MICRF008YM データシートの表示（PDF） - Micrel
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MICRF008YM Datasheet PDF : 13 Pages
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Micrel, Inc.
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)
315
390
418
433.92
ANT1 (mm)
231
185
173
168
ANT1 (inches)
9.1
7.3
6.8
6.6
Table 2. 1/4 Wave Antenna Lengths
The size of the antenna is calculated by the following
equation:
1 λAnt = 3 × 108 × 0.97 × 1000 [mm]
4
f×4
1 λAnt = 3 × 108 × 0.97 × 1000 [inches]
4
f × 101.6
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 wavelength 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
sensitive 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 approximately double the 1/4
wavelength and does not require a large ground plane.
One side of the antenna is connected to the RF input
MICRF008
and the other is connected to ground as seen in Figure
8.
Halfwave-Dipole Antenna
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 impedance
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 MICRF008YM 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.
Freq. (MHz)
315
390
418
433.92
ANT1 (mm)
231
185
173
168
ANT1 (inches)
9.1
7.3
6.8
6.6
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
August 2008
10
M9999-080108
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