NEC DB2000 Service Manual

Contents

NEC DB2000 Service Manual Manual pdf 24 page image

Part 3 : The P.A. Circuit and its ControlFig.8: The Power Amp CircuitThe P.A Circuit and its controlThe power amplifier control circuit ensures that the RF signal is regulated to therequired limits of operation.The TX RF signal is input on #1(IC106) or (IC107) and the power amplifier outputsthe signal at #4.IC105 and IC108 are op-amps, which output a control signal to the P.A.Power control is carried out by measuring the current drawn by the P.A (see Fig.9:P.A Circuit Control). The current drawn by the P.A. is directly proportional to theoutput power and is measured as a voltage drop across a known resistance pathor “Fixed Control Loop Stripline”. This consists of a 47mOhm resistor (R144) forGSM or two resistors in series, a 47mOhm (R144) and 27mOhm (R136) for PCN.The voltage drop (V_ERROR) is then integrated by the operational amplifier(IC108) to give an error voltage, which regulates the TXP signal.The TXP input signal to the power control circuit is generated by the IC27 #90. TXPprovides a limit for the frequency spectrum caused by burst modulation.BPFGSM = (890-915MHz)PCN = (1710-1785MHz)PA(FL106) (IC106) (IC103)BPFPA(IC104)(FL105)PCN_PRE_ONGSM_PRE_ONmechswitchTo AntennaDiplexor(IC105)(IC108)TXP_MODV_ERRORV_ERROR(IC107)From IC101TXP_MODFrom IC102(FL103)