User Guide

GM47/48 Design Guidelines
BA/SEM/MSC 02:0006 Rev PA1 30
Appendix III GSM transmit waveform characteristics
The simplified GSM loading characteristics and power supply reference
model in relation to the GM47 system connector is represented in
figure 1. J1 represents the interface to the GM47 system connector and
the voltage there is referenced Vcc.
Figure 2 shows a 1:8 cycle with the receive burst occurring in timeslot
1 and transmit in timeslot 6. Some of the quiet period is used by the
module to ‘listen’ to other GSM base-stations.
In simple terms the module can draw up to 2 Amps during the transmit
burst. The receive burst can draw up to 100mA. As the current
consumption of the receive burst does not normally cause problems for
power supply current drain, the simplified PSU model in figure 1
combines the receive, monitor and idle frames into an average current
consumption around 30mA. The transmit burst requires special
attention.
1 2 3 4 5 6 7 8 1 2
Receive Transmit Monitor Receive
577
µ
s
4.6 ms
timeslot
Figure 1 - Simplified GSM Timing Diagram
There are three main areas to consider in order to meet the transmit
burst requirements: PSU current capability, resistance between the
supply and module (R
LINE
), and bulk capacitance (C
BULK
) at the module
Vcc connection. The effect that each of these parameters has on the
waveform shape at Vcc is indicated in figure 3.