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46 02.09.12 ELECTRONIC DESIGN

LOUIS E. FRENZEL |COMMUNICATIONS EDITOR [email protected]

Fundamental to all wireless communications is modula-

tion, the process of impressing the data to be transmit-

ted on the radio carrier. Most wireless transmissions

today are digital, and with the limited spectrum avail-

able, the type of modulation is more critical than it has

ever been.

The main goal of modulation today is to squeeze as much

data into the least amount of spectrum possible. That objective,

known as spectral efficiency, measures how quickly data can

be transmitted in an assigned bandwidth. The unit of measure-

ment is bits per second per Hz (bits/s/Hz). Multiple techniqueshave emerged to achieve and improve spectral efficiency.

EngineeringEssentials

Todays designers can utilize myriad

modern modulation methods to pack

ever-increasing data into ever-decreasing

spectrum.


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ELECTRONIC DESIGNGO TO WWW.ELECTRONICDESIGN.COM 47

ASK AND FSK

There are three basic ways to modulate a sine wave radio

carrier: modifying the amplitude, frequency, or phase. More

sophisticated methods combine two or more of these variations

to improve spectral efficiency. These basic modulation forms

are still used today with digital signals.

Figure 1 shows a basic serial digital signal of binary zeros

and ones to be transmitted and the corresponding AM and

FM signals resulting from modulation. There are two types of

AM signals: on-off keying (OOK) and amplitude shift keying

(ASK). In Figure 1a, the carrier amplitude is shifted between

two amplitude levels to produce ASK. In Figure 1b, the binary

signal turns the carrier off and on to create OOK.

AM produces sidebands above and below the carrier equal

to the highest frequency content of the modulating signal. The

bandwidth required is two times the highest frequency content

including any harmonics for binary pulse modulating signals.

Frequency shift keying (FSK) shifts the carrier between twodifferent frequencies called the mark and space frequencies, or

fmand fs(Fig. 1c). FM produces multiple sideband frequencies

above and below the carrier frequency. The bandwidth pro-

duced is a function of the highest modulating frequency includ-

ing harmonics and the modulation index, which is:

m = f(T)

f is the frequency deviation or shift between the mark and

space frequencies, or:

f = fs fm

T is the bit time interval of the data or the reciprocal of the

data rate (1/bit/s).

Smaller values of m produce fewer sidebands. A popular

version of FSK called minimum shift keying (MSK) specifies

m = 0.5. Smaller values are also used such as m = 0.3.

There are two ways to further improve the spectral efficiency

for both ASK and FSK. First, select data rates, carrier frequen-

cies, and shift frequencies so there are no discontinuities in the

sine carrier when changing from one binary state to another.These discontinuities produce glitches that increase the har-

monic content and the bandwidth.

The idea is to synchronize the stop and start times of the

binary data with when the sine carrier is transitioning in ampli-

tude or frequency at the zero crossing points. This is called

continuous phase or coherent operation. Both coherent ASK/

OOK and coherent FSK have fewer harmonics and a narrower

bandwidth than non-coherent signals.

A second technique is to filter the binary data prior to modu-

lation. This rounds the signal off, lengthening the rise and fall

times and reducing the harmonic content. Special Gaussian

and raised cosine low pass filters are used for this purpose.

GSM cell phones widely use a popular combination, Gaussianfiltered MSK (GMSK), which allows a data rate of 270 kbits/s

in a 200-kHz channel.

BPSK AND QPSK

A very popular digital modulation scheme, binary phase

shift keying (BPSK), shifts the carrier sine wave 180 for

each change in binary state (Fig. 2). BPSK is coherent as the

phase transitions occur at the zero crossing points. The proper

demodulation of BPSK requires the signal to be compared to

a sine carrier of the same phase. This involves carrier recovery

and other complex circuitry.

A simpler version is differential BPSK or DPSK, where the

received bit phase is compared to the phase of the previous bit

signal. BPSK is very spectrally efficient in that you can trans-

mit at a data rate equal to the bandwidth or 1 bit/Hz.

In a popular variation of BPSK, quadrature PSK (QPSK),

the modulator produces two sine carriers 90 apart. The binary

data modulates each phase, producing four unique sine signals

shifted by 45 from one another. The two phases are added

together to produce the final signal. Each unique pair of bits

generates a carrier with a different phase (Table 1).

