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Micrel, Inc.
Application Information
ANT1
Helical PCB Antenna Pattern
MICRF213
C9
Y1
9.81563MHz
+3.3V
ANT2
J2
(np)SMA
C3
1.8pF
L3
75nH 2%
+3.3V
1
2
3
4
5
U1 MICRFAYQS
RO1 RO2
GNDRF NC
ANT
RSSI
GNDRF
CAGC
VDD
CTH
16
15
14
13
12
RSSI
R1
(np)
L1
39nH 5%
C8
6.8pF
L2
68nH 5%
C5
0.1μF
6
7
SQ
SEL0
SEL1
DO
11
10
DO
8
SHDN
GND
9
JP3
open
JP1
short
R3
JR2
short
C6
0.47μF
C4
4.7μF
R2
(np)
+3.3V
GND
D0
NC
SH
RSSI
1
2
3
4
5
6
+3.3V
DO
SH
RSSI
Figure 3. QR213HE1 Application Example, 315MHz
The MICRF213 can be fully tested by using one of the
many evaluation boards designed by Micrel and
intended for use with this device. As an entry level,
L1 and C8 form the pass-band-filter front-end. Its
purpose is to attenuate undesired outside band noise
which reduces the receiver performance. It is
the QR213HE1 (reference Figure 3) offers a good
calculated
by the parallel
resonance
equation
start for most applications. It has a helical PCB
antenna with its matching network, a band-pass-filter
f = 1/(2*PI*(SQRT(L1*C8)). Table 3 shows the most
used frequency values.
front-end as a pre-selector filter, matching network
and the minimum components required to make the
device work. The minimum components are a crystal,
Cagc, and Cth capacitors. By removing the matching
network of the helical PCB antenna (C9 and L3), a
Freq (MHz)
303.825
315
345
C8 (pF)
6.8
6.8
5.6
L1 (nH)
39
39
39
whip antenna (ANT2) or a RF connector (J2) can be
used instead. Figure 3 shows the entire schematic for
315MHz. Other frequencies can be used and the
values needed are listed in the tables below.
Capacitor C9 and inductor L3 are the passive
elements for the helical PCB matching network. It is
recommended that a tight tolerance be used for these
devices; such as 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, reference the: 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 the outside band noise
does not cause a problem. The MICRF213 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 ways can be used to find these
values, which are matched close to 50 ? . One method
Freq (MHz)
303.825
315
345
C9 (pF)
1.2
1.2
1.2
L3 (nH)
82
75
62
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, such as 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.
To calculate the matching values, one needs to know
the input impedance of the device. Table 4 shows the
input impedance of the MICRF213 and the suggested
matching values used for the most frequencies.
May 2007
8
M9999-052307-A
(408) 944-0800
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