September 2002

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 deter-

mined according to the table below.

Freq.

R

XL

Ind

Q

K

(MHz)

(ohms) (ohms)

(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 6. has an antenna

meeting this requirement.

Figure 6

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

shown in Figure 6.

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 7. 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 6. indicates an even more

uniform radiation pattern that the theoretical plot shown here.

Supply Bypassing

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

C4 = 0.1uFand C5 = 100pF

C4

0.1 F

16V

C5

100pF

50V

C3

4.7 F

16V

+5VTX

VDD

MICRF102BM

2

3

PC

1

REFOSC

4

Figure 8.

Example to Calculate C

Antenna Inductance 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

-12

C

-12

C

l

fL

C

T

T

=

×× ×

4

555 10

22

12

ππ

.

C

CC

C

SERIES

T

VAR

SERIES

=

−

1

11

82 10

12

.