MICRF218 datasheet(15/23 Pages) MICREL

15 / 23 page

Micrel

MICRF218

November 2011

M9999-111111

(408) 944-0800

Table 3 shows the most used frequency values.

Freq (MHz)

C8 (pF)

L1(nH)

315.0

6.8

390.0

6.8

418.0

6.0

433.92

5.6

Table 3. Band-Pass-Filter Front-End Values

There is no need for the bandpass-filter front-end for

applications where it is proven that the outside band

noise does not cause a problem. The MICRF218 has

image reject mixers which improve significantly the

selectivity and rejection of outside band noise.

Capacitor C3 and inductor L2 form the L-shape

matching network. The capacitor provides additional

attenuation for low frequency outside band noise, and

the inductor provides additional ESD protection for the

antenna pin. Two methods can be used to find these

values, which are matched close to 50Ω. One method

is done by calculating the values using the equations

below, and the other method uses a Smith chart. The

latter is made easier by using software that plots the

values of the components C8 and L1, like WinSmith

by Noble Publishing.

To calculate the matching values, one needs to know

the input impedance of the device. Table 4 4 shows

the input impedance of the MICRF218 and suggested

matching values for the most used frequencies. These

suggested values may be different if the layout is not

exactly the same as the one made here.

Freq (MHz)

C3 (pF)

L2(nH)

Z device (Ω)

315.0

1.5

16.3 -j210.8

390.0

1.2

8.26 – j163.9

418.0

1.2

11.1 – j161.9

433.92

1.1

9.54 – j152.3

Table 4. Matching values for the most used frequencies

For the frequency of 433.92MHz, the input impedance

is Z = 9.54 – j152.3Ω. The matching components are

calculated by:

Equivalent parallel = B = 1/Z = 0.410 + j6.54

msiemens

Rp = 1 / Re (B);

Xp = 1 / Im (B)

Rp = 2.44kΩ;

Xp = 345.8Ω

Q = SQRT (Rp/50 + 1)

Q = 7.06

Xm = Rp / Q

Xm = 345.8Ω

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 = 38.9nH

C3 = 1.06pF

Doing the same calculation example with the Smith

Chart, it would appear as follows,

First, the input impedance of the device is plotted,

(Z = 9.54 – j152)Ω @ 433.92MHz.(Figure 8).

Figure 8. Device’s Input Impedance, Z = 9.54-j152Ω

Second, the shunt inductor (39nH) and the series

capacitor (1.1pF) for the desired input impedance are

plotted (Figure 9). One can see the matching leading

to the center of the Smith Chart or close to 50Ω.

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