Thermistor; Umts Transmit Power Detector - LG U8290 Service Manual

3. TECHNICAL BRIEF
Figure 3.5.3-1 UMTS PA functional block diagram
An isolator between the Power Amplifier and the duplexer is highly recommended to provide constant
load and source impedances (respectively) to those devices. This improves PA stability, ACPR
performance, and harmonic suppression, as well as duplexer isolation, insertion loss, and ripple.
Transmitter performance is always improved when an isolator is included and is well worth the added
cost.
3.5.4 Thermistor (RT200 : 68K_2012_10%)
This thermistor senses temperature variations around UMTS PA to adjust PA gain deviation for assure
compliance with the applicable transmit power control standards. Negative temperature compensation
thermistor is used in the U8290.
3.5.5 UMTS transmit power detector (U203 : LMV225TLX)
This detector couples PA output power level to calibrate the transmitter characteristic over the channel
variation and temperature. Its detector coupling range and converted voltage is based on diode
sensitivity and transmitter power level.
The U8290 uses National Semiconductor LMV225TLX power detector IC. In Figure 3.5.5-1, R1 is set
to 1.8 resulting in an attenuation of 31.4dB. The output voltage is proportional to the logarithm of the
input power. Figure 3.5.5-2 shows the output voltage versus PA output power of the LMV225TLX
setup as depicted in Figure 3.5.5-1
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