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PBL385731SOS Datasheet(PDF) 9 Page - Ericsson |
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PBL385731SOS Datasheet(HTML) 9 Page - Ericsson |
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9 / 15 page  9 PBL 385 73 Battery feed R15 R16 R6 Regulation: 48V, 2 • 200 Ω 450k ∞ 75 Ω ∞∞ 47 Ω 48V, 2 • 400 Ω ∞ ∞ 75 Ω 48V, 2 • 800 Ω Sweden apply for spec. application No regulation: All feedings: Set for low gain ∞ 22k 47 - 75 Ω Set for high gain 75k ∞ 47 - 75 Ω Figure 13. Gain regulation principle. What is balancing the side tone? To understand that side tone balancing is to counteract the signal, that is transmitted via the microphone and trans- mitter to the line, returning to the earphone via the receiver. That presence of a strong side tone signal is disturbing in a way that one quite instictively lowers ones own voice level thus lowering the signal level for the other party. But again, if the balance is too good (seldom the case) the earphone will feel ”dead”. In practical terms what is expected is the same amplitude of ones own voice in the ear as when not talking in a telephone. The need to lower the side tone level where no balancing has been done is in the order of 6 - 12 dB. To understand that the side tone is influenced by other factors like, the impedance of the line and the signal that enters the ear acoustically directly from the mouth and from the mouth through the material in the handset. The signal that enters the microphone from the earphone acoustically will also influence the return loss factor to the telephone line. To understand that the side tone network can be trimmed to form a veritable ”distortion analyser”, so that the distortion that is present from the microphone, will be the only signal entering the earphone and this signal even being small will sound very bad. It is better to induce some of the fundamental frequency back by making the balance less perfect at that frequency. This is valid for a network that is trimmed to only one frequency. It is to strive to trim the network such that it will attenuate the fun- damental and the harmonic frequencies alike throughout the different line combinations. To understand that if one of the two signals entering the balancing system from either direction, direct from microphone or via the line, is clipped, will result in a very distorted signal entering the receiver amplifier and thus the earphone. Further , to remember that side tone is a small signal that is the difference of two large signals and that the amplitude of the distortion can be up to ten times the amplitude of the fundamental frequency. Figure 14. The side tone suppression principle. A short guidance for under- standing the side tone principle. (See fig.14.) Assuming the line impedance to be 600 Ω. ( theorethical value ) Z1 = Line impedance Z2 = The telephone set impedance 600 Ω Z1//Z2 = 300 Ω R6 will have a certain value 39 - 100 Ω to give the telephone a specified DC- characteristic and overcurrent protection. Assuming that this DC-characteristic requires R6=60 Ω, hence it will be 1/5 of the Z1//Z2. This will in transmitting mode result that 1/5 of the ac-signal that is on the line appear across R6. dB I L High limit Low limit a. b. c. Z1 Z2 PBL 38 573 Tx Rx a.) b.) c.) Zbal Line side Telephone set side 1 2 12 13 14 R6 R7 R9 C5 R10 R12 R11 C6 R8 { |
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