DESIGN IDEAS
`ADSL Line Driver/Receiver
`Design Guide, Part 1
`
`by Tim Regan
`
`Introduction
`Consumer desire for faster Internet
`access is driving the demand for very
`high data rate modems. A digital sub-
`scriber line (DSL) implementation
`speeds data to and from remote serv-
`ers with data rates of 512Kbps to
`8Mbps, much faster than current
`56Kbps modem alternatives. This
`speed of data communication is pro-
`viding the Internet with the capability
`to transfer information in new for-
`mats such as full-motion video, while
`
`greatly improving the timeliness of
`conventional information access.
`One very important feature of DSL
`technology is that the connection is
`handled through a normal telephone
`line; therefore, no special high speed
`cables or fiber optic links are required
`and every home and office is most
`likely DSL ready. Another feature is
`that the data interface can operate
`simultaneously with normal voice
`communication over the same tele-
`
`phone line. This allows the modem to
`be connected at all times and not
`interfere with the use of the same line
`for normal incoming and outgoing
`phone calls or faxes.
`The real “magic” of DSL technology
`stems from the application of digital
`signal processing (DSP) algorithms
`and data coding schemes. The imple-
`mentations have built-in intelligence
`to accommodate the wide variations
`of data transmission signal conditions
`
`15V
`
`RBT 12.4Ω
`
`1:2
`
`4
`
`6,7
`
`MIDCOM
`50215
`2
`
`9,10
`
`100Ω
`PHONE
`LINE
`
`2, 19 10
`SHDN
`3
`
`1/2 LT1795
`
`–+
`
`RBT 12.4Ω
`
`18
`11
`SHDNREF
`RADJ
`64.9k
`
`3
`
`2
`
`6
`
`5
`
`2R 2k
`
`R 1k
`
`2R 2k
`
`R 1k
`
`POSITIVE SUPPLY
`
`15V
`
`0.1µF
`
`0.1µF
`
`–15V
`
`+
`
`+
`
`10µF
`+
`
`10µF
`
`10µF
`
`NEGATIVE SUPPLY
`
`–+
`
`1/2 LT1361
`
`RFRX 3k
`
`RFRX 3k
`
`– +
`
`4
`
`–15V
`
`1/2 LT1361
`
`5V OR 3.3V
`
`AFE
`
`Tx FILTER
`
`Rx FILTER
`
`9
`
`8
`
`RG
`2k
`
`13
`
`12
`
`1
`
`7
`
`RF 1k
`
`RF 1k
`
`– +
`
`1/2 LT1795
`
`4–7,
`14–17
`–15V
`
`15V
`
`8
`
`26
`
`Linear Technology Magazine • February 2000
`
`Figure 1. Central-office ADSL transceiver
`
`TQ Delta Exhibit 2009
`Cisco Systems, Inc. v. TQ Delta LLC
`IPR2016-01021
`
`1
`
`

`

`DESIGN IDEAS
`
`EDOC
`I
`Q
`00
`00
`00
`10
`00
`01
`00
`11
`10
`00
`10
`10
`10
`01
`10
`11
`01
`00
`01
`10
`01
`01
`01
`11
`11
`00
`11
`10
`11
`01
`11
`11
`
`SLEVEL
`)BSM2(I
`)BSL2(Q
`3–
`3–
`3–
`1–
`3–
`1
`3–
`3
`1–
`3–
`1–
`1–
`1–
`1
`1–
`3
`1
`3–
`1
`1–
`1
`1
`1
`3
`3
`3–
`3
`1–
`3
`1
`3
`3
`
`ROTCEV
`GAM
`2.4
`2.3
`2.3
`2.4
`2.3
`4.1
`4.1
`2.3
`2.3
`4.1
`4.1
`2.3
`2.4
`2.3
`2.3
`2.4
`
` E (°)LGNA
`
`522
`891
`261
`531
`252
`522
`531
`801
`882
`513
`54
`27
`513
`243
`81
`54
`
`SYMBOL RATE
`(SYMBOLS/s)
`
`IN-PHASE
`DATA
`fB/2
`
`AMPLITUDE
`CONVERTER
`
`INCOMMING
`DATA
`BIT RATE fB
`(BITS/s)
`
`BIT-RATE
`DIVIDER
`
`TONE
`(CARRIRER)
`INPUT
`
`PHASE
`SPLITTER
`
`QAM
`OUTPUT
`SIGNAL
`
`QUADRATURE
`(90° PHASE-SHIFTED)
`DATA fB/2
`
`AMPLITUDE
`CONVERTER
`
`SYMBOL RATE
`(SYMBOLS/s)
`
`I0
`
`°, 360°
`
`11
`
`45°
`
`10
`
`Q 90°
`
`11
`
`10
`
`180°
`
`00
`
`01
`01
`
`00
`
`270°
`
`encountered with each connection
`through the telephone switching net-
`work. Sophisticated ASICs have been
`developed to provide small modems
`for PCs and handheld devices and the
`ability to compact many DSL lines on
`a single PCB card for telephone cen-
`tral-office deployment.