Figure 3a illustrates QPSK with a phasor diagram where the

phasor represents the carrier sine amplitude peak and its posi-

tion indicates the phase. A constellation diagram in Figure 3b

shows the same information. QPSK is very spectrally efficientsince each carrier phase represents two bits of data. The spec-

Binarydata

0 0

1 1

Carriersine

Higherfrequency

Lower frequencyLower frequency

(a) ASK

(b) OOK

(c) FSK

1. Three basic digital modulation formats are still very popular with low-

data-rate short-range wireless applications: amplitude shift keying (a), on-

off keying (b), and frequency shift keying (c). These waveforms are coher-ent as the binary state change occurs at carrier zero crossing points.

Serialbinarydata

BPSK

Phase changes

1 1 10 00 2. In binary phase shift

keying, note how a

binary 0 is 0 while a

binary 1 is 180. The

phase changes when the

binary state switches so

the signal is coherent.


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tral efficiency is 2 bits/Hz, meaning twice the data rate can be

achieved in the same bandwidth as BPSK.

DATA RATE AND BAUD RATE

The maximum theoretical data rate or channel capacity (C)

in bits/s is a function of the channel bandwidth (B) channel in

Hz and the signal-to-noise ratio (SNR):

C = B log2(1 + SNR)

This is called the Shannon-Hartley law. The maximum data

rate is directly proportional to the bandwidth and logarithmi-

cally proportional the SNR. Noise greatly diminishes the data

rate for a given bit error rate (BER).

Another key factor is the baud rate, or the number of modu-

lation symbols transmitted per second. The term symbol in

modulation refers to one specific state of a sine carrier signal. Itcan be an amplitude, a frequency, a phase, or some combination

of them. Basic binary transmission uses one bit per symbol.

In ASK, a binary 0 is one amplitude and a binary 1 is another

amplitude. In FSK, a binary 0 is one carrier frequency and a

binary 1 is another frequency. BPSK uses a 0 shift for a binary

0 and a 180 shift for a binary 1. In each of these cases there is

one bit per symbol.

Data rate in bits/s is calculated as the reciprocal of the bit

time (tb):

bits/s = 1/tb

With one symbol per bit, the baud rate is the same as the bit

rate. However, if you transmit more bits per symbol, the baud

rate is slower than the bit rate by a factor equal to the number of

bits per symbol. For example, if 2 bits per symbol are transmit-

ted, the baud rate is the bit rate divided by 2. For instance, with

QPSK a 70-Mbit/s data stream is transmitted at a baud rate of

35 symbols/s.

M-PSK

QPSK produces two bits per symbol, making it very spec-

trally efficient. QPSK can be referred to as 4PSK because

there are four amplitude-phase combinations. By using smaller

phase shifts, more bits can be transmitted per symbol. Somepopular variations are 8PSK and 16PSK.

8PSK uses eight symbols with constant carrier amplitude

45 shifts between them, enabling 3 bits to be transmitted for

each symbol. 16PSK uses 22.5 shifts of constant amplitude

carrier signals. This arrangement results in a transmission of 4bits per symbol.

While M-PSK is much more spectrally efficient, the greater

the number of smaller phase shifts, the more difficult the sig-

nal is to demodulate in the presence of noise. The benefit of

M-PSK is that the constant carrier amplitude means that more

efficient nonlinear power amplification can be used.

QAM

The creation of symbols that are some combination of

amplitude and phase can carry the concept of transmitting

more bits per symbol further. This method is called quadrature

amplitude modulation (QAM). For example, 8QAM uses four

carrier phases plus two amplitude levels to transmit 3 bits persymbol. Other popular variations are 16QAM, 64QAM, and

256QAM, which transmit 4, 6, and 8 bits per symbol respec-

tively (Fig. 4).

While QAM is enormously efficient of spectrum, it is more

difficult to demodulate in the presence of noise, which is most-

ly random amplitude variations. Linear power amplification

is also required. QAM is very widely used in cable TV, Wi-Fi

wireless local-area networks (LANs), satellites, and cellular

telephone systems to produce maximum data rate in limited

bandwidths.

APSK

Amplitude phase shift keying (APSK), a variation of both

M-PSK and QAM, was created in response to the need for an

improved QAM. Higher levels of QAM such as 16QAM and

above have many different amplitude levels as well as phase

shifts. These amplitude levels are more susceptible to noise.

