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Micrel, Inc.
Please keep in mind that these suggested values may
MICRF213
Second, we plot the shunt inductor (68nH) and the
be different if the layout is not exactly the same as the
series
capacitor
(1.8pF)
for the desired
input
one depicted here.
impedance (Figure 5). We can see the matching
Freq (MHz)
303.825
315
345
C3 (pF)
1.8
1.8
1.8
L2 (nH)
72
68
56
Z device ( ? )
34.6– j245.1
32.5 – j235
25.3 – j214
leading to the center of the Smith Chart or close to
50 ? .
Table 4. Matching Values for the
Most Used Frequencies
For the frequency of 315MHz, the input impedance is
Z = 32.5 – j235 ? , then the matching components are
calculated by:
Equivalent parallel = B = 1/Z = 0.577 + j4.175
msiemens
Rp = 1 / Re (B);
Rp = 1.733 k ? ;
Xp = 1 / Im (B)
Xp = 239.5 ?
Q = SQRT (Rp/50 + 1)
Q = 5.972
Xm = Rp / Q
Xm = 290.21 ?
Resonance Method For L-shape Matching Network
Lc = Xp / (2.Pi.f);
L2 = (Lc.Lp) / (Lc + Lp);
Lp = Xm / (2.Pi.f)
C3 = 1 / (2.Pi.f.Xm)
L2 = 66.3nH
C3 = 1.74pF
Doing the same calculation example with the Smith
Chart, it would appear as follows,
First, we plot the input impedance of the device,
(Z = 32.5 – j235) ? @ 315MHz.(Figure 4).
Figure 5. Plotting the Shunt Inductor
and Series Capacitor
Crystal Y1 or Y1A (SMT or leaded respectively) is the
reference clock for all the device internal circuits.
Desired
crystal
characteristics
are:
10pF load
capacitance, 30ppm, ESR < 50 ? and a -40oC to
+105oC temperature range. Table 5 shows the crystal
frequencies and one of Micrel’s approved crystal
manufactures (www.hib.com.br).
Figure 4. Device’s Input Impedance, Z = 32.5 – j235 ?
The oscillator of the MICRF213 is a Colpitts type. It is
very sensitive to stray capacitance loads. Thus, very
good care must be taken when laying out the printed
circuit board. Avoid long traces and ground plane on
May 2007
9
M9999-052307-A
(408) 944-0800
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