`However, as is the case with almost
`any system, DSL still requires funda-
`mental operational amplifier functions
`to put the signal on to the phone line
`and to pick off the small signals
`received at the other end. Although
`many system designers are compe-
`tent and comfortable with DSP and
`all things digital, they often find their
`understanding of analog issues to be
`a bit rusty when it comes to imple-
`menting the physical connection to
`and from the telephone line. This
`series of articles will provide an over-
`view of the requirements placed on
`the amplifiers and provide guidelines
`to component selection and the impli-
`cations on distortion performance and
`power consumption and dissipation,
`the most important system issues
`related to the analog components.
`Figure 1 shows a complete central
`office DSL line driver/receiver. This is
`the basic circuit topology that pro-
`vides differential transmit signal drive
`to the line and detection of the differ-
`ential received signal. The full
`requirements of DSL are easily met by
`using devices from Linear Tech-
`
`Linear Technology Magazine • February 2000
`
`Figure 2. Quadrature amplitude modulation
`nology’s broad line of high speed power
`amplifiers for the driver and high
`speed, low noise dual amplifiers for
`the receiver. Using either current feed-
`back or voltage feedback topologies,
`the family of drivers consists of
`amplifiers with bandwidths from
`35MHz to 75MHz, slew rates in excess
`of 200V/m s with output current
`capability from 125mA to over 1 amp.
`The receiver family combines similar
`high speed performance with low
`noise, less than 10nV/(cid:214) Hz, and low
`quiescent operating current, less than
`10mA. The devices shown in Figure 1
`are the LT1795 500mA output cur-
`rent, 50MHz bandwidth dual op amp
`and the LT1361 50MHz dual ampli-
`fier with input noise voltage of
`9nV/(cid:214) Hz and total supply current of
`only 10mA.
`Although there are several varia-
`tions of DSL technology (SDSL, HDSL,
`HDSL2, VDSL and ADSL, to name a
`few) the requirements placed on the
`amplifiers for these different stan-
`dards are very similar. The major
`difference between the approaches,
`as they affect the line driver, is the
`amount of power actually put on to
`the phone line by the line-driver
`amplifier. For simplicity, these articles
`will focus on the most recently
`approved standard, ADSL (asymmet-
`ric DSL), but the concepts discussed
`apply equally to any of the other
`standards.
`
`This first installment will provide
`an overview of the requirements of
`ADSL and how it is done, as well as a
`discussion of the circuit topology and
`the requirements for the components
`used for implementation.
`The Requirements for ADSL
`The full specifications for ADSL are
`contained in two ITU (International
`Telecommunications Union) docu-
`ments called G.992.1, for systems
`often referred to as Full-Rate ADSL or
`G.dmt, and G.992.2, a lower data rate
`approach often called G.Lite. Both
`systems use a technique called
`discrete multitone, or DMT, for trans-
`mitting data. With DMT, a frequency
`band up to 1.2MHz is split up into
`256 separate tones (also call sub-
`carriers) each spaced 4.3125kHz
`apart. With each tone carrying sepa-
`rate data, the technique operates as if
`256 separate modems were running
`in parallel. To further increase the
`data transmission rate, each indi-
`vidual tone is quadrature amplitude
`modulated (QAM). As shown in Fig-
`ure 2, the data to be transmitted is
`used to create a unique amplitude
`and phase-shift characteristic for each
`carrier tone, through the combina-
`tion of I and Q data, called a symbol.