Furthermore, these multiple levels require linear power

amplifiers (PAs) that are less efficient than nonlinear (e.g.,

class C). The fewer the number of amplitude levels or the

smaller the difference between the amplitude levels, the greater

the chance to operate in the nonlinear region of the PA to boost

power level.

APSK uses fewer amplitude levels. It essentially arranges

the symbols into two or more concentric rings with a constantphase offset . For example, 16APSK uses a double-ring PSK



(a) (b)

0

90

180

270

0

90

180

270

0001

11 10

3. Modulation can be represented without time domain waveforms. For example, QPSK can be

represented with a phasor diagram (a) or a constellation diagram (b), both of which indicate phase

and amplitude magnitudes.

0180

90

270

4. 16QAM uses a mix of amplitudes and phas-

es to achieve 4 bits/Hz. In this example, there

are three amplitudes and 12 phase shifts.


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format (Fig. 5). This is called 4-12 16APSK with four symbols

in the center ring and 12 in the outer ring.

Two close amplitude levels allow the amplifier to operate

closer to the nonlinear region, improving efficiency as well as

power output. APSK is used primarily in satellites since it is a

good fit with the popular traveling wave tube (TWT) PAs.

OFDM

Orthogonal frequency division multiplexing (OFDM) com-

bines modulation and multiplexing techniques to improve

spectral efficiency. A transmission channel is divided into

many smaller subchannels or subcarriers. The subcarrier fre-

quencies and spacings are chosen so theyre orthogonal to one

another. Their spectra wont interfere with one another, then,

so no guard bands are required (Fig. 6).

The serial digital data to be transmitted is subdivided into

parallel slower data rate channels. These lower data rate sig-

nals are then used to modulate each subcarrier. The most com-

mon forms of modulation are BPSK, QPSK, and several levelsof QAM. BPSK, QPSK, 16QAM, and 64QAM are defined

with 802.11n. Data rates up to about 300 Mbits/s are possible

with 64QAM.

The complex modulation process is only produced by digital

signal processing (DSP) techniques. An inverse fast Fourier

transform (IFFT) generates the signal to be transmitted. An

FFT process recovers the signal at the receiver.

OFDM is very spectrally efficient. That efficiency level

depends on the number of subcarriers and the type of modula-

tion, but it can be as high as 30 bits/s/Hz. Because of the wide

bandwidth it usually occupies and the large number of subcar-

riers, it also is less prone to signal loss due to fading, multipath

reflections, and similar effects common in UHF and micro-

wave radio signal propagation.

Currently, OFDM is the most popular form of digital modu-

lation. It is used in Wi-Fi LANs, WiMAX broadband wire-

less, Long-Term Evolution (LTE) 4G cellular systems, digital

subscriber line (DSL) systems, and in most power-line com-

munications (PLC) applications. For more, see Orthogonal

Frequency-Division Multiplexing (OFDM): FAQ Tutorial, at

http://mobiledevdesign.com/tutorials/ofdm.

DETERMINING SPECTRAL EFFICIENCY

Again, spectral efficiency is a measure of how quickly

data can be transmitted in an assigned bandwidth. The unitof measurement for spectral efficiency is bits/s/Hz (b/s/Hz).

Each type of modulation has a maximum theoretical spectral

efficiency measure(Table 2).

SNR is another important factor that influences spectral

efficiency. It also can be expressed as the carrier to noise power

ratio (CNR). The measure is the BER for a given CNR value.

BER is the percentage of errors that occur in a given number of

bits transmitted. As the noise becomes larger compared to the

signal level, more errors occur.

Some modulation methods are more immune to noise than

others. Amplitude modulation methods like ASK/OOK and

QAM are far more susceptible to noise so they have a higher

BER for a given modulation. Phase and frequency modulation

(BPSK, FSK, etc.) fare better in a noisy environment so they

require less signal power for a given noise level(Fig. 7).

OTHER FACTORS AFFECTING SPECTRAL EFFICIENCY

While modulation plays a key role in the spectral efficiency

you can expect, other aspects in wireless design influence it aswell. For example, the use of forward error correction (FEC)

techniques can greatly improve the BER. Such coding methods

add extra bits so errors can be detected and corrected.

These extra coding bits add overhead to the signal, reduc-

ing the net bit rate of the data, but thats usually an acceptable

tradeoff for the single-digit dB improvement in CNR. Such

coding gain is common to almost all wireless systems today.

