LMH2110_15 Datasheet, PDF (19/36 Pages) Texas Instruments – LMH2110 8-GHz Logarithmic RMS Power Detector with 45-dB Dynamic Range

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LMH2110
SNWS022D – JANUARY 2010 – REVISED JUNE 2015
Feature Description (continued)
RMS detectors are in particular suited for the newer communication standards like W-CDMA and LTE that exhibit
large peak-to-average ratios and different modulation schemes (signal shapes). This is a key advantage
compared to other types of detectors in applications that employ signals with high peak-to-average power
variations or different modulation schemes. For example, the RMS detector response to a 0-dBm modulated W-
CDMA signal and a 0-dBm unmodulated carrier is essentially equal. This eliminates the need for long calibration
procedures and large calibration tables in the baseband due to different applied modulation schemes.
7.3.3 LMH2110 RF Power Detector
For optimal performance of the LMH2110, the device must to be configured correctly in the application (see
Functional Block Diagram).
For measuring the RMS (power) level of a signal, the time average of the squared signal needs to be measured
as described in Accurate Power Measurement. This is implemented in the LMH2110 by means of a multiplier and
a low-pass filter in a negative-feedback loop. A simplified block diagram of the LMH2110 is depicted in Functional
Block Diagram. The core of the loop is a multiplier. The two inputs of the multiplier are fed by (i₁, i₂):
i₁ = i_LF + i_RF
(4)
i₂ = i_LF – i_RF
where
• i_LF is a current depending on the DC output voltage of the RF detector, and
• i_RF is a current depending on the RF input signal.
(5)
The output of the multiplier (i_OUT) is the product of these two current and equals:
i_LF2 i_RF2
i_out =
I₀
where
• I₀ is a normalizing current.
(6)
By a low-pass filter at the output of the multiplier the DC term of this current is isolated and integrated. The input
of the amplifier A acts as the nulling point of the negative feedback loop, yielding:
³ ³ iLF2dt = iRF2dt
(7)
which implies that the average power content of the current related to the output voltage of the LMH2110 is
made equal to the average power content of the current related to the RF input signal.
For a negative-feedback system, the transfer function is given by the inverse function of the feedback block.
Therefore, to have a logarithmic transfer for this RF detector, the feedback network implements an exponential
function resulting in an overall transfer function for the LMH2110 of:
V_out
=
V₀
log
¨©§
1
V_x
³^VRF2dt ¸¹·
where
• V₀ and V_X are normalizing voltages.
(8)
As a result of the feedback loop a square-root is also implemented, yielding the RMS function.
Given this architecture for the RF detector, the high-performance of the LMH2110 can be understood. In theory
the accuracy of the logarithmic transfer is set by:
• The exponential feedback network, which basically needs to process a DC signal only.
• A high loop gain for the feedback loop, which is specified by the amplifier gain A.
The RMS functionality is inherent to the feedback loop and the use of a multiplier; thus, a very accurate LOG-
RMS RF power detector is obtained.
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