MICRF008 データシートの表示（PDF） - Micrel
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MICRF008 Datasheet PDF : 12 Pages
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MICRF008
Input Connector/Antenna
Starting at J2, a SMA connector to connect to a RF signal
generator. Levels above –20dBm should be avoided. An
antenna pad is also provided. Solder a piece of solid wire for
the frequency of interest. See Table 2 for 1/4 wavelength
antennas. It should be noted that 1/4 wavelength antennas
work best ONLY with ground planes of an equivalent size of
the antenna. This is not the case here since the board size is
very small.
Freq. (MHz)
ANT1 (mm)
ANT1 (inches)
315
231
9.1
390
185
7.3
418
173
6.8
433.92
168
6.6
Table 2. 1/4 Wave Antenna Lengths
The size of the antenna is calculated by the following equa-
tion,
1
4
λ Ant =
3 × 108
×
f
0.97
×4
×
1000
[mm]
1
4
λ Ant =
3
×
108
f
× 0.97 ×
× 101.6
1000
[inches]
where,
• 3 x 108 is the speed of light in vacuum in meter
per seconds.
• 0.97 is a correction factor for the difference
between electrical length and physical length
and varies depending on ground plane, and
objects next to the antenna.
• f is the frequency of interest in Hertz.
1/4 wavelength antennas are good for initial tests, but should
never be used in a final product, unless the PCB area has a
large ground plane. Small ground planes makes 1/4 wave-
length monopole antennas unstable, and any object close to
the antenna changes its impedance. It is completely useless
if the antenna is not straight up or if any object touches it.
Other antennas, such as half-wave dipole, or 1/8 wavelength
monopole are better solutions for small ground planes. A
1/8 wavelength antenna has approximately half the size of a
1/4 wavelength antenna, which makes it very attractive for
small designs. The 1/8 wavelength antenna is not as sensi-
tive to proximity effects as the 1/4 wavelength antenna.
However, the 1/8 wavelength antenna has a reactive element
in its impedance that needs to be taken into account in the
matching network. The halfwave dipole has a size approxi-
mately double the 1/4 wavelength and does not require a
large ground plane. One side of the antenna is connected to
the RF input and the other is connected to ground as seen in
Figure 8.
Halfwave-Dipole Antenna
Micrel
one side connected to the RF input
one side connected to ground
Figure 8. Halfwave-Dipole Antenna
Matching Network
Z1 to Z5 are used for the matching network. Not all parts are
used at the same time. Several types of matching network
can be implemented: T, π, or a L network. Here a matching
network has two purposes. The main purpose is to match the
antenna to the antenna pin. By matching it is meant to reduce
the loss there is between two circuits; here the antenna being
used or signal generator and the antenna pin. The secondary
purpose is to filter undesired signals, which will reduce the
receiver performance. Since it is a secondary function, more
filtering may be required, depending on the amount of noise
or adjacent signal interference present in the antenna. A
T-Network was chosen assuming that an 1/4 wavelength
antenna is being used, which has impedance close to 50Ω in
free air or the receiver is being connected to a RF signal
generator that has 50Ω output impedance. By using the
proper matching elements, almost 9dB better sensitivity can
be obtained. There are several techniques used for imped-
ance matching. One frequently used is to plot impedance
values in the Smith Chart. However, one needs to know the
antenna impedance and the antenna pin impedance in the
MICRF008BM to perform the matching. Table 3 below shows
the antenna pin input impedance versus frequency and the
matching elements needed to match 50Ω to the antenna pin.
Antenna Pin Input
Impedance
(ohms)
23 - j207
14 - j166
12 - j159
10 - j151
Frequency
(MHz)
315
390
418
433.92
Z1, Z4, Z2(Note 1)
8.2pF, 18nH, 82nH
3.9pF, 16nH, 43nH
4.7pF, 12nH, 39nH
4.7pF, 11nH, 39nH
Note 1.
All inductors used are Coilcraft 0603 size, and the capacitors
are Vishay COG 5%. Z3 and Z5 are not used.
Table 3. Antenna Input, Frequency and
Matching Values
July 2003
9
MICRF008
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