December 2006

9

MICRF102

MICRF102

Micrel

Design Examples

Complete reference designs including gerber files can be

downloaded from Micrel’s website at: www.micrel.com/prod-

uct-info/qwikradio.shtml.

Antenna Characteristics

In this design, the desired loop inductance value is determined

according to the table below.

Freq.

R

XL

Ind

Q

K

(MHz)

(

Ω)

(

Ω)

(nH)

(XL/R)

300

1.7

84.5

44.8

39.72

0.83

315

2.34

89.3

45.1

39.65

0.85

390

3.2

161

47.4

52.00

0.90

434

2.1

136

50.0

78.33

0.96

The reference design, shown in Figure 7, has an antenna

meeting this requirement.

Figure 7. Demo Board PCB.

Loop antennas are often considered highly directional. In

fact small loop antennas can achieve transmit patterns close

in performance to a Dipole antenna. The radiation pattern

below is the theoretical radiation pattern for the antenna, as

shown in Figure 8.

E-total, phi = 0¡

E-total, phi = 90¡

(180-phi) direction

phi direction

30.0

180.0

60.0

120.0

150.0

150.0

120.0

60.0

30.0

0.0

Figure 8. Polar Elevation Pattern at 315MHz.

The 0 degree plot is the radiation pattern in the plane of the

transmitter PCB, the 90 degree plot represents the plane

perpendicular to the PCB. Micrel’s evaluation of the perfor-

mance of the board in Figure 8 indicates an even more uniform

radiation pattern that the theoretical plot shown here.

Supply Bypassing

Supply bypassing consists of three capacitors; C3 = 4.7µF,

C4 = 0.1µFand C5 = 100pF

C4

0.1

µF

16V

C5

100pF

50V

C3

4.7

µF

16V

+5VTX

VDD

MICRF102

2

3

PC

1

REFOSC

4

Figure 9. Supply Bypassing

Example to Calculate C

Calculation

Length_mils = 2815

dmils = 70

k = 0.85

Length

Length_mils 25.4

1000

Length 71.501

=

×

()

=

d

dmils

d

=

×

()

=

25 4

1000

1 778

.

.

L

0.2 Length ln

Length

d

1.6

10

k

L44 10

9

9

=×

×

−

⎛

⎝⎜

⎞

⎠⎟

××

=×

−

−

Where Length and d are in mm and L is in H;

Where k is a constant dependent on PCB material, copper

thickness, etc.

MICRF102 Series Capacitor Calculation:

f = 315

L = 46

C

fL

C

T

T

=

××

×

=×

−

1

4

2 587 10

22

12

ππ

.

C

CC

C

SERIES

TVAR

SERIES

=

−

=×

−

1

11

.

MICRF102 Series Capacitor Calculation:

f = 433.92

L = 52

C

fL

C

T

T

=

××

×

=×

−

1

4

2 587 10

22

12

ππ

.