MICRF219_11 datasheet(13/23 Pages) MICREL

13 / 23 page

Micrel

MICRF219

June 2011

M9999-060811

(408) 944-0800

ape

additional

nd noise.

tection for

ed to find

50Ω. One

using the

mith chart

easier via a software plot

y, the user

n for best

hing point,

ne needs to know

the input impedance of the device. Table 3 shows the

suggested

matching val

for the most of

d frequencies.

These sugge

d value

ay be

t if the layout

exactly

same

he on

here:

Z devic

Low-Noise Amplifier Input Matching

Capacitor C3 and inductor L2 form the “L” sh

matching network. The capacitor provides

attenuation for low-frequency outside ba

The inductor provides additional ESD pro

the antenna pin. Two methods can be us

these values that best matched near

method is done by calculating the values

equations below and the other is using a S

utility. The latter is made

input

where components are added on. In this wa

can see the impedance moving directio

values of C8 and L1 toward to central matc

like WinSmith by Noble Publishing.

To calculate the matching values, o

input im dance

the M RF219 and

ues

ten use

ste

s m

differen

not

the

as t

e made

req (MHz)

C3 (pF)

L2(nH)

e (Ω)

315

33 - j2

1.8

390

1.5

23 – j199

418

1.5

21 – j186

433.92

1.5

19 – j174

Table 3. Matching Values for the Most Used

Frequencies

equency of 433.92MHz, the input impedance

– j174.2Ω, then the matching components

allel = B =

.68msiemens

5kΩ;

Xp = 176.2Ω

Q = SQRT (Rp/50 + 1)

Q = 5.831

Xm = Rp / Q

82.98Ω

Resonance Method for L-shape Matching Network

Lc = Xp / (2×Pi×f);

Lp = Xm / (2×Pi×f)

L2 = (Lc×Lp) / (Lc + Lp);

C3 = 1 / (2×Pi×f×Xm)

L2 = 39.8nH

C3 = 1.3pF

with the Smith

First, one plots the input impedance of the device,

(Z = 18.6 – j174.2)Ω @ 433.92MHz.(Figure 5):

For the fr

is Z = 18.6

are calculated by:

Equivalent par

1/Z = 0.606 + j5

Rp = 1 / Re (B); Xp = 1 / Im (B)

Rp = 1.6

Xm = 2

Doing the same calculation example

Chart, would appear as follows,

Figure 5. Device’s Input Impedance, Z = 19 – j174Ω

Second, one plots the shunt inductor (39nH) and the

series capacitor (1.5pF) for the desired input

impedance (Figure 6). One can then see the matching

leading to the center of the Smith Chart or close to

50Ω.

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