LMV221SDX National Semiconductor, LMV221SDX Datasheet - Page 20

• Manufacturer Part Number: LMV221SDX
• Manufacturer: National Semiconductor

Specifications of LMV221SDX

• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 6
• Package Type: LLP EP
• Lead Free Status / RoHS Status: Not Compliant

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tion is based on direct determination of the average power
and not – like the diode detector – of the peak power.
The advantages and disadvantages can be summarized as
follows:
•
•
•
•
Logarithmic Detectors
The transfer function of a logarithmic detector has a linear in
dB response, which means that the output voltage changes
linearly with the RF power in dBm. This is convenient since
most communication standards specify transmit power levels
in dBm as well.
The advantages and disadvantages can be summarized as
follows:
•
•
•
•
1.1.3 Characteristics of the LMV221
The LMV221 is a Logarithmic RF power detector with ap-
proximately 40 dB dynamic range. This dynamic range plus
its logarithmic behavior make the LMV221 ideal for various
applications such as wireless transmit power control for CD-
MA and UMTS applications. The frequency range of the
LMV221 is from 50 MHz to 3.5 GHz, which makes it suitable
for various applications.
The LMV221 transfer function is accurately temperature com-
pensated. This makes the measurement accurate for a wide
temperature range. Furthermore, the LMV221 can easily be
connected to a directional coupler because of its 50 ohm input
termination. The output range is adjustable to fit the ADC input
range. The detector can be switched into a power saving
shutdown mode for use in pulsed conditions.
1.2 Applications of RF Power Detectors
RF power detectors can be used in a wide variety of applica-
tions. This section discusses two application. The first exam-
ple shows the LMV221 in a transmit power control loop, the
second application measures the voltage standing wave ratio
(VSWR).
The temperature stability of (R)MS detectors is almost as
good as the temperature stability of the diode detector;
only a small part of the circuit operates at RF frequencies,
while the rest of the circuit operates at low frequencies.
The dynamic range of (R)MS detectors is limited. The
lower end of the dynamic range is limited by internal device
offsets.
The response of (R)MS detectors is highly waveform
independent. This is a key advantage compared to other
types of detectors in applications that employ signals with
high peak-to-average power variations. For example, the
(R)MS detector response to a 0 dBm WCDMA signal and
a 0 dBm unmodulated carrier is essentially equal.
The transfer shape of R(MS) detectors has many
similarities with the diode detector and is therefore subject
to similar disadvantages with respect to the ADC
resolution requirements (See Figure 1, left side).
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
1, right side).
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1.2.1 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 trans-
fer function. Instead, the overall transfer function is deter-
mined by the power detector. The overall transfer function
accuracy depends thus on the RF detector accuracy. The
LMV221 is especially suited for this application, due to the
accurate temperature stability of its transfer function.
Figure 3 shows a block diagram of a typical transmit power
control system. The output power of the PA is measured by
the LMV221 through a directional coupler. The measured
output voltage of the LMV221 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 50Ω, the LMV221 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 appropri-
ate coupling factor.
1.2.2 Voltage Standing Wave Ratio Measurement
Transmission in RF systems requires matched termination by
the proper characteristic impedance at the transmitter and
receiver side of the link. In wireless transmission systems
though, matched termination of the antenna can rarely be
achieved. The part of the transmitted power that is reflected
at the antenna bounces back toward the PA and may cause
standing waves in the transmission line between the PA and
the antenna. These standing waves can attain unacceptable
levels that may damage the PA. A Voltage Standing Wave
Ratio (VSWR) measurement is used to detect such an occa-
sion. It acts as an alarm function to prevent damage to the
transmitter.
VSWR is defined as the ratio of the maximum voltage divided
by the minimum voltage at a certain point on the transmission
line:
Where Γ = V
efficient.
This means that to determine the VSWR, both the forward
(transmitted) and the reflected power levels have to be mea-
FIGURE 3. Transmit Power Control System
REFLECTED
/ V
FORWARD
denotes the reflection co-
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