LMH2120 Datasheet, PDF (26/34 Pages) National Semiconductor (TI) – Linear RMS power detector particularly suited for accurate

LMH2120
SNWS021C – JULY 2010 – REVISED FEBRUARY 2013
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2.0
1.5
1.0
85°C
0.5
0.0
-0.5
-40°C
-1.0
-1.5
-2.0
-50 -40 -30 -20 -10
0
RF INPUT POWER (dBm)
Figure 65. 10 dB Step Error vs.
RF Input Power at 1900 MHz
Variation due to Modulation
RMS power detectors, such as the LMH2120 are inherently insensitive to different modulation schemes. This in
contrast to traditional detectors like peak detectors and LOG AMP detectors, where modulation forms with high
peak-to-average ratios (PAR) can cause significant output variation. This is because the measurement of these
detectors is not an actual RMS measurement and is therefore waveform dependent.
To be able to compare the various detector types on modulation sensitivity, the variation due to modulation
parameter is used. To calculate the variation due to modulation (E_MOD), the measurement result for an
unmodulated RF carrier is subtracted from the measurement result for a modulated RF carrier. The calculations
are similar to those for variation over temperature. The variation due to modulation can be calculated by:
E_MOD = (V_{OUT_MOD} - V_{OUT_CW}) / K_SLOPE
(17)
where V_{OUT_MOD} is the measured output voltage for an applied power level of a modulated signal, V_{OUT_CW} is the
output voltage as a result of an applied un-modulated signal having the same power level.
Figure 66 shows the variation due to modulation for W-CDMA, where a E_MOD of 0.16 dB is obtained for a
dynamic range from -34 dBm to -2 dBm.
1.5
1.0
W-CDMA, REL8
0.5
0.0
W-CDMA, REL6
-0.5
W-CDMA, REL7
-1.0
-1.5
-50 -40 -30 -20 -10 0 10
RF INPUT POWER (dBm)
Figure 66. Variation due to Modulation for W-CDMA at 1900 MHz
26
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