulation). PBL3853 4 I=0.3mA VF +Line A complex network 220 Ω + 820Ω//115nF Example: b) R1 C3 Rs ≈1Ω a) C2 b) a) Real impedance b) Complex impedance The complex network should be connected to the speech circuit like shown in c). See text. c) Circuit supply |
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