`The symbols represented by each tone
`are updated at a 4kHz rate or 4000
`symbols per second. Full Rate ADSL
`uses up to 15 bits of data to create
`
`27
`
`2
`
`S
`

`

`DESIGN IDEAS
`
`–37dBm/Hz
`
`–40dBm/Hz
`
`POTS
`
`UPSTREAM
`
`DOWNSTREAM
`
`26kHz
`
`130kHz
`
`134kHz
`
`552kHz
`
`1.104MHz
`
`BOTH
`
`G.LITE
`
`FULL RATE
`
`Figure 3. DMT channel allocation
`
`route to and from the central office.
`The total power required can be
`determined from the following
`equation:
`LINE POWER (dBm) = PSD (dBm/Hz)
`+ 10 • Log(FMAX – FMIN)
`The downstream power require-
`ments are much higher than the
`upstream requirements because of
`the wider bandwidth used for the
`transmission. For this reason, Full
`Rate ADSL requires more line power
`than G.Lite for downstream trans-
`missions. Upstream power is the same
`for both Full Rate and G.Lite
`
`each symbol. This results in a theo-
`retical maximum of 60Kb/s for each
`tone. If all 256 tones are used in
`parallel, the total theoretical data rate
`can be as fast as 15.36Mb/s. For
`G.Lite, only 8 bits are used per sym-
`bol with only half of the carrier tones
`used for a theoretical maximum data
`rate of 4.096Mb/s.
`In an actual DSL application, the
`tones are allocated for use depending
`on the direction of communication,
`as shown in Figure 3. Most of the
`tones are used for communication
`from the central office (CO) to an end
`user’s PC modem (often referred to as
`the CPE or customer premises
`equipment). This direction of com-
`munication is called “downstream.”
`The direction of communication from
`a PC modem to the central office (and,
`ultimately, to an Internet server) is
`called “upstream.” The use of more
`tones for the downstream direction
`makes sense from an Internet-access
`point of view, because most users
`download more information than they
`upload. Most upstream communica-
`tion with a server is simply to request
`information to be sent quickly down-
`stream. This difference in data rates
`up- and downstream is the reason
`ADSL is called asymmetric DSL.
`Also indicated in Figure 3 is the
`power spectral density (PSD) of all of
`the tones used. This determines the
`amount of signal power that needs to
`be put on to the phone line. The power
`levels are restricted to minimize cross-
`talk and interference into other phone
`lines contained in wire bundles en
`
`Table 1. ADSL requirements
`
`scitsiretcarahC
`
`lacirtcelE
`stnemeriuqeR
`
`retemaraP
`desUslennahC
`dnaBycneuqerF
`)zHk(
`)zHk(htdiwdnaB
`lartcepSrewoP
`DSP,ytisneD
`)zH/mBd(
`rewoPeniL
`)mBd(
`eniLSMR
`)Wm(rewoP
`ecnadepmIeniL
`)Ω(
`eniLSMR
`)V(egatloV
`eniLSMR
`)Am(tnerruC
`egarevA-ot-kaeP
`RAP,oitaR
`eniLkaeP
`)V(egatloV
`kaeP-ot-kaeP
`)V(egatloVeniL
`eniLkaeP
`)Am(tnerruC
`eniLkaeP
`)Wm(rewoP
`
`ataDlaciteroehT
`setaR
`
`lobmyS/stiB
`
`lennahC/stiB
`)s/stiBK(
`etaRataDxaM
`slennahCrof
`desU
`
`LSDAetaRlluF
`maertsnwoD
`652ot13
`
`etiL.GLSDA
`maertsnwoD
`821ot13
`
`LSDAetaRlluF
`etiL.Gro
`maertspU
`03ot6
`
`4011ot7.331
`
`255ot7.331
`
`4.921ot8.52
`
`3.079
`
`3.814
`
`5.301
`
`04–
`
`02
`
`001
`
`001
`
`1.3
`
`13
`
`3.5
`
`5.61
`
`33
`
`071
`
`5272
`
`51
`
`06
`
`0–4
`
`3.61
`
`34
`
`001
`
`2
`
`12
`
`3.5
`
`11
`
`22
`
`011
`
`5711
`
`8
`
`23
`
`s/bM5.31
`
`s/bM1.3
`
`73–
`
`31
`
`02
`
`001
`
`4.1
`
`51
`
`3.5
`
`6.7
`
`2.51
`
`67
`
`085
`
`)lluF(51
`)etiL.G(8
`)lluF(06
`)etiL.G(23
`
`)lluF(s/bM4.1
`)etiL.G(s/bK867
`
`28
`
`Linear Technology Magazine • February 2000
`
`3
`
`

`

`DESIGN IDEAS
`
`which channels are best suited for
`use. The DSP algorithms will auto-
`matically pack the most data into the
`best transmission channels to maxi-
`mize the data rate for a particular
`connection. Figure 4 illustrates a typi-
`cal line spectrum during a training-up
`interval in a G.Lite example, as mea-
`sured at the central office end.
