User Guide

System Module LG4 and Grip Module LS4 CCS Technical Documentation
Page 48 ¤Nokia Corporation Issue 2 11/02
for the RC-network comes from signaling, i.e. how often PSW slots occur. They are
repeated after 10 frames. The AFC tracks the base station frequency continuously which
enables the transceiver to have a stable frequency reference.
The settling time requirement is also determined from the allowed start up-time. When
the transceiver is in sleep mode and ”wakes-up" to receive mode, there is only about 5
ms for the AFC voltage to settle. When the first burst is received, the system clock has to
be settled into r 0.1 PPM frequency accuracy. The VCTCXO module also requires 5 ms to
settle into the final frequency. The amplitude rises into full swing in 1 to 2 ms, but the
frequency settling time is higher so this oscillator must be powered up early enough.
DC-compensation
DC compensation is made during DCN1 and DCN2 operations (controlled via serial bus).
DCN1 is carried out by charging the large external capacitors in AGC stages to a voltage
which cause a zero dc-offset. DCN2 set the signal offset to constant value (RXREF 1.35
V). The RXREF signal is used as a zero level to RX ADCs.
Power control with analog temperature compensation scheme
The detected voltage level is compared by the HAGAR internal error-amplifier to the cur-
rent TXC voltage level, which is generated by a DAC in BB. The TXC line is a so-called
'raised cosine' shaped function, the effect of which is to minimize the switching tran-
sients during the power ramp/decay phase. Because the dynamic range of the detector is
not wide enough to control the power ( more precisely, RF output voltage ) over the
whole range, there is an additional control line named TXP to work below detectable lev-
els. The burst is enabled and set to rise with TXP until such time as the output level is
high enough for feedback loop to kick-in.
The feedback loop controls the output level via a control pin in PA to the desired output
level and burst has got the waveform of TXC-ramps. Because feedback loops could be
unstable, this loop is compensated with a dominating pole. This pole serves firstly to
decrease gain of the error amplifier at higher frequencies which in turn increases the
phase margin ( stability ). Secondly, it also provides for noise filtering on the TXC line.
Before power ramp the temperature information from detector is stored to C
temp
. This
temperature information is used during the burst to compensate power levels at differ-
ent temperatures. The TXP signal enables the antenna switch module to TX mode. There
are two separate power control loops in HAGAR, one for E-GSM900 and the other
GSM1800.