Nokia Mobile Phones NSE-5 Series Service Manual page 75

PAMS
Technical Documentation
TXC–voltage as function of time has a raised cosine form (cos
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.
AGC
The purpose of the AGC-amplifier is to maintain a constant output level
from the receiver. To accomplish this, pre-monitoring is used. This
pre-monitoring is done in three phases and determines the settling time
for the RX AGC. The receiver is switched on approximately 150 s before
the burst begins and DSP measures the received signal level. The DSP
then adjusts the AGC-DAC in accordance with the measured signal level
and/or switches on/off the LNA with the front–end amplifier control line
(FRACTRL). The AGC amplifier has a 57 dB continuos controllable gain
(–17 dB to 40 dB) while the gain control of the LNA has two steps. That is
the gain in the LNA is either –16 dB or 15 dB.
The requirement for the received signal level under static conditions is that
the MS shall measure and report to the BS over the range -110 dBm to
-48 dBm. For RF levels above -48 dBm, the MS must report the same
signal level to the BS. Because of those requirements, the LNA is turned
"ON" (FRACTRL = "0") for received levels below -48 dBm. This leaves the
AGC in the SUMMA to adjust the gain to desired output value (56mVpp).
This is accomplished in DSP by measuring the received IQ level after the
selectivity filtering (IF-filters, Σ∆ converter and FIR-filter in DSP). For RF
levels below -94 dBm, the output level of the receiver drops dB by dB with
a level of 9 mVp-p @-110 dBm for GSM900 and 7.1 mVp-p @ -110 dBm
for GSM1800.
This strategy is chosen as a compromise between avoiding saturation
when strong interfering signals are present and not sacrificing the signal to
noise ratio. The 56 mVpp target level is set, because the RX-DAC in the
COBBA in baseband will saturate at 1.4 Vpp. This results in a headroom
of 28 dB which is sufficient for the +/- 200 kHz faded adjacent channel
(approximately 19 dB) and an extra 9 dB for pre-monitoring.
Issue 1 07/99
NSE–5
System Module
- function).
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