Micrel, Inc.

MICRF213

May 2007

9

M9999-052307-A

(408) 944-0800

Please keep in mind that these suggested values may

be different if the layout is not exactly the same as the

one depicted here.

Freq (MHz)

C3 (pF)

L2 (nH)

Z device (

Ω)

303.825

1.8

72

34.6– j245.1

315

1.8

68

32.5 – j235

345

1.8

56

25.3 – j214

Table 4. Matching Values for the

Most Used Frequencies

For the frequency of 315MHz, the input impedance is

Z = 32.5 – j235

Ω, then the matching components are

calculated by:

Equivalent parallel = B = 1/Z = 0.577 + j4.175

msiemens

Rp = 1 / Re (B);

Xp = 1 / Im (B)

Rp = 1.733 k

Ω;

Xp = 239.5

Ω

Q = SQRT (Rp/50 + 1)

Q = 5.972

Xm = Rp / Q

Xm = 290.21

Ω

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 = 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)

Ω @ 315MHz.(Figure 4).

Figure 4. Device’s Input Impedance, Z = 32.5 – j235

Ω

Second, we plot the shunt inductor (68nH) and the

series

capacitor

(1.8pF)

for

the

desired

input

impedance (Figure 5). We can see the matching

leading to the center of the Smith Chart or close to

50

Ω.

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

Ω 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).

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