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Texas Instruments AN-2064 LMH2120 Evaluation Board (Rev. A) User guides
User's Guide
SNWA012A – July 2010 – Revised April 2013
AN-2064 LMH2120 Evaluation Board
1
General Description
The LMH2120 is a 40 dB Linear RMS power detector particularly suited for accurate power measurement
of modulated RF signals that exhibit large peak-to-average ratios; i.e., large variations of the signal
envelope. Such signals are encountered in W-CDMA and LTE cell phones. The RMS measurement
topology inherently ensures a modulation insensitive measurement.
The device has an RF frequency range from 50 MHz to 6GHz. It provides an accurate temperature- and
supply- insensitive output voltage that relates linearly to the RF input power (see Figure 8). The
LMH2120's excellent conformance to a linear response enables an easy integration by using slope and
intercept only, reducing calibration effort significantly. The device operates with a single supply from 2.7V
to 5V. The LMH2120 has an RF power detection range from -35 dBm to 5dBm and is ideally suited for
use in combination with a directional coupler. Alternatively, a resistive divider can be used.
The device is active for EN=High − otherwise it is in a low power-consumption shutdown mode. To save
power and prevent discharge of an external filter capacitance, the output (OUT) is high impedance during
shutdown.
The LMH2120 power detector is offered in a tiny 6-bump DSBGA package.
Figure 1 shows the LMH2120 Evaluation Board.
Figure 1. LMH2120 Evaluation Board
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1
Basic Operation
2
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Basic Operation
The circuit operates with a single supply form 2.7V to 5V and has an RF power detection range from −35
dBm to 5dBm. The board consist of a single LMH2120 along with external components soldered on a
printed circuit board. External supply voltages and input signals can be applied to the on-board
connectors. The supply voltage is applied with connectors P21 (VDD) and P22 (GND). The RF input
signal is applied by SMA connector P1. This RF signal is applied through an RF generator and is
connected with a 50Ω SMA cable. The detector output can be measured via BNC connector P3.
3
Configuration
The LMH2120 evaluation board can be configured via jumper settings. The device is active when EN =
High. This can be accomplished by setting the jumper J4 to VDD or by using external control on P4 by
setting the jumper J4 to EN. Since the device has an internal operating voltage of 2.5V, the voltage level
on the enable should not be higher than 3V to prevent damage to the device. Also enable voltage levels
lower than 400 mV below GND should be prevented. In both cases the ESD devices start to conduct
when the enable voltage range is exceeded and excessive current will be drawn. To guarantee a correct
operation, a voltage divider formed by R2 and R3 is present on the evaluation board. The absolute
maximum ratings are also exceeded when the enable (EN) is switched to HIGH (from shutdown to active
mode) while the supply voltage is switched off. This situation should be prevented at all times. A solution
to protect the device is the resistor R1 of 1kΩ in series with the enable input to limit the current.
An overview of the various jumper positions on the board is given in Figure 2. The settings of these
jumpers and their functions are listed in Table 1.
2
AN-2064 LMH2120 Evaluation Board
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Configuration
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Figure 2. Jumper Positions
Table 1. Jumper and Header Overview
Jumper
J4
Function
Enable
Jumper
Position
Description
1–2
Active, Connects Enable Pin to VDD (factory default configuration)
3–4
External Control, Connects Enable Pin to Enable P4
5-6
Shutdown, Connects Enable Pin to GND
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3
Schematic
4
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Schematic
The schematic of the evaluation board is shown in Figure 3.
Figure 3. Evaluation Board Schematic
5
Bill of Materials
The Bill of Material (BOM) of the evaluation board is listed in Table 2.
Table 2. Bill of Materials of the Evaluation Board
Designator
Description
Comment
C1, C2
0603 Capacitor
10 nF
C3, C4, C5
0603 Capacitor
10 pF
C6
0603 Capacitor
10 µF
J4
Header
2×3
P1
Connector
SMA
P21, P22
Connector
Banana
P3, P4
Connector
BNC
R1
0603 Resistor
1 kΩ
R2, R3
0603 Resistor
100 kΩ
R4
0603 Resistor
0Ω
TP1
Test Point
GND
U1
DSBGA
LMH2120
4
AN-2064 LMH2120 Evaluation Board
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Board Layout
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6
Board Layout
As with any other RF device, careful attention must be paid to the board layout. If the board layout isn’t
properly designed, performance might be less than can be expected for the application. The LMH2120 is
designed to be used in RF applications, having a characteristic impedance of 50Ω. To achieve this
impedance, the input of the LMH2120 needs to be connected via a 50Ω transmission line. Transmission
lines can be created on PCBs using microstrip or (grounded) coplanar waveguide (GCPW) configurations.
In order to minimize injection of RF interference into the LMH2120 through the supply lines, the PCB
traces for VDD and GND should be minimized for RF signals. This can be done by placing a small
decoupling capacitor between the VDD and GND. It should be placed as close as possible to the VDD
and GND pins of the LMH2120.
Figure 4 shows the component locations of the LMH2120 evaluation board, and Figure 5 shows the board
layout of the LMH2120 evaluation board.
Figure 4. Component Locations of Evaluation Board
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Board Layout
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Figure 5. Board Layout of Evaluation Board
6
AN-2064 LMH2120 Evaluation Board
SNWA012A – July 2010 – Revised April 2013
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Measurement Setup
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7
Measurement Setup
The performance of the LMH2120 can be measured with the setup shown in Figure 6.
An external power supply provides a voltage of 2.7V to 5V to the evaluation board. An accurate and stable
RF Signal Generator is used to produce the test signal. Use of low loss cables is recommended to ensure
reliable measurement data. The detected output voltage can be measured with a Digital Voltage Meter
(DVM).
VDD
Power
Supply
GND
VDD
OUT
RFIN
RF Signal
Generator
LMH2120
Eval
Board
Digital
Volt
Meter
GND
Figure 6. Measurement Setup
8
Measurement Results
Figure 7 shows the output voltage versus frequency for various power levels on RFIN. The frequency
range is from 10 MHz to 10 GHz. Figure 8 shows the output voltage versus RF input power for various
frequencies.
10
10
3.5 GHz
RFIN = 0 dBm
RFIN = -5 dBm
RFIN = -15 dBm
RFIN = -20 dBm
0.1
2.6 GHz
1
RFIN = -10 dBm
VOUT (V)
VOUT (V)
1
RFIN = -25 dBm
1.9 GHz
900 MHz
0.1
50 MHz
RFIN = -30 dBm
RFIN = -35 dBm
0.01
10M
5.8 GHz
0.01
RFIN = -40 dBm
100M
1G
10G
-50
-30
-20
-10
0
10
RF INPUT POWER (dBm)
FREQUENCY (Hz)
Figure 7. Output Voltage vs. Freqency
-40
Figure 8. Output Voltage vs. RF Input Power
SNWA012A – July 2010 – Revised April 2013
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7
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