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Micrel
Table 3 shows the most used frequency values.
Q = SQRT (Rp/50 + 1)
MICRF218
Freq (MHz)
315.0
390.0
418.0
433.92
C8 (pF)
6.8
6.8
6.0
5.6
L1(nH)
39
24
24
24
Q = 7.06
Xm = Rp / Q
Xm = 345.8 ?
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)
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 50 ? . 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.
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) ? @ 433.92MHz.(Figure 8).
Freq (MHz)
315.0
390.0
418.0
433.92
C3 (pF)
1.5
1.2
1.2
1.1
L2(nH)
68
47
43
39
Z device ( ? )
16.3 -j210.8
8.26 – j163.9
11.1 – j161.9
9.54 – j152.3
Figure 8. Device’s Input Impedance, Z = 9.54-j152 ?
Table 4. Matching values for the most used frequencies
For the frequency of 433.92MHz, the input impedance
is Z = 9.54 – j152.3 ? . 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 = 2.44k ? ; Xp = 345.8 ?
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 50 ? .
November 2011
15
M9999-111111
(408) 944-0800
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