AN-1434 [NSC]
Crest Factor Invariant RF Power Detector; 波峰因数不变RF功率检测器
| 型号： | AN-1434 |
| 厂家： | 
National Semiconductor |
| 描述： | Crest Factor Invariant RF Power Detector |
| 文件： | 总6页 (文件大小：527K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
National Semiconductor
Application Note 1434
Barry Yuen
Crest Factor Invariant RF
Power Detector
January 2006
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.
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.
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.
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.
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.
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.
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.
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.
TABLE 1. Theoretic Bandwidth Efficiency
Theoretical Bandwidth/Spectral
Modulation Scheme
Comments
Constant Envelope
Efficiency (bit/second/Hz)
GMSK
QPSK
8PSK
1
2
3
4
Non-constant Envelope
Non-constant Envelope
Non-constant Envelope
16QAM
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.
© 2006 National Semiconductor Corporation
AN201772
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Overview of Digital Modulation in Cellular Phones (Continued)
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20177202
GMSK
QPSK
1 bits per symbol
2 bits per symbol
20177204
16QAM
4 bits per symbol
20177203
8PSK
3 bits per symbol
FIGURE 1. Constellation Diagram of Digital Modulation Used in Cellular Phones
The other way to look at the bandwidth efficiency is the
symbol rate (or Baud Rate) because the signal bandwidth for
the communications channel needed depends on the sym-
bol rate, not on the bit rate.
(Gaussian Minimum Shift Keying with BT = 0.3). In this
modulation format, the amplitude of the modulating carrier is
kept constant (as shown in Figure 1) while its frequency is
varied by the modulating message signal. This is a desirable
characteristic for using a power efficient RF amplifier in the
transmitter.
Symbol rate = bit rate / the number of bits transmitted with
each symbol
Amplitude variations can exercise nonlinearities in an ampli-
fier’s amplitude-transfer function, generating spectral re-
growth and unwanted adjacent channel power. Since there
is no amplitude variation in MSK, more efficient amplifiers
(which tend to be less linear) can be used with constant
envelope signals. This reduces power consumption.
Bit rate is the frequency of a system bit stream or raw data
stream. The symbol rate is the bit rate divided by the number
of bits that can be transmitted with each symbol on each time
interval.
If one bit is transmitted per symbol, as in MSK, then the
symbol rate would be the same as the bit rate. If two bits are
transmitted per symbol, as in QPSK, then the symbol rate
would be half of the bit rate. In comparing the MSK and
QPSK, we can easily find that the symbol rate of QPSK is
lower for transmitting the same amount of information. This
is why modulation formats that are more complex and use a
higher number of states can send the same information over
a narrower piece of the RF spectrum.
In the GSM/GPRS standard, the modulated waveforms are
filtered with a Gaussian filter, which results in a narrow
spectrum. In addition, the Gaussian filter has no time-
domain overshoot and therefore the peak deviation is not
increased.
QPSK and 16QAM Modulation in
3G CDMA
16-ary Quadrature Amplitude Modulation (16QAM) is going
to be used in the HSPDA of the W-CDMA standard. The
GMSK Modulation in GSM/GPRS
The GSM/GPRS cellular phone network uses a variation of
the constant-envelope modulation format called 0.3 GMSK
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2
QPSK and 16QAM Modulation in
3G CDMA (Continued)
symbol rate is one fourth of the bit rate for 16QAM because
16 = 24 as seen in Figure 1. So this modulation format
produces a more spectrally efficient transmission. It is more
efficient than MSK, QPSK or 8PSK. Note that QPSK is the
same as 4-ary Quadrature Amplitude Modulation.
Crest Factor (CF) or
Peak-to-Average Ratio (PAR) in
Digitally-Modulated RF
The crest factor is defined as the ratio of peak amplitude to
average amplitude (Peak-to-Average Ratio) as shown in
Figure 2. For narrow band QPSK modulation, the crest factor
is 6 dB and for offset QPSK modulation, the crest factor is
5.1 dB.
In linear schemes, the amplitude of the transmitted signal
varies with the modulating digital signal as in BPSK or
QPSK. In systems where bandwidth efficiency is more im-
portant than power efficiency, constant envelope modulation
is not as well suited.
20177205
FIGURE 2. Crest Factor Definition
In cellular phone systems, the Complementary-Cumulative-
Probability-Distribution Function (CCDF) is used to measure
the statistics of a transmitted radio frequency signal. The
CCDF statistically represents how often a radio frequency
signal is above a specific power level or threshold. This
CCDF is the authentic method to characterize the crest
factor (CF) or peak-to-average (PAR) of WCDMA or
CDMA2000 signals in the 3G network.