`A Typical ADSL Line Driver/
`Receiver Circuit
`Referring to Figure 1, the compo-
`nents shown will implement a Full
`Rate ADSL central office (downstream)
`port. A discussion of the circuit topol-
`ogy and aspects important for
`component selection follow.
`Transformer Coupling
`A transformer is used to connect the
`transceiver to the phone line, mainly
`to provide isolation from the line. The
`turns ratio of the transformer can be
`used to provide gain to the transmit-
`ted signal. This turns ratio has a
`major effect on the power supply volt-
`ages for the line-driver amplifiers. By
`stepping up the signal from the driver
`to the line via the transformer, the
`amount of voltage swing needed by
`the amplifiers is reduced. As an ideal
`transformer has equal power in the
`primary and secondary, while the
`voltage is stepped up, the current is
`stepped down. The consequences of
`using a step-up transformer are ben-
`eficial in that lower, more conventional
`supply voltages can be used, but the
`amplifiers must have higher current
`driving capability.
`The limit on the turns ratio is
`primarily a function of the sensitivity
`of the receive circuitry. Step-up
`transformers will, unfortunately, step-
`down the signal received from the
`phone line. Further attenuation of
`the received signal by the transformer
`in addition to the inherent transmis-
`sion line attenuation can cause the
`receiver to stop functioning. If this
`occurs, the modem will disconnect
`from the line.
`A transformer should be selected
`for a flat, distortion-free frequency
`response from 20kHz to 2MHz to cover
`the full frequency spectrum for an
`
`29
`
`Although the data rates shown in
`Table 1 are impressively fast, they
`are, indeed, theoretical. In an actual
`connection over the phone line, all
`manner of interference sources will
`alter the frequency response over the
`1.2MHz band. These interference
`sources can contaminate or attenu-
`ate many of the carrier tones to render
`them completely unusable, or useful
`but with less than the maximum pos-
`sible number of data bits encoded.
`Additionally, higher frequency tones
`are attenuated more than the lower
`ones, particularly over longer lengths
`of phone line used to make the
`connection.
`Another issue that can render par-
`ticular tones unusable or create
`transmission errors is distortion from
`the amplifier driving the line. Distor-
`tion products, whether harmonic,
`intermodulation or from signal clip-
`ping, from any of the carrier tones,
`create signal energy in the frequency
`spaces used by other tones. This
`energy also contaminates the data
`content of the tones and can result in
`fewer tones being used for data trans-
`mission. If many tones are unusable
`or their data handling capability is
`reduced, the actual data rate for any
`given connection can be significantly
`less than the theoretical maximum.
`One of the best features of a DSL
`modem is the intelligence built in to
`obtain the fastest data rate for any set
`of line conditions. When a connection
`between a modem and the telephone
`central office is initiated, the first
`action to occur is called “training-
`up.” During this interval, both ends
`transmit maximum power in each
`channel in an effort to determine
`
`–20dBm
`
`10dBm/DIV
`
`–120dBm
`0Hz
`
`56kHz/DIV
`
`560kHz
`
`Figure 4. G.Lite training-up spectrum
`
`implementations. As will be seen, the
`line power requirement is the most
`significant factor in designing a line
`driver for a particular application.
`Table 1 is a summary of the char-
`acteristics, electrical requirements
`and maximum data rates for ADSL
`modems.
