Figure 127 Power control signal usage in GSM (GMSK) and EDGE (8PSK) transmission
Note:
Timings are not shown accurately in the previous figure.
GSM PA module
A single GSM/EDGE PA module contains two separate amplifier chains, one for EGSM900 (and GSM850) and
another for GSM1800/1900. Both amplifiers have a battery supply connection and two bias current inputs. The
bias current for final amplifier stage is adjusted according the power level in use in order to optimise efficiency.
The bias currents are also used as on/off switching signals for PAs.
In the EDGE mode, PA linearity has to be higher than in GMSK mode because of envelope variations of the 8-PSK
modulations. This is achieved by increasing the bias currents compared to the GMSK mode and setting a
dedicated Vmode control signal up. Increasing bias currents improves the linearity of the amplifiers, but it also
tends to unnecessarily increase the gain of the PA. Vmode control aims to keep the gain of the amplifiers down.
The bias current needed for the maximum and the lowest output powers is specified by a PA manufacturer.
The current for the intermediate power levels is then linearly adjusted between these two values.
Frequency synthesizers
RF has separate synthesizers for Rx and Tx. Both synthesizers consist of:
• PLL
• loop filter
• VCO
• balun
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