Digital compression is another useful technique. The digital

data to be sent is subjected to a compression algorithm that

greatly reduces the amount of information. This allows digital

signals to be reduced in content so they can be transmitted as

shorter, slower data

streams.F o r ex amp l e ,

voice signals are

compressed for dig-

ital cell phones and

voice over Internet

protocol (VoIP)

phones. Music is

compressed in MP3

TABLE 1: CARRIER PHASESHIFT FOR EACH PAIR OF BITS

REPRESENTED

Bit pairs Phase (degrees)

0 0 45

0 1 135

1 1 225

1 0 315

TABLE 2: SPECTRAL EFFICIENCY FOR POPULAR

DIGITAL MODULATION METHODS

Type of modulation Spectral efficiency (bits/s/Hz)

FSK 10 (depends on the type of modulationand the number of subcarriers)

270

180

90

0

A1

A2

5. 16APSK uses two

amplitude levels, A1 and

A2, plus 16 different

phase positions with an

offset of . This tech-

nique is widely used in

satellites.



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or AAC files for faster transmission and less storage. Video

is compressed so high-resolution images can be transmitted

faster or in bandwidth-limited systems.

Another factor affecting spectral efficiency is the utiliza-

tion of multiple-input multiple-output (MIMO), which is the

use of multiple antennas and transceivers to transmit two or

more bit streams. A single high-rate stream is divided into

two parallel streams and transmitted in the same bandwidth

simultaneously.

By coding the streams and their unique path characteristics,

the receiver can identify and demodulate each stream and reas-

semble it into the original stream. MIMO, therefore, improves

data rate, noise performance, and spectral efficiency. Newerwireless LAN (WLAN) standards like 802.11n and 802.11ac/

ad and cellular standards like LTE and WiMAX use MIMO.

For more, see How MIMO Works at http://electronicdesign.

com/article/communications/how-mimo-works12998.aspx.

IMPLEMENTING MODULATION AND DEMODULATION

In the past, unique circuits implemented modulation and

demodulation. Today, most modern radios are software-

defined radios (SDR) where functions such as modulation

and demodulation are handled in software. DSP algorithms

manage the job that was previously assigned to modulator and

demodulator circuits.

The modulation process begins with the data to be trans-

mitted being fed to a DSP device that generates two digital

outputs, which are needed to define the amplitude and phase

information required at the receiver to recover the data. The

DSP produces two baseband streams that are sent to digital-to-

analog converters (DACs) that produce the analog equivalents.

These modulation signals feed the mixers along with the

carrier. There is a 90 shift between the carrier signals to the

mixers. The resulting quadrature output signals from the mix-

ers are summed to produce the signal to be transmitted. If the

carrier signal is at the final transmission frequency, the com-

posite signal is ready to be amplified and sent to the antenna.

This is called direct conversion. Alternately, the carrier signalmay be at a lower intermediate frequency (IF). The IF signal

is upconverted to the final carrier frequency by another mixer

before being applied to the transmitter PA.

At the receiver, the signal from the antenna is amplified

and downconverted to IF or directly to the original baseband

signals. The amplified signal from the antenna is applied to

mixers along with the carrier signal. Again, there is a 90 shift

between the carrier signals applied to the mixers.

The mixers produce the original baseband analog signals,

which are then digitized in a pair of analog-to-digital convert-

ers (ADCs) and sent to the DSP circuitry where demodulation

algorithms recover the original digital data.

There are three important points to consider. First, the

modulation and demodulation processes use two signals in

quadrature with one another. The DSP calculations call for two

quadrature signals if the phase and amplitude are to be pre-

served and captured during modulation or demodulation.

Second, the DSP circuitry may be a conventional program-

mable DSP chip or may be implemented by fixed digital logicimplementing the algorithm. Fixed logic circuits are smaller

and faster and are preferred for their low latency in the modula-

tion or demodulation process.

Third, the PA in the transmitter needs to be a linear amplifier

if the modulation is QPSK or QAM to faithfully reproduce the

amplitude and phase information. For ASK, FSK, and BPSK, a

more efficient nonlinear amplifier may be used.

THE PURSUIT OF GREATER SPECTRAL EFFICIENCY

With spectrum being a finite entity, it is always in short sup-

ply. The Federal Communications Commission (FCC) and

other government bodies have assigned most of the electro-

magnetic frequency spectrum over the years, and most of thatis actively used.



