Two Point Modulation

Another_{way to}extend the FM _responseofaPLL downto

DC,

^involves

the

application

of modulationtoboth the VCO and the reference oscil¬

lator

simultaneously.

This scheme is called two

point

modulation. The block

diagram

^of^a transmitter basedon a

single loop

^PLL

synthesizer

modulated

by

way oftwo

point

modulation is

depicted

ⁱⁿ

Figure

^{3 7.}

The

single loop

^PLL

synthesizer

^consists ^of ^a

VCO,

^two

digital frequency dividers,

phase-frequency

^detector

(PFD),

loop filter,

and a reference oscillator. The _purpose of the

loop

^is ^to ^{make the}

phase

^{of the VCO}

precisely

^{tracks the}

phase

of the reference oscillator.

Although

^{the PLL is}

nonlinear,

^{when the}

loop

^is

locked,J

^it ^can ^be

accurately

^described

by

^a linear model

[/ '].

5A PLL is said tobelocked, ^{when the}frequencyof the divided VCO signal ^is thesame asthefrequencyof the reference oscillator and the outputof the PFD is proportional^to^thephasedifference of thetwosignals.

Chapter

^3. Transceiver Architectures

Shaping VCXO Filter

Figure

^3.7: ^Twopoint ^modulation transmitter architecture.

The transfer function

describing

the instantaneous VCO

angular frequency

io0 when the

modulating signal Vfm

^is

applied

^to^{the VCO}

input (path ^Phf

ⁱⁿ

Figure 3.7)

^is

Thf(s)

KvcoKPFDF{s)^sK^FM

(3.6)

where

Kyco

^is^{the VCO}

gain

when driven from the

input

^connected^to the

loop filter, Kfm

^is ^{the VCO}

gain

when driven from the modulation

input (including ^amplifier Aca\), ^Kppp

^is ^the

phase-frequency

^detector

gain factor,

N is the divider constant of the divider

dividing

^{the VCO}

frequency,

^and

F(s)

^is ^the

^loop

filter transfer

function,

^{which has} ^a

low-pass

characteristic and often is chosento have the

following

^form:

F(s) ^Ko

^{1 +}^STZ

S 1 + STp

(3.7)

Thf(s)

^has ^a

high-pass

characteristic: the output

frequency

^can

only

be modulated at rates greater than the PLL

loop

^bandwidth. ^For

modulation rateslower than the PLL

loop bandwidth,

^the

phase

^error

introduced

by

^the

modulating signal

^is ^corrected

by

the action of the

loop

^{and does} not appear at the output.

On the other

hand,

the transfer function

describing

the instanta¬

neous VCO

angular frequency

io0 when the

modulating signal Vfm

3.2. Transmitter Architectures 23

1 10

Frequency (kHz)

100

Figure

^3.8: ^Twopoint ^modulator

partial

and total FM

transfer func¬

tions normalized to

Kfm-

applied

^tothe reference oscillator

(path Plf)

^is

Tlf(s)

⁼

KycoKpFDKRF(s)

, KvcoKPFDF{s)

b T N

(3.8)

where R is the dividerconstant of the divider

dividing

the referenceos¬

cillator

frequency,

^and

Kr

^is ^the

gain

of the reference oscillator whose output

frequency

^must ^{also be}

capable

^of

being

^modulated

by

voltage signal. Tpp(s)

^has ^a

low-pass

characteristic whichmeans that

through

this

path

^{the PLL} output

frequency

^can

only

be modulated at rates slower than the

loop

bandwidth. For rates faster than

this,

^the

synthe¬

sizer is not fast

enough

^to ^{follow the}

changes.

To obtain a modulator with a modulation bandwidth

extending

from DCto

frequencies higher

^{than the}

loop bandwidth,

^the

synthesizer

has tobe modulated

through

^both

paths simultaneously:

T(s)

⁼

THf(s)A-Tlf(s) (3.9)

Chapter

^3. Transceiver Architectures

Inserting (3.6)

^and

(3.8)

ⁱⁿ^{the above}

expression

^it ^can be shown that if

Kfm-R

⁼

KR-N (3.10)

T(s) ^simplify

^to

T(s)

⁼

^Kfm (3.11)

That

is,

^if

Equation (3.10)

^is

^satisfied,

^the^two

point

modulator becomes

a modulator witha flat_response

extending

^{from DC} ^to^anupper limit determined

by

the VCO modulation

bandwidth, independently

^from

the PLL

loop

^bandwidth. ^The

loop

^bandwidth ^can thus be chosen

according

^to^other

specifications

^such ^as the maximum lock time.

The

partial

FM transfer functions

Tpp(s)

^and

Thf(s) together

^with

the total FM transfer function

T(s)

^of^a^two

point

modulator basedon a

type²third-order

loop

^with^abandwidth of 20 kHz anda

phase margin

of 55° are illustrated in

Figure

^3.8. ^The ^curves have been normalized to

Kfm-

One

disadvantage

of this architecture is the need for a

voltage

^con¬

trolled

crystal

^oscillator

(VCXO)

ⁱⁿ

^place

^of^a conventional

crystal

^os¬

cillator.

