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PBL388131N Datasheet(PDF) 9 Page - Ericsson |
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PBL388131N Datasheet(HTML) 9 Page - Ericsson |
9 / 16 page 9 PBL 388 13 Control F2 F3 F5 F4 F1 PBL 388 13 Power amplifier input P1 C 11 16 14 13 C C C R 7 68 9 CMP R5 C2 C1 C3 C4 + Transmitter channel output R C CTR 24 4 15 + 5 3 1 2 +L R + C R C R C C C + GND Receiver channel input +Rx in + N Det Tx Det Rx Det R Rxout + Txout Tx in + Mic. F6 Tx in Rx in 12 10 Ref. + + decreases with factor of 10, thus increasing the detectors dynamic range. See fig.10. Background Noise Detector The general function of the backgro- und noise detector in the transmitting channel is to create a positive signal ( in respect to the internal reference) so that, when coupled to the summing point at the CMP input, will counteract the continuous type signal from the transmitter level detector representing the actual sound pressure level at the microphone. This counteracts the noise from influencing the switching characteristics. The input signal to the back ground noise level detector is taken from the output of the transmitter detector, a voltage representing the envelope of the amplified microphone sig- nal. The detector inverts and amplifies this signal 2 x (transmitting mode) and has on it´s output a RC network consisting of an internal resistor of 100k and an external capacitor C4. The voltage across C4 is connected to the CMP input (summing point) via a resistor R5. The extent to which the NDet output will influence the potential at CMP input is set by the gain of the detector, the maximum swing and R5. If a continuous input signal is received from the microphone ( > 10sec.) the voltage across C4 is pulled positive (relative to the reference) with a time constant set by C4 to e.g. 5 sec. A continuous input signal is thus treated as noise. Since the output of the noise detector is going negative it thereby counteracts the signal from the transmitter detector and thus helping the receiver detector signal to maintain a set relation to the transmitter detector signal. If the transmitter input signal contains breaks like breath pauses the voltage at TxDetout decreases. If the voltage across C3 gets less than the inverted voltage across C4 divided by the detector gain a rapid charge of C4 towards reference will follow (all levels referred to the reference). If the breaks are frequent as in speech the background detector will not influence the switching characteristic of the system. See fig. 11. There is a threshold of approx. 50mV at TxDetout to prevent the CTR Input For full speech control (50dB attenuation between the channels) this in- put can be left unconnected. To set the function to 25dB attenuation the input has to be higher than 600mV below V+. See figure 15. To set the circuit into a mute state (results in, reduced gain in receiver channel for the DTMF confidence tone in the loudspeaker and closed transmitter channel) a voltage below Vref has to be connected to the input. By lowering the voltage at the input below 0.9V a condition will emerge where both receiver and trans- mitter channels are closed. See fig. 13 and 15. Figure 16. Speech switching arrangement. activation of background noise detection in noiseless environment. In the receiver mode some of the loudspeaker output signal will be sensed by the microphone. In order not to treat this input signal as noise, the noise detector goes into a hold state and ”remembers” the level from the previous transmitting mode periode. |
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