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PDF AN-1434 Data sheet ( Hoja de datos )
| Número de pieza | AN-1434 | |
| Descripción | Crest Factor Invariant RF Power Detector | |
| Fabricantes | National Semiconductor | |
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Crest Factor Invariant RF
Power Detector
National Semiconductor
Application Note 1434
Barry Yuen
January 2006
Introduction
Over the past few years cellular phone system have made a
major transition from power efficient digital modulation
schemes to bandwidth efficient digital modulation tech-
niques. This is because new cellular phone systems need to
provide a high transmission rate capability to satisfy the
needed broadband applications.
Power efficiency modulation schemes provides reliable
transmission of information in a communications system at
the lowest practical power level. One of the very successful
examples is the GSM/GPRS network. The binary signaling
GMSK modulation is used in the GSM/GPRS network.
A bandwidth efficient modulation scheme delivers a higher
data rate within a limited spectrum bandwidth. All the initial
phases of 3G networks take advantage of this kind of modu-
lation. A Sixteenth-ary Quadrature Amplitude Modulation
(16QAM) scheme is even used in the latest High Speed
Downlink Packet Access (HSDPA) of W-CDMA air interface.
The 16QAM is used in the downlink to provide mobile users
with the capability to download information much quicker.
The move to bandwidth efficient modulation provides more
information capacity with varieties of 3G cellular phone ser-
vices, higher data security, better quality of services (QoS),
and quicker system availability.
The high level M-ary modulation schemes, like 8PSK,
16QAM, etc, have a greater capacity to convey large
amounts of information than low level binary modulation
schemes, like GMSK.
The gain of greater capacity comes at the expense of more
complex hardware in the radio and DSP. Alternatively, more
complex transmitters and receivers can be used to transmit
the same information over less bandwidth.
In summary, the transition to more and more spectrally effi-
cient transmission techniques requires more and more com-
plex hardware. This complex hardware may include a better
DSP, faster signal processing algorithms, high linear RF
power amplifiers, and more accurate RF power detectors,
etc.
The objective of this article is to demonstrate and briefly
explain how the LMV232 Mean Square Power Detector from
National Semiconductor can be used as an accurate RF
power detector for bandwidth efficiency modulated RF trans-
mission in a handset or mobile unit.
Overview of Digital Modulation in
Cellular Phones
A digital modulation scheme is more spectral efficient if it can
transmit a greater amount of data or bits per second in a
given bandwidth. Therefore, we define the bandwidth or
spectral efficiency of a modulation to be “transmission bit
rate divided by the occupied channel bandwidth, bit/second/
Hz.” Table 1 indicates that a higher M-ary modulation
scheme has a large number of output levels and, therefore,
has better spectral efficiency.
TABLE 1. Theoretic Bandwidth Efficiency
Modulation Scheme
GMSK
QPSK
8PSK
16QAM
Theoretical Bandwidth/Spectral
Efficiency (bit/second/Hz)
1
2
3
4
Comments
Constant Envelope
Non-constant Envelope
Non-constant Envelope
Non-constant Envelope
Figure 1 shows the constellation diagram of each modula-
tion. In these diagrams, we can see that only the GMSK has
a constant RF envelope.
www.DataSheet4U.com
© 2006 National Semiconductor Corporation AN201772
www.national.com
1 page  
3G Cellular Phone Applications (Continued)
20177209
FIGURE 5. Applications Block Diagram for the LMV232 in a 3G Multi-band Handset
In this design, a 16 dB directional coupler is used at the
output of the power amplifier (PA). This sets the maximum
input RF power to the LMV232 to
PIN (dBm) = +28 dBm − 16 dB = +12 dBm
When the output power of PA is +11 dBm, the input power of
the LMV232 would be:
PIN (dBm) = +11 dBm − 16 dB = −5 dBm
In this design example, we have set the operating range of
the LMV232 to be from –5 dBm to +12 dBm so that we have
enough room for coupling factor variations.
Detection Error Over Temperature
The LMV232 mean square RF power detector is used to
detect the transmit power in a 3G mobile unit. In a real
application, the detected voltage VDETECTED has to be cali-
brated to a known reference before the detection method
can be used in normal phone operation.
Because of
VDETECTED ∝ PIN (mW) = POUT (mW) − Coupling (dB)
wwwT.DhearteaSisheaetli4nUea.crormesponse from –7 dBm to +13 dBm when
the power is represented in mW. This linear characteristic
provides an added advantage for power amplifier detection
voltage calibration.
In the production process, VDETECTED is measured at two
different power levels (2-Point), say at the power amplifier’s
POUT = +14 dBm and POUT = +24 dBm. (This corresponds to
PIN = −2 dBm and PIN = +8 dBm respectively with a 16 dB
coupler.) Based on this measurement data, we can create a
linear equation for VDETECTED and the PA’s POUT.
Figure 6 is the measurement results based on the applica-
tions circuit in Figure 5. We also tested the RF power detec-
tor circuit through –40˚C to +85˚C. The 2-Point test data was
taken at room temperature and its estimated equation is
used to predict the PA’s POUT at any temperature.
If VDETECTED = 1V, then the mobile unit will estimate that its
PA’s POUT = +12.3 dBm disregarding the temperature con-
dition. In a hypothetical situation, the power amplifier’s out-
put power would be POUT = +12.65 dBm if the mobile unit
was at a temperature of −40˚C.
The detection error in the previous prediction would be 12.65
dBm − 12.3 dBm = 0.35 dB.
Figure 7 is the same kind of measurement results as Figure
6, but the graph is zoomed into the small signal region.
Again, the detection error based on the 2-Point test equation
will be less than 0.65 dB over the temperature range from
–40˚C to +85˚C.
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