Moreover,

ⁱⁿ ^order ^to

satisfy Equation (3.10),

^the

gain

^{of the} VCXO or the

gain

of the VCO needs to be calibrated. This can be achieved

by inserting

^a ^variable

gain amplifier (or attenuator)

ⁱⁿ ^one

of the two

paths Ppp

Phf

exemplified by

^the

amplifier Acai

ⁱⁿ

Figure

^3.7.

Fortunately,

^{the BER}

performance

^seems ^{not to} ^be very sensitive to

gain

mismatches between the two

paths [23.].

Often,

ⁱⁿ ^a

transceiver,

the reference

frequency

of the PLL is also used asbasic clock

frequency

for the baseband_processor. The effects of the

slight frequency change

^introduced

by

^the^two

point

^modulator ^on the

operation

of the latter have thus tobe taken intoaccount. In case

problems,

the reference

frequency

for the main

synthesizer

^has ^to^be

generated

^with^aseparate

crystal

^oscillator^or^{with the}

help

^of^a^second

PLL

using

the baseband

crystal

^oscillator^as^reference

frequency.

^{In the}

latter _case, the reference

frequency

of the main PLL canbe modulated

by choosing

^a verylow

secondary loop

bandwidth and

by applying

^the

low-frequency

components ^{of the}

modulating signal

^tothe VCO of the

secondary loop.

The reference oscillator isnotthe

only place

^{where the}

low-frequency

modulation components ^can ^be

injected

^into ^the

loop.

An alternative

3.2. Transmitter Architectures 25

point

^ise.g. infront of the

loop

^filter. ^A

simple analysis

however shows that toachieve

frequency

modulation down to

DC,

^an ^ideal

integrator

is needed. Even

though

TDD/TDMA

^systemlike Bluetooth

provides enough

^time ^intervals

during

^{which the}

integrator

^can^be reset, ^modu¬

lation

through

the reference oscillator is

particularly

^attractive ^as ^the

VCXO

automatically performs

^{the needed}

integration

without the need forextra

circuitry.

Theuseofa

fractional-N

PLL allows foranextramethodtomodu¬

late the

frequency

^of^a

synthesizer.

^As^a fractional-N PLL allowa very fine control of the

frequency,

the modulation information can

directly

be added tothe nominal divide value

controlling

the carrier

frequency.

By including

digital compensation filter,

this method allows to

dig¬

itally

modulate the carrier at rates slower and faster than the

loop

bandwidth

[21].

^{The main}

disadvantage

of this method is the fact that the

compensation

^filter

depends

^on ^{the PLL}

loop

^filter.

Chapter

^3. Transceiver Architectures

Chapter ⁴

System Planning

In the

previous chapter

^we have discussed in

general

^terms^receiver^and

transmitterarchitectures

capable

^of

providing

^the

performance required by

Bluetooth. For each_one, we have

pointed

^outthe various

trade-offs, merits,

and weaknesses.

Here,

^we ^first

analyze

^the

requirements

relevant for the RF trans¬

ceiver dictated

by

the Bluetooth standard. The Bluetooth type ap¬

proval specify

^test conditions for a

complete

system. ^Some

specifica¬

tions as e.g. the nominal output power ^are

directly applicable

^as ^re¬

quirements

^for ^one ^or ^more parts of the transceiver. Others need to be translated in a metric

appropriate

^{for the}

design

of the transceiver front-end. For

instance,

the receiver

sensitivity

^is

specified

^as ^a ^maxi¬

mal BER

given

^a ^reference

signal.

The BER in its turn

depends

^from

the type ^of

demodulator/detector

used and from the noise

generated by

the front-end. Thus after

having specified

the kind of detector to be

used,

^{the noise}

figure

^can^{be used}^{as a} better metric for the receiver front-end.

Next,

^we^will

identify

^the^most

appropriate

architecture

compatible

with the target

applications,

^and

finally

^we ^will

assign

^the

required performance figures

^toeach block of thesystem.

Chapter

^4.

System Planning

4.1 Transceiver Requirements

No documento A low-power CMOS Bluetooth transceiver (páginas 34-41)

DC,

application

simultaneously.

point

diagram

single loop

synthesizer

by

point

depicted

Figure

single loop

synthesizer

VCO,

digital frequency dividers,

phase-frequency

(PFD),

loop filter,

loop

phase

precisely

phase

Although

nonlinear,

loop

locked,J

accurately

by

[/ '].

Chapter

Figure

describing

angular frequency

modulating signal Vfm

applied

input (path Phf

Figure 3.7)

Thf(s)

(3.6)

Kyco

gain

input

loop filter, Kfm

gain

input (including amplifier Aca\), Kppp

phase-frequency

gain factor,

dividing

frequency,

F(s)

loop

function,

low-pass

following

F(s) Ko

(3.7)

Thf(s)

high-pass

frequency

only

loop

loop bandwidth,

phase

by

modulating signal

by

loop

hand,

describing

angular frequency

modulating signal Vfm

Frequency (kHz)

Figure

partial

transfer func¬

Kfm-

applied

(path Plf)

Tlf(s)

input (path ^Phf

input (including ^amplifier Aca\), ^Kppp

^loop

F(s) ^Ko

T(s) ^simplify

^Kfm (3.11)

^satisfied,

^place