56 subcarriers

20-MHz channel

312.5-kHzsubcarrierspacing

312.5-kHzsubcarrierbandwidth

Each subcarriermodulated byBPSK, QPSK,16QAM, or 64QAM

6. In the OFDM signal for the IEEE 802.11n Wi-Fi standard, 56 subcarriers

are spaced 312.5 kHz in a 20-MHz channel. Data rates to 300 Mbits/s can

be achieved with 64QAM.

BER

103

104

105

106

107

108

1096 10 14 18 22 26 30

CNR (dB)

BPSK

QPSKQPSK

8QAM8QAM

8PSK

16QAM16QAM

64QAM64QAM

7. This is a comparison of several popular modulation methods and their

spectral efficiency expressed in terms of BER versus CNR. Note that for a

given BER, a greater CNR is needed for the higher QAM levels.



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Shortages now exist in the cellular and

land mobile radio sectors, inhibiting the

expansion of services such as high data

speeds as well as the addition of new sub-

scribers. One approach to the problem

is to improve the efficiency of usage by

squeezing more users into the same or

less spectrum and achieving higher data

rates. Improved modulation and access

methods can help.

One of the most crowded areas of spec-

trum is the land mobile radio (LMR) and

private mobile radio (PMR) spectrum

used by the federal and state governments

and local public safety agencies like fire

and police departments. Currently theyre

assigned spectrum by FCC license in the

150- to 174-MHz VHF spectrum and the421- to 512-MHz UHF spectrum.

Most radio systems and handsets use

FM analog modulation that occupies a

25-kHz channel. Recently the FCC has

required all such radios to switch over

to 12.5-kHz channels. This conversion,

known as narrowbanding, doubles the

number available channels.

Narrowbanding is expected to improve

a radios ability to get access to a chan-

nel. It also means that more radios can

be added to the system. This conversion

must take place before January 1, 2013.Otherwise, an agency or business could

lose its license or be fined. This switcho-

ver will be expensive as new radio sys-

tems and handsets are required.

In the future, the FCC is expected to

mandate a further change from the 12.5-

kHz channels to 6.25-kHz channels,

again doubling capacity without increas-

ing the amount of spectrum assigned. No

date for that change has been assigned.

The new equipment can use either ana-

log or digital modulation. It is possible

to put standard analog FM in a 12.5-kHz

channel by adjusting the modulation

index and using other bandwidth-nar-

rowing techniques. However, analog FM

in a 6.25-kHz channel is unworkable, so

a digital technique must be used.

Digital methods digitize the voice sig-

nal and use compression techniques to

produce a very low-rate serial digital sig-

nal that can be modulated into a narrow

band. Such digital modulation techniques

are expected to meet the narrowbanding

goal and provide some additional perfor-mance advantages.

New modulation techniques and pro-

tocolsincluding P25, TETRA, DMR,

dPMR, and NXDNhave been devel-

oped to meet this need. All of these new

methods must meet the requirements of

the FCCs Part 90 regulations and/or the

regulations of the European Telecommu-

nications Standards Institute (ETSI) stan-

dards such as TS-102 490 and TS-102-

658 for LMR.

The most popular digital LMR tech-

nology, P25, is already in wide use in

the U.S. with 12.5-kHz channels. Its fre-

quency division multiple access (FDMA)

method divides the assigned spectrum

into 6.25-kHz or 12.5-kHz channels.

Phase I of the P25 project uses a

four-symbol FSK (4FSK) modulation.Standard FSK, covered earlier, uses two

frequencies or tones to achieve 1 bit/

Hz. However, 4FSK is a variant that

uses four frequencies to provide 2-bit/

Hz efficiency. With this scheme, the stan-

dard achieves a 9600-bit/s data rate in a

12.5-kHz channel. With 4FSK, the car-

rier frequency is shifted by 1.8 kHz or

600 Hz to achieve the four symbols.

In Phase 2, a compatible QPSK modu-

lation scheme is used to achieve a simi-

lar data rate in a 6.25-kHz channel. The

phase is shifted either 45 or 135 toget the four symbols. A unique demodu-

lator has been developed to detect either

the 4FSK or QPSK signal to recover the

digital voice. Only different modulators

on the transmit end are needed to make

the transition from Phase 1 to Phase 2.

