MICRF102_06 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 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¡
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 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.
Micrel
Supply Bypassing
Supply bypassing consists of three capacitors; C3 = 4.7µF,
C4 = 0.1µFand C5 = 100pF
+5VTX
1 PC
ASK 8
C4
0.1µF
16V
C5
100pF
50V
C3
4.7µF
16V
2 VDD
ANTP 7
MICRF102
3 V_SS
ANTM 6
4 REFOSC
SB 5
Figure 9. Supply Bypassing
Example to Calculate C_S and C_P Antenna Inductance
Calculation
Length_mils = 2815
dmils = 70
k = 0.85
Length = ^{(Length_mils × 25.4 )} d = ^{(dmils × 25.4)}
1000
1000
Length = 71.501
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
1
×
f²
×L
C_T = 2.587 × 10^−12
C_SERIES =
1
1_− 1
C_T C_VAR
C_SERIES = 8.2 × 10^−12
MICRF102 Series Capacitor Calculation:
f = 433.92 × 10⁶
L = 52 × 10^-9
C_VAR = 5 × 10^-12
C_P = 2.7 × 10^-12
C_T
=
4
×
π²
1
×
f²
×L
C_T = 2.587 × 10^−12
December 2006
9
MICRF102