Power Definition and Measurement
in Cellular Phone Systems
In the DC world, the power dissipating through a circuit
element is calculated by taking the product of the current
through this element and the voltage drop across the ele-
ment,
PDC = V • I (Watts or mW)
In IS-95, the PAR is found to be 3.9 dB and the PAR in
CDMA 1X RTT can be as high as 5.4 dB. In W-CDMA, the
PAR can vary from 2 dB to 11 dB. In summary, the PAR in
CDMA2000 or W-CDMA depends on the radio configura-
tions, spreading code combinations and channels used.
Sometimes, crest factor reduction techniques are used to
reduce the effective PAR in base station transmitters.
In the analog and RF world, either peak power or average
power could be used to represent the transmitted or received
energy level. Figure 3 show a typical RF signal with ampli-
tude variation on its envelope. The power level of the signal
is defined by:
20177207
FIGURE 3. Peak Power and Average Power Measurement in a Digitally-Modulated RF Signal
3
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Peak Power Detector
3G Cellular Phone Applications
Peak power measurement has been successfully used in
GSM and AMP cellular phone networks. Typically, peak RF
power measurement is performed by circuits with a pair of
Schottky diodes, where one diode is used for RF rectification
and the other is used for temperature compensation.
In today’s market, the maximum output power of a 3G RF
power amplifier is slightly higher than +28 dBm and the
maximum output power of a 3G mobile unit is +27 dBm. One
extra decibel of power is needed to compensate for the
intrinsic loss of circuits between the antenna and power
amplifier. A 3G RF power amplifier usually has two modes; a
high power mode recommended for use from +16 dBm and
above; and a low power mode recommended for use from
+16 dBm and below.
As we mentioned in the previous section, 3G CDMA RF
signals have high crest factors and peak power measure-
ment will result in values that are higher than the average
power of the CDMA signal by a factor of PAR. Using a peak
power detector requires a proprietary calibration approach to
correct the measurement uncertainties caused by the vary-
ing crest factor.
Figure 4 is the estimated curve of output power probability in
a 3G mobile unit. Since the highest probability of output
power occurrence in 3G is from +10 dBm and above in a 3G
mobile unit, it is very critical to tightly control its output RF
power from +10 dBm to a maximum output signal level of
+27 dBm.
Average Power Detector
Average power measurement is to measure the average
power of the RF signal. This method can handle signals with
changing modulation envelopes such as QPSK or 16QAM.
The measurement result provides a true measure of the
average power in the signal, regardless of modulation type
and peak-to-average ratio (PAR or CF). As a result, average
power measurement is ideal to handle signals in which the
amplitude of the envelope of modulation changes with time
such as IS-95, CDMA2000, W-CDMA or TD-sCDMA.
As mentioned in the previous section, 3G CDMA RF signals
have high crest factors and the peak power measurement
will result in values that are higher than the average power of
the CDMA signal by a factor of PAR.
To resolve these uncertainties caused by the crest factor, a
mean square radio frequency power detector, like the
LMV232, can be used to measure the average power of a
changing modulation signal.
20177208
Mean Square Detection
The LMV232 is a trans-linear device and makes use of the
exponential Ic-Vbe relation of a bipolar transistor:
FIGURE 4. Probability of PA Output Level in a 3G
CDMA Mobile Unit
y = e2
= x2
•
1n(x)
Since the LMV232 follows the square law, we can observe
that the output voltage VOUT (Volts) of the LMV232 is pro-
portional to the input power PIN (Watts or miliWatts).
The LMV232 has a 20+dB linear detection range from +13
dBm and below. Figure 5 is a recommended application
block diagram for use in detecting the transmitted RF signal
level in a multi-band 3G mobile unit.
VOUT (Volts) = k • PIN (mW)
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4
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
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
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:
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.
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.
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.
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.
The detection error in the previous prediction would be 12.65
dBm − 12.3 dBm = 0.35 dB.
Because of
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.
∝
VDETECTED PIN (mW) = POUT (mW) − Coupling (dB)
There is a linear response 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.
5
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20177210
20177211
FIGURE 6. Over Temp Data of the LMV232 in a
Handset to Demonstrate Measurement Accuracy
FIGURE 7. Over Temp Data to Show the LMV232’s
Accurate Performance at Small Signals
Summary
The LMV232 is optimized for applications in 3G mobile units.
Together with a directional coupler, the LMV232 can be used
to detect accurately the handset’s transmit power level at the
most often used range of +10 dBm and above. This is shown
in the probability curve. Its crest factor invariant detection
characteristic eliminates the need for the proprietary peak-
to-average ratio correction process.
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
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