The most widespread digital LMR

technology outside of the U.S. is TET-

RA, or Terrestrial Trunked Radio. This

ETSI standard is universally used in

Europe as well as in Africa, Asia, and

Latin America. Its time division multiple

access (TDMA) approach multiplexes

four digital voice or data signals into a

25-kHz channel.

A single channel is used to support a

digital stream of four time slots for the

digital data for each subscriber. This is

equivalent to four independent signals in

adjacent 6.25-kHz channels. The modu-

lation is /4-DQPSK, and the data rate is

7.2 kbits/s per time slot.

Another ETSI standard, digital mobile

radio (DMR), uses a 4FSK modulation

scheme in a 12.5-kHz channel. It canachieve a 6.25-kHz channel equivalent

02.09.12 ELECTRONIC DESIGN


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ELECTRONIC DESIGNGO TO WWW.ELECTRONICDESIGN.COM

in a 12.5-kHz channel by using two-slot

TDMA. The voice is digitally coded with

error correction, and the basic rate is 3.6

kbits/s. The data rate in the 12.5-kHz

band is 9600 kbits/s.

A similar technology is dPMR, or digi-

tal private mobile radio standard. This

ETSI standard also uses a 4FSK modula-

tion scheme, but the access is FDMA in

6.25-kHz channels. The voice coding rate

is also 3.6 kbits/s with error correction.

LMR manufacturers Icom and Ken-

wood have developed NXDN, another

standard for LMR. It is designed to oper-

ate in either 12.5- or 6.25-kHz channels

using digital voice compression and a

four-symbol FSK system. A channel may

be selected to carry voice or data.The basic data rate is 4800 bits/s. The

access method is FDMA. NXDN and

dPMR are similar, as they both use 4FSK

and FDMA in 6.25-kHz channels. The

two methods are not compatible, though,

as the data protocols and other features

are not the same.

Because all of these digital techniques

are similar and operate in standard fre-

quency ranges, Freescale Semiconductor

was able to make a single-chip digital

radio that includes the RF transceiver

plus an ARM9 processor that can be pro-grammed to handle any of the digital

standards. The MC13260 system-on-

a-chip (SoC) can form the basis of a

handset radio for any one if not multiple

protocols. For more, see Chip Makes

Two-Way Radio Easy at http://electron-

icdesign.com/article/communications/

Chip-Makes-Two-Way-Radio-Easy.aspx.

Another example of modulation tech-

niques improving spectral efficiency and

increasing data throughput in a given

channel is a new technique from Novel-

Sat called NS3 modulation. Satellites are

positioned in an orbit around the equator

about 22,300 miles from earth. This is

called the geostationary orbit, and satel-

lites in it rotate in synchronization with

the earth so they appear fixed in place,

making them a good signal relay plat-

form from one place to another on earth.

Satellites carry several transponders

that pick up the weak uplink signal from

earth and retransmit it on a different fre-

quency. These transponders are linear

and have a fixed bandwidth, typically 36MHz. Some of the newer satellites have

72-MHz channel transponders. With a

fixed bandwidth, the data rate is some-

what fixed as determined by the modula-

tion scheme and access methods.

The question is how one deals with the

need to increase the data rate in a remote

satellite as required by the ever increas-

ing demand for more traffic capacity.

The answer lies in simply creating and

implementing a more spectrally efficient

modulation method. Thats what Nov-

elSat did. Its NS3 modulation method

increases bandwidth capacity up to 78%.

That level of improvement comes from

a revised version of APSK modulation

covered earlier. One commonly used sat-

ellite transmission standard, DVB-S2, is

a single carrier (typically L-band, 950 to1750 MHz) that can use QPSK, 8PSK,

16APSK, and 32APSK modulation with

different forward error correction (FEC)

schemes. The most common application

is video transmission.

NS3 improves on DVB-S2 by offer-

ing 64APSK with multiple amplitude

and phase symbols to improve efficien-

cy. Also included is low density parity

check (LDPC) coding. This combination

provides a maximum data rate of 358

Mbits/s in a 72-MHz transponder.

Because the modulation is APSK, theTWT PAs dont have to be backed off to

preserve perfect linearity. As a result, they

can operate at a higher power level and

achieve the higher data rate with a lower

CNR than DVB-S2. NovelSat offers its

NS1000 modulator and NS2000 demod-

ulator units to upgrade satellite systems

to NS3. In most applications, NS3 pro-

vides a data rate boost over DVB-S2 for

a given CNR.

