User Guide

PAMS Technical Documentation System Module
NSB-5
Issue 1 03/01 Nokia Mobile Phones Ltd. Page 57
After filtering, the signal goes to the final amplifier, which is a MMIC PA (N500) with an
input impedance of 50 ohms. The MMIC contains three amplifier stages with interstage
matching. The first amplifier stage is variable and is control by the TX power control cir-
cuitry. An external driver is required to supply the necessary current to the TX power
control circuitry. The PA has over 45 dB power gain and is capable of producing an out-
put of 32.5 dBm with an input of 0 dBm. Harmonics generated by the nonlinear PA
(class AB) are attenuated with the output external matching net work and the low-
pass/bandstop filtering in the duplexer (Z502).
Power control circuitry consists of a power detector, an error amplifier in SUMMA and
the A/D converter in CCONT (N100). The directional coupler is situated between the
power amplifier and duplex filter. The power detector is a combination of a directional
coupler and a diode rectifier. The directional coupler converts the forward going power
with a certain ratio into a signal which is rectified by a schottky diode and a filter to cre-
ate a DC voltage. This DC voltage is fed to
1. A/D converter in CCONT which holds a sample of the detector output (no RF signal);
then MCU/DSP sets the TXC voltage accordingly for the following burst.
2. The error amplifier in SUMMA
The error amplifier in SUMMA compares the detected voltage and the TXC voltage, which
is generated by a D/A converter in COBBA_GJ. This creates a closed control loop and
since the gain control characteristics of the PA are linear in the absolute scale, the out-
put burst of the PA tracks the TXC voltage linearity.
Power Detection Circuit
The power detector gives an indication of output RF power by rectifying the RF voltage
to a DC voltage. Ideally the output voltage of this peak envelope detector is the peak
value of the RF voltage but in real world the output voltage is somewhat smaller
depending on the quality of the detector diode.
A bias current is driven through the detector diode, which causes an additional voltage
component to the output of the detector. The output voltage is then a sum of the recti-
fied voltage and the bias voltage. This bias voltage is a function of biasing resistors, sup-
ply voltage and the voltage knee of the diode. At small RF power levels the rectified
voltage can be only a few millivolts/dB which means that all other voltage components
should remain very stable to achieve a reliable indication of the output power.
However, the variation of the knee voltage of the diode alone causes more than 100 mV
variation in the output voltage over the specified temperature range. Furthermore, the
temperature variation varies the rectifying sensitivity of the detector diode but this
effect is less significant. With a simple passive bias network, the bias current of the diode
will also change with temperature and this effect can be used to partially cancel the
variation of the sensitivity.
In order to avoid the bias voltage variation ruining the accuracy of the power control
loop, the bias voltage of the detector has to be monitored and included in the power
control voltage (TXC), which determines the output power. The detector bias voltage
monitoring is accomplished by periodically measuring the output voltage of the detector