The RF detect circuit is use d to detect the RF amplitud e level of the TX signal. RF detect reports back to U1800(DCI) pin 31, using a dc level, for amplitude stabi lization. The RF

detect circuit is RF coupled with the TX sign al from either band, 8 00MHz or 1900MHz. The RF input is then conver ted into a dc level an d sent to the RF_DETECT line. There are

two stages in the RF detect circuit which are used to increase the dynamic range of the RF detect output. The two stages are controlled by the TX_STEP line input. TX _STEP is

high when power steps 0 through 5 are used . TX_STEP is low when power steps 6 through 10 are used .

When TX_STEP is low Q503-A is switched on and Q503-B is swi tched off. Q501 inverts the state of Q503-B. This operation forward biases diode CR504. A t the same time it cuts

off the supply to diode CR502, th us not forward biasing CR502. Whe n TX_STEP is hig h Q503-A is switc hed off and Q503 -B is switched on. This operation cuts off the supply to

diode CR504. At the same time current is supplied to amplifier Q502 and CR502. During low power steps the signal is se nt directly to the RF _DET output through CR504. During

high power steps the signal passes through amplifier Q502 and then sent to the RF _DET output through CR502.

Proper PA loads need to be defined when op erating in 80 0MHz analog and 800MHz digit al mode. This is done by making Load B active or inactive on the PA output path. When

transmitting in 800MHz an alog mode, LOAD A is used and LOAD B is bypassed. LOAD B is bypassed by allowing CR8 22 to be forward biased. Forward biasing CR822 w ill pro-

vide a ground state at the anode side, thus allowing only LOAD A(C823) to be present.

CR822 is forward biase d by having a supply voltage present at the AD_SW line. When the A_D line is pulled d own, Q505 is switched on, forcing the base of Q821 low. PNP tran-

sistor Q821 will then b e switched on, allowin g the output(AD_SW) to be pulled high from SW _QMOD_KEY.

In 800MHz digital mode, CR821 i s not forward biased, thus having CR821 in an open state. Dur ing this state LOAD A and LOAD B are used for the PA load. To have CR822 in an

open state, AD_SW is not pulled high by SW_QMOD_KEY. The AD_SW state is determined by the A_D line . Having a high state at A_D would switch Q505 off, thus not switch-

ing Q821 oon.

RF Detect/A_D Switch

Q821

SW_QMOD_KEY

A_D

TL901

TL900

TX_1900

Load

SW_QMOD_KEY

CR822

Q505