User Guide

NHM-8NX
System Module & UI PAMS Technical Documentation
Page 76 ãNokia Corporation Issue 1 05/02
The outputs from the individual PA chains is feed to individual couplers, that means one
for GSM and one for PCN, before entering the RX/TX switch. The harmonics from the PA
is filtered by filters located in the RX/TX switch.
The output sample from the directional coupler's is rectified by a dual shottky-diode,
with common output, and filtered before being fed to the error amplifier in Mjoelner
which again is controlling the gain of the individual PA chain relative to the TXC voltage.
Power control
The detected voltage is compared in the error-amplifier in Mjoelner to TXC- voltage,
which is generated by a DA-converter in BB. TXC has got a raised cosine form (cos
4
-
function), which reduces switching transients, when ramping power 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 named TXP to work
below detectable levels. Burst ramping is enabled and set to rise with TXP until the out-
put level is high enough for the feedback loop to work. The output of the loop error
amplifier controls the PA output level via the gain-control pin in the PA to the desired
output level and the burst has are thereby shaped by the waveform of the TXC-ramp.
Because feedback loops could be unstable, this loop is compensated with a dominating
pole. This pole decreases gain on higher frequencies to get phase margins high enough.
Also this pole filter out the noise which is coming from TXC line. Each band has its own
control line from the loop amplifier.
Figure 31: Power Loop
Synthesizer and RF Control
All control of the synthesizer, LNA, modulators on/of e.g. (all functionality that is
POWER AMPLIFIER
EGSM900
GSM1800
22k
4.7k
100p
10p
Detector diodes
Directional couplers
Ant.switch
VPD_900
VPD_1800
12k
8k2
10p
To mecanial att.sw.
12k
47R
TXP
Rfbl
TXC
DET
PLFB2
VPCL
VPCH
-
+
-
+
Mjoelner
RFIC
EGSM900
GSM1800
Cfbl
3k3
Vant_900
TXC
47R
47R
Vant_1800
5k6
10p
10n
10n
100p
47R
82k