LMX2240 datasheet(10/12 Pages) NSC

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Applications Information

THE INTERMEDIATE FREQUENCY LIMITER

The IF limiter has a large amount of gain at high enough

frequency to cause concern about oscillation To ensure

that the limiter does not oscillate a few precautions should

be taken

The compensation capacitors that are used

should be chosen to roll off any unwanted frequencies be-

low the band of interest The capacitor should be a high Q

RF type ceramic chip capacitor For DECT the capacitor

value should be 100 pF and the capacitors should be sol-

dered as close to the LMX2240 as possible This will create

a pass band from 40 MHz to 150 MHz The AC coupling

capacitor at the input to the limiter (from the SAW filter)

should be the same value as the compensation capacitors

THE DISCRIMINATOR

There are two types of discriminator that can be used to

demodulate FM signals The first is a delay line discrimina-

tor which uses a delay in one path of the received signal to

introduce a phase difference between it and the received

signal The operation of the delay line discriminator is de-

rived in the inset box The other type of discriminator relies

on a quadrature tank to directly introduce a phase shift in

the received signal This is the type of implementation that

is commonly used in mobile communications because of its

relative ease of construction and low cost

The discriminator operates best when the inputs to it are

hard-limited (ie square edges) If the input signal is small

enough such that the IF amplifier cannot limit it the output

voltage swing of the limiter will suffer Typically the mini-

mum voltage swing the discriminator can see and still fully

switch is about 100 mVPP The two inputs to the discrimina-

tor can be of different peak-to-peak voltage swings as long

as both are over the lower limit This allows the quadrature

tank circuit to have some insertion loss In fact up to 8 dB

insertion loss can be tolerated while still ensuring that the

discriminator output won’t suffer

The quadrature circuit can also affect the discriminator out-

put voltage swing The discriminator output voltage swing

specified assumes perfect quadrature at the frequency of

interest (mixer operation) With available analog compo-

nents perfect quadrature is not possible This is due in part

to the high frequency of the IF and the proportionally very

narrow bandwidth of the desired signal For example a

DECT signal is about 1 MHz wide which is k 1% of the IF

at which the demodulation occurs This makes the quadra-

ture circuit difficult to achieve With moderately high Q com-

ponents

however

a reasonable phase shift can be

achieved with a single pole tank This is illustrated by the

following equation the output of the discriminator is given

(1)

DISCout e cos(0c t ) cos(0c t aw)

which results in

(2)

DISCout e cos(0c t a0c t aw) a cos(0c t b0c t bw)

When the double frequency component is filtered out with a

low pass filter the cosine of the phase remains

(3)

DISCout e cos(bw) e cos(w)

It can be seen that at 90 phase shift the output will be zero

At 0 the output will be 05 and at 180 it will be b05 The

output swing is then set by the multiplication of the cosine

term with the discriminator output amplifier’s gain

With a circuit that gives an output peak-to-peak voltage of

10 VPP (min) with ideal quadrature the slope is seen to be

55 mVdegree With a practical quadrature tank circuit at

1106 MHz the phase shift over a 1 MHz bandwidth is about

45 –50 which translates to an output peak-to-peak voltage

of about 250 mVPP

Assume the FM modulated signal is denoted as

s(t) e cos (0ct a m(t))

(4)

where

m(t) e m

b(t) dt

and b(t) is the modulating baseband signal The con-

stant m is defined as m e 2DfTb The signal s(t) must

be delayed by some u so that

l(t) e s(tau) e cos (0c(tau) a m(tau))

(5)

If the delay u is such that

0ct e 2nq a

n e 0 1 2 3

(6)

then

s(tau) e sin(0ct a m(tau))

(7)

and multiplying (4) and (7) yields

s(t) l(t) e cos (0ct a m(t)) sin (0ct a m(tau))

sin (20ct a m(t) a m(tau))

(8)

sin (m(tau) b m(t))

The double frequency component can be filtered off

with a lowpass filter If u is kept small

sin (m(tau) b m(t))

m(tau) b m(t)

tau

b(t) dt b

b(t) dt

(9)

tau

b(t) dt

b(t)

The object for a delay line then is to maximize the de-

lay while retaining the approximations necessary to sat-

isfy (9) u k 01 Tb

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