LMH2110 Datasheet, PDF (26/33 Pages) National Semiconductor (TI) – 8 GHz Logarithmic RMS Power Detector with 45 dB Dynamic Range

LMH2110
SNWS022C – JANUARY 2010 – REVISED MARCH 2013
V2
V1
25°C response
Temp (T)
response
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RF_IN (dBm)
P P+10 dB
P_T
P_T+X
Figure 56. Graphical Representation of 10 dB Step calculations
Figure 57 shows the typical 10 dB step error at 1900 MHz, where a dynamic range of 30 dB is obtained for E_10dB
= ±1 dB.
2.0
1.5
1.0
85°C
0.5
0.0
-0.5
-40°C
-1.0
-1.5
-2.0
-40 -35 -30 -25 -20 -15 -10 -5 0
RF INPUT POWER (dBm)
Figure 57. 10 dB Step Error vs. RF Input Power at 1900 MHz
Variation due to Modulation
The response of an RF detector may vary due to different modulation schemes. How much it will vary depends
on the modulation form and the type of detector. Modulation forms with high peak-to-average ratios (PAR) can
cause significant variation, especially with traditional RF detectors. This is because the measurement is not an
actual RMS measurement and is therefore waveform dependent.
To calculate the variation due to modulation (E_MOD), the measurement result for an un-modulated RF carrier is
subtracted from the measurement result of 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
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 (18)
Figure 58 shows the variation due to modulation for W-CDMA, where a E_MOD of 0.09 dB in obtained for a
dynamic range from -38 dBm to –5 dBm.
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