User Guide

PAMS
Technical Documentation
NSB–1
System Module
Original 06/98
Page 3 – 32
Power Detection Circuit
The power detector gives an indication of output RF power by rectifying
the RF voltage to a DC voltage. Ideally the output voltage of this peak
envelope detector is the peak value of the RF voltage but in real world the
output voltage is somewhat smaller depending on the quality of the detec-
tor diode. Due to low supply voltage used in the phone the maximum en-
velope voltage of the detector is limited to about 1.5V. Over 30 dB power
range this would yield a very low voltage at the lowest power level. The
problem is circumvented by having a controllable attenuator limiting the
detector input power at high power levels. A voltage doubling detector is
being used to further increase the envelope voltage without need to in-
crease the coupler coupling factor and antenna path losses excessively.
RF part of the power detector consists of schottky diode V501, bias resis-
tors R531, 532, 502, capacitors C502, 535, 500, and 549. The bias volt-
age at diode output varies considerably with temperature. To eliminate
this variation the detector output is coupled to the error amplifier through
capacitor C520. Before transmission and between each burst the output
end of the capacitor is connected to a stable reference potential with FET
V506. The detector reference potential is formed from the regulated 2.8V
supply with resistors R515 and 516. The FET is controlled according to
VTX voltage by transistor V507. When VTX is down the FET is closed
and C520 is charged with the potential difference between the detector
bias potential and the detector reference voltage. Upon rise of VTX the
FET is opened and the output end of C520 is allowed to follow the RF en-
velope voltage from the detector.
The detector reference voltage is about 0.5V. The bias voltage at the
diode output is set 0.2 – 0.3V below the reference voltage at room tem-
perature in order to avoid reverse voltage across tantalum capacitor
C520 in cold temperature. A relatively large value of 4u7 was chosen for
C520 for the case that the error amplifier gain would need to be limited
with feedback resistors which would cause a current flowing through
C520 changing the potential across it during the TX burst. A lower value
will suffice if the error amplifier doesn’t need current sourcing or sinking at
the input.