`The following are important items
`to note:
`The phone line characteristic
`impedance for ADSL is 100W
`. This is
`used to determine the voltage and
`current required to provide the proper
`line-power level.
`The term PAR stands for peak-to-
`average ratio. This term is similar to
`the more common term of crest fac-
`tor. This determines the peak value of
`the voltage put on the line over time
`with respect to the RMS voltage level:
`VPEAK = PAR • VRMS
`The DMT signal placed on the line
`looks basically like white noise,
`because many different frequencies
`of rapidly changing amplitude and
`phase are combined simultaneously.
`The changes of each tone are consid-
`ered random as they result from an
`arbitrary sequence of data bits com-
`prising the transmitted information.
`Over time, the signals can align and
`stack up to create a large peak signal.
`If this large peak is not processed
`cleanly (for example, if the line-driver
`amplifier clips) data errors can occur,
`which must be detected and resent.
`Transmission errors, particularly over
`a noisy environment such as phone
`lines, are inevitable. These errors are
`identified by a term called the bit-
`error rate (BER); an acceptable level
`to maintain fast and accurate data
`transmission is one error per every
`107 symbols. The PAR is determined
`by the probability of the random line
`signal reaching a certain peak voltage
`during the time interval required for
`107 symbols. For the DMT signal, this
`peak value is 5.3 times the RMS sig-
`nal level. This factor is very important
`in determining both the minimum
`supply voltage required to prevent
`clipping of the signal and also the
`peak output current capability of the
`line driver.
`
`Linear Technology Magazine • February 2000
`
`4
`
`

`

`DESIGN IDEAS
`
`+
`VO
`22VP-P
`
`RBT
`
`11VP-P
`
`1:1
`
`VO DIFF
`
`22VP-P
`
`ZLINE
`22VP-P
`
`11V
`
`A1
`
`–11V
`
`RF
`
`RF
`
`11V
`
`–+
`
`RG
`
`+EIN
`
`44VP-P
`
`RBT
`
`1:1
`
`22VP-P
`
`ZLINE
`22VP-P
`
`22V
`
`A1
`
`–22V
`
`R4
`
`–+
`
`R2
`
`R1
`
`R3
`
`+EIN
`
`–EIN
`
`Figure 5a. A single-ended driver requires a high supply voltage to produce
`the desired peak-to-peak swing of the DMT signal on the phone line.
`
`RBT
`
`_
`VO
`22VP-P
`
`A2
`
`– +
`
`–EIN
`
`RPRIMARY =
`
`RECEIVED PRIMARY POWER =
`2
`eRX
`
`2
`
`ADSL transmission. Minimal inser-
`tion loss in the transformer over the
`same frequency range is also desir-
`able. Insertion loss, usually specified
`in dBm, is power lost in the trans-
`former. The driver amplifier must
`provide this additional power in order
`to maintain the required signal power
`level on the phone line.
`Transformer
`Termination Resistors
`The two resistors (called back-termi-
`nation resistors) shown between the
`amplifier outputs and the primary of
`the transformer are inserted for two
`reasons: to provide a means for
`detecting the received signal and to
`make the impedance of the modem
`match the impedance of the phone
`line. The receiver circuit is two differ-
`ence amplifiers that provide gain to
`the small signals that appear across
`the termination resistors. The con-
`nection and scaling of the input
`resistors to the receiver amplifiers are
`purposely set to provide a first-order
`cancellation of the simultaneously
`occurring transmit signal. This
`technique is called “echo cancellation”
`and the circuit topology is called a “2-
`wire to 4-wire hybrid” (the 2-wire
`phone line interfaces with four wires,
`the two differential driver lines and
`the two receive signal lines). The can-
`cellation of the transmitted signal from
`the received signal path is not perfect.
`Due to signal phase shifts and resis-
`tor mismatching, a factor of 6dB to
`20dB of attenuation is typical, with
`higher frequencies being cancelled
`less. The amount of transmitted sig-
`nal that remains is cancelled digitally
`by DSP echo-canceling algorithms.
`The value of the termination resis-
`tors is a function of the line impedance
`
`30
`
`and the transformer turns
`ratio. The turns ratio, n, is
`defined by the number of
`turns of the winding con-
`nected to the phone line
`–11V
`(the secondary) divided by
`Figure 5b. A differential driver achieves the same swing
`with half the supply voltage of the single-ended driver.
