Nokia NSB-5 Series Technical Documentation Manual page 58

NSB-5
System Module
at a moment when no RF power is being transmitted. This measured voltage is con-
verted into a digital signal by an A/D converter where it is used by DSP as part of the
control voltage. Ideally the control voltage is formed as a sum of exactly the same com-
ponents as the output voltage of the detector, the rectified voltage and the bias voltage.
The rectified voltage component sets the output power and should obey the peak enve-
lope sensitivity curve of the detector diode offset with the coupling factor of the direc-
tional coupler. The bias voltage is measured and updated in the control voltage often
enough so that no remarkable temperature drift has time to occur. The bias voltage must
be measured before the first burst of the transmission period. The detector diode is
located close to the receiver so that the bias voltage measurement can also be used to
indicate the receiver temperature as well if needed (RSSI correction).
The third voltage component affecting the operation of the power control loop in addi-
tion to the rectified RF and bias voltages is the offset voltage of the error amplifier. An
operational amplifier is integrated in SUMMA and is used as the error amplifier. The
input offset voltage should remain relatively stable with temperature but the variation
from device to device can be several tens of millivolts.
Therefore the offset voltage must to be taken into account when tuning the power con-
trol loop in operation. This means adding or subtracting an offset correction to the power
control voltage. A fixed correction will probably suffice, although the input offset volt-
age is actually dependent on the common mode input voltage of the loop amplifier. The
value of the offset correction should then be defined at a low power control voltage
where the error due to the offset voltage is the most significant.
The power control voltage has the following formula:
U = U + k * U
txc rf
U = power control voltage
txc
U = RF output level setting voltage
rf
k = constant
U = bias voltage at the output of the detector
bias
U = correction voltage due to loop amplifier input offset.
offset
The RF output level setting U
the applied power level. The voltages at each power level can be predetermined if the
variation between the individual detector diodes is not too large. If the peak envelope
sensitivity of the detector varies considerably with temperature a temperature depen-
dent correction must to be added to the value of U
be obtained from the detector output bias voltage measurement.
The constant coefficient k is needed to compensate the voltage division from the output
of the COBBA D/A converter to the input of the loop amplifier. This is due to output/input
resistances of the devices. A proper selection of k also reduces the error due to detector
peak envelope sensitivity variation with temperature. The value of k is likely to be slightly
above 1.
The bias voltage U
Page 58
+ U , where
bias offset
has values approximately from 20mV to 2V according to
rf
at the output of the detector is measured with an A/D converter
bias
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PAMS Technical Documentation
. An indication of temperature can
rf
Issue 1 03/01