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Micrel
Application Information
Figure 3 – QR211HE1 Application Example, 433.92 MHz
MICRF211
The MICRF211 can be fully tested by using one of
which reduces the receiver performance. It is
many evaluation boards designed at Micrel for this
calculated
by the parallel resonance
equation
device. As an entry level, the QR211HE1 (Figure 3)
offers a good start for most applications. It has a
helical PCB antenna with its matching network, a
f = 1/(2×PI×(SQRT L1×C8)). Table 3 shows the most
used frequency values.
band-pass-filter front-end as a pre-selector filter,
matching network and the minimum components
required to make the device work, which are a crystal,
Cagc, and Cth capacitors. By removing the matching
network of the helical PCB antenna (C9 and L3), a
whip antenna (ANT2) or a RF connector (J2) can be
Freq (MHz)
390.0
418.0
433.92
C8 (pF)
6.8
6.0
5.6
L1(nH)
24
24
24
used instead. Figure 3 shows the entire schematic of
it for 433.92MHz. Other frequencies can be used and
the values needed are in the tables below.
Capacitor C9 and inductor L3 are the passive
elements for the helical PCB matching network. A
tight tolerance is recommended for these devices, like
2% for the inductor and 0.1pF for the capacitor. PCB
variations may require different values and
optimization. Table 2 shows the matching elements
for the device frequency range. For additional
information look for Small PCB Antennas for Micrel RF
Products application note.
Table 3. Band-Pass-Filter Front-End Values
There is no need for the band-pass-filter front-end for
applications where it is proven that the outside band
noise does not cause a problem. The MICRF211 has
image reject mixers which improve significantly the
selectivity and rejection of outside band noise.
Capacitor C3 and inductor L2 form the L-shape
matching network. The capacitor provides additional
attenuation for low frequency outside band noise and
the inductor provides additional ESD protection for the
antenna pin. Two methods can be used to find these
values, which are matched close to 50 ? . One method
Freq (MHz)
390.0
418.0
433.92
C9 (pF)
1.2
1.2
1.5
L3(nH)
43
36
30
is done by calculating the values using the equations
below and another by using a Smith chart. The latter
is made easier by using software that plots the values
of the components C8 and L1, like WinSmith by Noble
Publishing.
Table 2. Matching Values for the Helical PCB Antenna
To use another antenna, like the whip kind, remove
C9 and place the whip antenna in the hole provided in
the PCB. Also, a RF signal can be injected there.
L1 and C8 form the pass-band-filter front-end. Its
purpose is to attenuate undesired outside band noise
To calculate the matching values, one needs to know
the input impedance of the device. Table 4 shows the
input impedance of the MICRF211 and suggested
matching values for the most used frequencies. These
suggested values may be different if the layout is not
exactly the same as the one made here.
March 2007
8
M9999-030107
(408) 944-0800
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