User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 84
Issue 1 07/99
After the TX/RX switch the signal is fed to the diplexer. The TX/RX switch
is set to transmit position with BAND_SEL = low and VTX = high. There is
a directional coupler connected between the PA output and the input of
the TX/RX switch to provide feedback for the power loop.
Transmitter Power Control for GSM900 and GSM1800
The power control circuit consists of the gain control stage of the PA, a
power detector at the PA output and an error amplifier in the SUMMA.
There is a directional coupler connected after the PA output in both
chains, but the power sensing line and detector are common for both
bands. The GSM900 feedback signal is attenuated to the same level as
the GSM1800 feedback signal. The combining of the two feedback signals
is achieved with a diplexer. A sample is taken from the forward going
power. This signal is rectified with a schottky-diode and after RC-filtering a
DC-voltage is available. The DC–voltage reflects the output power. This
power detector is linear on absolute scale, with the exception that it
saturates on very low and high power levels, i.e. it forms an S-shaped
curve.
The detected voltage is compared in the error-amplifier in the SUMMA to
the TX power control voltage (TXC), which is generated by the
DA-converter in the COBBA. The output of the error amplifier is fed to the
gain control input of the PA. Because the gain control characteristics in the
PA are linear in absolute scale, the control loop defines a voltage loop,
when closed. The closed loop tracks the TXC-voltage. The shape of the
TXC–voltage as function of time has a raised cosine form (cos
4 - function).
This shape reduces the switching transients, when the power is pulsed up
and down.
Because the dynamic range of the detector is not wide enough to control
the power (actually RF output voltage) over the whole range, there is a
control signal named TXP (TX power enable) to work under detected
levels. The burst is enabled and set to rise with TXP until the output level
is high enough for the feedback loop to work. The loop controls the output
power via the control pin on the PA to the desired output level.
Because the feedback loop could be unstable, it is compensated with a
dominating pole. This pole decreases the gain on higher frequencies to
get the phase margins high enough.