LMH2100 Datasheet, PDF (33/49 Pages) National Semiconductor (TI) – 50 MHz to 4 GHz 40 dB Logarithmic Power Detector for CDMA and WCDMA

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LMH2100
SNWS020C – NOVEMBER 2007 – REVISED OCTOBER 2015
Application Information (continued)
The advantages and disadvantages can be summarized as follows:
• The temperature stability of the LOG detector transfer function is generally not as good as the stability of
diode and R(MS) detectors. This is because a significant part of the circuit operates at RF frequencies.
• The dynamic range of LOG detectors is usually much larger than that of other types of detectors.
• Since LOG detectors perform a kind of peak detection their response is wave form dependent, similar to
diode detectors.
• The transfer shape of LOG detectors puts the lowest possible requirements on the ADC resolution (See
Figure 77).
8.2 Typical Applications
RF power detectors can be used in a wide variety of applications. The first example shows the LMH2100 in a
Figure 79, the second application measures the Figure 88.
8.2.1 Application With Transmit Power Control Loop
The key benefit of a transmit power control loop circuit is that it makes the transmit power insensitive to changes
in the Power Amplifier (PA) gain control function, such as changes due to temperature drift. When a control loop
is used, the transfer function of the PA is eliminated from the overall transfer function. Instead, the overall
transfer function is determined by the power detector. The overall transfer function accuracy depends thus on the
RF detector accuracy. The LMH2100 is especially suited for this application, due to the accurate temperature
stability of its transfer function.
Figure 79 shows a block diagram of a typical transmit power control system. The output power of the PA is
measured by the LMH2100 through a directional coupler. The measured output voltage of the LMH2100 is
filtered and subsequently digitized by the ADC inside the baseband chip. The baseband adjusts the PA output
power level by changing the gain control signal of the RF VGA accordingly. With an input impedance of 50 Ω, the
LMH2100 can be directly connected to a 30 dB directional coupler without the need for an additional external
attenuator. The setup can be adjusted to various PA output ranges by selection of a directional coupler with the
appropriate coupling factor.
B
A
RF _VGA
S
E
B
GAIN
A
N
D
ADC
R_S
C_S
COUPLER
PA
ANTENNA
50 :
OUT RFIN
LMH2100
LOGIC
EN
GND
Figure 79. Transmit Power Control System
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