`the number of turns of the
`proper value, one-half of the power
`driver side winding (the primary). To
`delivered by the amplifiers is dissi-
`make the modem impedance match
`pated in these resistors. To deliver
`the line impedance, the total imped-
`100mW of signal power to the phone
`ance across the primary winding is
`line, for example, requires the driver
`deter mined by the following
`amplifiers to output at least 200mW
`relationship:
`of power.
`ZLINE
`Why Differential Drive?
`ni2
`Two amplifiers configured as a differ-
`To provide balanced drive to the
`ential gain stage are typically used to
`primary of the transformer, so that
`provide signal drive to the primary of
`each power amplifier shares the work
`the transformer. There are two rea-
`load evenly, each termination resis-
`sons for this configuration: it reduces
`tor is set to a value of one-half of
`the supply voltage to the amplifiers by
`RPRIMARY.
`a factor of two and also cancels any
`This value of termination resis-
`even harmonic distortion nonlinear-
`tance on the primary is also optimal
`ity contributed by the amplifiers.
`for receiving maximum power from
`With single-ended drive of the pri-
`the line. The received signal on the
`mary, the supply voltage for the
`phone line, eRX, driving the secondary
`amplifier must be large enough to
`through the line impedance, ZLINE
`(nominally 100W
`provide the full peak-to-peak signal
`) will develop signal
`swing of the DMT signal placed on to
`power in the primary per the follow-
`the phone line. With differential drive,
`ing relationship:
`each amplifier contributes just one-
`half of the peak signal amplitude;
`therefore, the total supply voltage is
`only one half the peak-to-peak volt-
`age level placed on the line. This is
`shown conceptually in Figure 5. This
`reduction in supply voltage allows
`the use of the standard power supply
`voltages available in computers for
`the high speed DSL modem card.
`A differential amplifier will ideally
`cancel all even harmonic distortion
`products. This is due to the applica-
`tion of a signal that is the difference
`between two signals, one signal being
`
`+ 2 • ZLINE + ni2 • RPRIMARY
`
`RPRIMARY =
`
`ZLINE
`ni2 • RPRIMARY
`which is also at a maximum when
`ZLINE
`ni2
`While the termination resistors
`serve an important purpose, they also
`create significant signal and power
`loss. With the resistors set to their
`
`Linear Technology Magazine • February 2000
`
`5
`
`

`

`DESIGN IDEAS
`
`2-BIT THRESHOLD (DMT)
`–140dBm/Hz BACKGROUND NOISE
`
`9000'
`
`12,000'
`
`15,000'
`
`21,000'
`
`18,000'
`
`–50
`
`–60
`
`–70
`
`–80
`
`–90
`
`–100
`
`–110
`
`–120
`
`-130
`
`–140
`
`–150
`
`POWER SPECTRAL DENSITY (dBm/Hz)
`
`0
`
`200
`
`800
`600
`400
`FREQUENCY (kHz)
`Figure 6. Typical received signal power
`spectral density, AWG26 loops
`
`1000
`
`1200
`
`power spectral density of –140dBm/
`Hz. This is equivalent to a noise volt-
`age of 31nV/(cid:214) Hz. The receiver
`amplifier should have a noise spec-
`tral density in the band between
`20kHz and 1MHz lower than this level.
`Linear Technology provides several
`fast amplifiers with noise voltage spec-
`tra of less than 10nV/(cid:214) Hz. Lower noise
`is required in inverse proportion to
`the turns ratio of the transformer
`used to address the attendant reduc-
`tion in both the noise floor and the
`received signal.
`The amount of signal received is a
`function of the length of phone line
`used to make the connection, as
`shown in Figure 6. This is referred to
`as the loop length. Very long loop
`lengths can severely attenuate the
`transmitted signal, particularly at the
`higher channel frequencies. The
`greater the attenuation of a channel,
`the fewer data bits can be transmitted
`in that channel, which affects the
`overall communication data rate. As
`a rule of thumb, a received signal-to-
`noise ratio of 18dB allows two data
`bits to be used in a channel. With
`each 3dB of additional signal above
`the noise floor, an extra bit of data can
`be used. With 45dB to 50dB signal-
`to-noise ratio, a full 12 bits of data
`can be exchanged in one channel
`frequency.
