MICRF102 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – QwikRadio UHF ASK Transmitter

MICRF102
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¡
0.0
30.0
30.0
60.0
60.0
(180-phi) direction
phi direction
120.0
120.0
150.0
180.0
150.0
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.
Micrel
Supply Bypassing
Supply bypassing consists of three capacitors; C3 = 4.7uF,
C4 = 0.1uFand C5 = 100pF
+5VTX
1 PC
ASK 8
C4
0.1 F
16V
C5
100pF
50V
C3
4.7 F
16V
2 VDD
ANTP 7
MICRF102BM
3 V_SS
ANTM 6
4 REFOSC
SB 5
Figure 8.
Example to Calculate C_S and C_P
Antenna Inductance Calculation
Length_mils = 2815
dmils = 70
k = 0.85
Length = ^{(Length_mils × 25.4)}
1000
Length = 71.501
d = ^{(dmils × 25.4)}
1000
d = 1.778
L
=
0.2
×
Length ×
ln^{}
Length
d
− 1.6^{}
×
10^−9
×
k
L = 44 × 10^−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 × 10⁶
L = 46 × 10^-9
C_VAR = 5 × 10^-12
C_P = 12 × 10^-12
C_T
=
4×
π²
l
×
f²
×L
C_T = 5.55 × 10^−12
C_SERIES =
1
1_− 1
C_T C_VAR
C_SERIES = 8.2 × 10^−12
September 2002
9
MICRF102