ACKNOWLEDGMENT

Special thanks to marketing director

Debbie Greenstreet and technical mar-

keting manager Zhihong Lin at Texas

Instruments as well as David Fursten-

berg, chairman of NovelSat, for their

help with this article.


MORE FROM LOU FRENZEL

SEE MORE communications coveragefrom Lou at http://electronicdesign.

com/author/1843/LouisEFrenzel.


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FACTORPFC INTO YOUR

POWER-SUPPLYDESIGN

SAM DAVIS |CONTRIBUTING EDITOR [email protected]

Before the latest IEC61000-3-2 standard took effect in 2005, most power

supplies for PCs, monitors, and TVs generated excessive line harmonics

when operating from single-phase, 110- to 120-V, 60-Hz ac. Spurred on by

this newer and stricter IEC standard, power-supply manufacturers aim to

minimize power-line harmonics by adding power factor correction (PFC).

To understand the impact of IEC61000-3-2, its best to first look at the

ideal situation, which places a load resistor (R) directly across the power line (Fig. 1).

Here, the sinusoidal line current, IAC, is directly proportional to and in phase with the

line voltage VAC. Therefore:

I (t) =V (t)

R (1)

This means that for the most efficient and distortion-free power-line operation, all

loads should behave as an effective resistance (R), whereby the power used and deliv-

ered is the product of the RMS line voltage and line current.


St r ic ter gu ide l ines

imposed by version

3 of the IEC standard

for harmonic current

emissions push design-

ers to embrace power-

factor-correction meth-

odologies.


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However, loads for many electronic systems require an ac-

to-dc conversion. In this case, the load on the power line from

a typical power supply consists of a diode bridge driving a

capacitor (Fig. 2).

Its a nonlinear load for the power line because two diodesof the bridge rectifier lie in the direct power path for either the

positive or negative half-cycle of the input ac line voltage. This

nonlinear load draws line current only during the peak of the

sinusoidal line voltage, resulting in the peaky input line cur-

rent that causes line harmonics (Fig. 3).

A nonlinear load causes harmonics comparable in magni-

tude to the fundamental harmonic current at l ine frequency.

Figure 4 shows the magnitude of higher-order harmonics cur-

rents normalized with respect to the magnitude of the funda-

mental harmonic at line frequency.

However, only the harmonic current at the same frequency

as the line frequency and in phase with the line voltage (in this

case, the fundamental harmonic at line frequency) given in

Figure 1 contributes to the average power delivered to the load.

These harmonics currents can affect operation of other equip-

ment on the same utility line.

The magnitude of line harmonics depends on a power sup-

plys power factor, which varies from 0 to 1. A low power-factor

value causes higher harmonics, while a high power-factor value

produces lower harmonics. Power factor (PF) is defined as:

PF =P

I V (2)

where P = real power in watts; IRMS= RMS line current; VRMS

= RMS line voltage; and VRMS

IRMS= apparent power involt-amperes (VA). PF also equals the cosine of the phase

angle () between line current and voltage; in that regard,

Equation 2 can be rewritten as:

P = (I V )cos (3)

The value of cosis a number between 0 and 1.

If = 0, cos= 1 and P = IRMSVRMS, which is the same

as for a resistor load. When the PF is 1, the load consumes all

of the energy supplied by the source.

If = 90, then cos= 0; therefore, the load receives zero

power. The generator thats providing the power must deliver

IRMSVRMSpower, even though no power is used for useful

work.

Thus, for the diode bridge-capacitor case in Figure 2, the

only variable left in the PF definition of Equation 2 is the line

current IRMS, since line voltage (VRMS) is fixed by power-line

generators to 120 V. The higher the IRMSthe power line drawsfor the given average power delivered to the load, the lower the

power factor (PF).

The ac-dc converter in Figure 2, which operates from 120-V

ac line voltage and delivers 600 W to the load while drawing

10 A of the line current, has a PF = 0.5. However, Figure 1s

resistive load with a PF of 1, which draws 600 W from the 120-

V ac line, draws only 5 A from the line.

The electric utility suffers from low PF loads because it

must provide higher generating capability to support demands

for increased line current due to poor load PF. Nonetheless, it

charges the user only for delivery of average power in watts

not the generation of volt-amperes.

This difference between volt-amperes and watts eitherappears as heat or is reflected back to the ac power line. The

most common means of correcting this condition is to employ

power factor correction.