`The next installment in this series
`will provide the design calculations to
`determine the minimum requirements
`for supply voltage, current drive
`capability and resultant power con-
`sumption and dissipation. In addition,
`heat management issues will be
`discussed.
`
`31
`
`2 + a3EIN
`
`
`
`3 + a4EIN4 +
`
`an inverted version of the other, to the
`primary of the transformer. This can
`be shown mathematically by repre-
`senting the linear output signals of
`the amplifiers as a power series:
`Each output is a linear function of
`the input signal:
`VO = f(EIN)
`which, represented as a power series,
`is
`VO = a1EIN + a2EIN
`5 …
`a5EIN
`The inputs to the differential
`–; therefore:amplifier are EIN+ and EIN
`
`
`
`
`4
`VO(+) = a1EIN + a2EIN2 + a3EIN3 + a4EIN
`5 …
`+ a5EIN
`and
`
`VO(–) = –a1EIN + a2EIN2 – a3EIN
`5 …
`4 – a5EIN
`a4EIN
`The differential output of the ampli-
`fier stage is
`VODIFF = VO(+) – VO(–)
`therefore:
`5 + …
`VODIFF = 2a1EIN + 2a3EIN3 + 2a5EIN
`
`which does not contain any even har-
`monic products. The complete
`cancellation of even harmonics
`depends on the gain and phase-shift
`matching of the amplifiers and the
`signal paths over the frequency range
`of concern.
`
`3 +
`
`Bandwidth, Slew Rate
`and Noise Requirements
`of the Amplifiers
`High speed amplifiers with band-
`widths much wider than the
`transmitted signal bandwidth should
`be used to maintain flat gain and
`constant phase shift of the DMT sig-
`nals. The amount of gain required in
`the transmit power amplifiers is
`dependant on the signal levels pro-
`vided by the analog front end (AFE),
`which is a circuit block that provides
`the interface between the line trans-
`ceiver and the DSP processor. The
`gain must be sufficient to put the
`proper amount of power on the phone
`line for the DSL standard being imple-
`mented (refer to Table 1). The
`
`Linear Technology Magazine • February 2000
`
`maximum frequency to be processed
`by the amplifiers is also a function of
`the standard being applied; this, in
`turn, sets the minimum bandwidth
`required. As a rule of thumb, the gain
`bandwidth product specification of
`the amplifiers used should be at least
`five times the required value to
`maintain linear accuracy over the
`transmitted signal spectrum. This
`specification provides an indication
`of the distortion-free, high speed sig-
`nal processing capability of the
`amplifier. For example, a Full Rate
`ADSL downstream transmitter with a
`gain of four and a maximum frequency
`of 1.1MHz requires a gain-bandwidth
`of 4.4MHz; therefore, amplifiers
`should be chosen that have a gain-
`bandwidth specification of at least
`22MHz. Parts with higher bandwidths
`are even better for preserving excel-
`lent gain and phase shift matching
`over the 1.1MHz band of operation.
`The slew rate of the amplifiers used
`is not so critical, because the signal
`spectrum is typically band-limited by
`filter networks. The step response of
`these filters slows down the rise and
`fall times of the signals presented to
`the amplifiers. A slew rate of at least
`10V/m s is usually adequate. How-
`ever, very fast slew rates are essentially
`free in wideband amplifier designs.
`Internal biasing currents charging
`and discharging internal compensa-
`tion capacitors and individual node
`capacitances of the circuit determine
`the slew rate of an amplifier. To pro-
`duce a high frequency amplifier,
`circuit-biasing currents are increased
`to minimize impedances at critical
`circuit nodes and small geometry tran-
`sistor structures are used to minimize
`stray capacitance. This results in very
`fast slew rates for the amplifier as an
`inherent byproduct of a high gain-
`bandwidth product characteristic.
`Faster slew rates ensure very fast
`dynamic response and reduced sig-
`nal distortion.
`Low noise characteristics, together
`with a wide gain bandwidth capabil-
`ity are most important for the
`amplifiers used in the receive cir-
`cuitry. On a typical connection, a
`phone line will have a noise floor
`
`6
`
`