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PBL386112SHS Datasheet(PDF) 11 Page - Ericsson |
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PBL386112SHS Datasheet(HTML) 11 Page - Ericsson |
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11 / 18 page  PBL 386 11/2 11 Figure 10. Hybrid function. VT Combination CODEC/Filter RTX RFB ZB ZRX ZT VTX RSN VRX PBL 386 11/2 Hybrid Function The hybrid function can easily be imple- mented utilizing the uncommitted amplifier in conventional CODEC/filter combinations. Please, refer to figure 10. Via impedance Z B a current proportional to VRX is injected into the summing node of the combination CODEC/filter amplifier. As can be seen from the expression for the four-wire to four-wire gain a voltage proportional to V RX is returned to V TX. This voltage is converted by R TX to a current flowing into the same summing node. These currents can be made to cancel by letting: V TX + V RX = 0 (E L = 0) R TX Z B The four-wire to four-wire gain, G 4-4, in- cludes the required phase shift and thus the balance network Z B can be calculated from: Z B = - RTX • V RX = V TX Z T - G 2-4S • ( ZL + 2RF) - R TX • Z RX • α RSN Z T G 2-4S • ( ZL + 2RF) When choosing R TX, make sure the output load of the VTX terminal is (R TX//RT in figure 13) > 20 k Ω. If calculation of the Z B formula above yields a balance network containing an inductor, an alternate method is recom- mended. Contact Ericsson Microelectron- ics for assistance. The PBL 386 11/2 SLIC may also be used together with programmable CODEC/ filters. The programmable CODEC/filter allows for system controller adjustment of hybrid balance to accommodate different line impedances without change of hard- ware. In addition, the transmit and receive gain may be adjusted. Please, refer to the programmable CODEC/filter data sheets for design information. Longitudinal Impedance A feed back loop counteracts longitudinal voltages at the two-wire port by injecting longitudinal currents in opposing phase. Thus longitudinal disturbances will ap- pear as longitudinal currents and the TIPX and RINGX terminals will experience very small longitudinal voltage excursions, leav- ing metallic voltages well within the SLIC common mode range. The SLIC longitudinal impedance per wire, Z LoT and ZLoR, appears as typically 20 Ω to longitudinal disturbances. It should be not- ed that longitudinal currents may exceed the dc loop current without disturbing the vf transmission. Capacitors C TC and CRC If RFI filtering is needed, the capacitors designated C TC and CRC in figure 13, con- nected between TIPX and ground as well as between RINGX and ground, may be mounted. C TC and CRC work as RFI filters in con- junction with suitable series impedances (i.e. resistances, inductances). Resistors R F1 and RF2 may be sufficient, but series inductances can be added to form a sec- ond order filter. Current-compensated in- ductors are suitable since they suppress common-mode signals with minimum influ- ence on return loss. Recommended values for C TC and CRC are below 1 nF. Lower values impose smaller degradation on re- turn loss and longitudinal balance, but also attenuate radio frequencies to a smaller extent. The influence on the impedance loop must also be taken into consideration when programming the CODEC. C TC and C RC contribute to a metallic impedance of 1/( π•f•C TC) = 1/(π•f•CRC), a TIPX to ground impedance of 1/(2• π•f•C TC) and a RINGX to ground impedance of 1/(2• π•f•C RC). AC - DC Separation Capacitor, C HP The high pass filter capacitor connected between terminals HP and RINGX pro- vides the separation of the ac and dc signals. C HP positions the low end frequen- cy response break point of the ac loop in the SLIC. Refer to table 1 for recommended value of C HP. Example: A C HP value of 68 nF will position the low end frequency response 3dB break point of the ac loop at 13 Hz (f 3dB) according to f 3dB = 1/(2•π•RHP•CHP) where R HP = 180 k Ω. |
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