POWER-FACTOR CORRECTION

The IEC-61000-3-2 standard defines the maximum har-

monic current allowed for a given power level. Initial versions

of the standard in 1995 and 2001 were changed by the 2005

Edition 3. It imposed stricter requirements on power-line

harmonic currents for (Class D) PCs, monitors, and TVs con-

suming between 75 and 600 W and16 A per phase. To meet

those requirements, designers must employ active power-

factor correction (PFC) in Class D power supplies.

Many PFC circuits employ a boost converter. One limitation

in the conventional

boost PFC converter

is that it can operate

only from the recti-

fied ac line, which

involves two-stage

V

R

(a) (b)

VAC

IACVAC, IAC

IAC

VAC

t

1. With a resistive load on the power line (a), line current is proportional

and in phase with the line voltage (b).

C

+

V

IAC

VAC

VAC

2. A diode bridge and capacitor across the power line results in a nonlin-

ear load.

IAC

3. Line current is

peaky and out of

phase with the diode

bridge-capacitor loadsline voltage.



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power processing (Fig. 5). Waveforms generated by the con-

verter better illustrate this problem (Fig. 6). In addition, theres

no simple and effective way to introduce isolation in a conven-

tional boost converter.

Using a full-bridge extension of the boost converter, which

is then controlled as a PFC converter, is one way to introduce

isolation (Fig. 7). However, this adds the complexity of fourtransistors on the primary side and four diode rectifiers on the

secondary, both operating at the switching frequency of, say,

100 kHz. Plus, four more diodes are in the input bridge rectifier

operating at the line frequency of 50/60 Hz.

Besides low-frequency sinusoidal current, the line current

will have superimposed input inductor ripple current at the

high switching frequency, which needs to be filtered out by an

additional high-frequency filter on the ac line. The presence

of 12 switches operating in the hard-switching mode results

in high conduction and switching losses. The best efficiency

reported for this two-stage approach and its supplementary

switching devices is 87%.

Such a method also suffers from the startup problem due tostep-up dc conversion gain. It needs additional circuitry to pre-

charge the output capacitor so the converter can start up.

To achieve 1 kW or higher power, designers often employ

a three-stage approach (Fig. 8). Here, the standard boost PFC

converter and an isolated step-down converter follow the inputs

bridge rectifier. This requires a total of 14 switches. At least six

of those switches are high voltage, further decreasing efficien-

cy and increasing the cost. Still, with the

highest efficiency based on best present

switching devices reaching about 90%,

its better than the two-stage approach.

For medium and low power, theres an alternative approach

that reduces the amount of switches by using a forward con-

verter for the isolation stage (Fig. 9). Before going this route,

one must be aware that although there are now 10 switches, the

four switching devices in the forward converter impose greater

voltage stresses on both primary and secondary side switches

than the full-bridge solution. In addition, the full-bridge solu-tion requires four magnetic components.

BRIDGELESS PFC CONVERTER

Breaking new ground in this arena, Dr. Slobodan Cuk, presi-

dent of Teslaco, developed a bridgeless PFC converter (patent

pending) that operates directly from the ac line. Its claimed to

be the first true single-stage bridgeless ac-dc PFC converter.

To accomplish this feat, Cuk employs a new switching pow-

er-conversion method, termed hybrid-switching. It employs

a converter topology consisting of only three switches: one

controllable switch S and two passive current rectifier switches

(CR1 and CR2) (Fig. 10).

The two rectifiers turn on and off in response to the state ofthe main switch (S) for either positive or negative polarity of

the input ac voltage. This topology consists of an inductor in

series with the input, the floating energy-transferring capacitor

that acts as a resonant capacitor for the part of the switching

cycle, and a resonant inductor.

Because the conventional converters based on PWM square-

wave switching use inductors and capacitors, they require



C

+

VAC

VR

IR

O

VAC

S

97% 97% = 94%

L CR

Harmonic number(f = fundamental)

1.0

0.8

0.6

0.40.2

0Harmo

nicamp

litude

(nor

ma

lize

dto

fun

damental)

1 5 9 13 17 21 25 29

4. Peaky line

current generates

current harmonics

comparable in mag-

nitude to the fun-

damental harmonic

current at the line

frequency. 5. Two-stage power processing is required in this simplified conventional

PFC boost converter.

VR

IR

6. Shown are voltage and current waveforms

from conventional PFC boost converter.

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