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Selection of CTH and CAGC Capacitors
Capacitors C6 and C4, Cth and Cagc respectively
provide time-based reference for the data pattern
received. These capacitors are selected according to
data profile, pulse duty cycle, dead time between two
received data packets, and if the data pattern has or
does not have a preamble. See Figure 11 for an
example of a data profile.
MICRF218
determine the signal to noise ratio of the RF link,
crude range estimate from the transmitter source and
AM demodulation, which requires a low Cagc
capacitor value.
Shut Down Control
The shut down pin (SHDN) is useful to save energy.
When its level close to Vdd (SHDN = 1), the device is
not in operation. Its DC current consumption is less
Preamble
Header
PW1 PW2
than 1μA (do not forget to remove R3). When toggling
from high to low, there will be a time required for the
device to come to steady state mode, and a time for
data to show up in the DO pin. This time will be
1
2
3
4 5 6 7 8 9 10
t1 t2
PW2 = Narrowest pulse width
t1 & t2 = data period
Figure 11. Example of a Data Profile
dependent upon many things such as temperature,
choice of crystal used, and if the there is an external
oscillator with faster startup time. Normally, with the
crystal vendors suggested, the data will show up in
the DO pin around 1msec time, and 2msec over the
For best results, the capacitors should always be
optimized for the data pattern used. As the baud rate
increases, the capacitor values decrease. Table
shows suggested values for Manchester Encoded
data, 50% duty cycle.
temperature range of the device. See Figures 12.
SEL0
JP1
Short
Open
Short
Open
SEL1
JP2
Short
Short
Open
Open
Demod.
BW
(hertz)
1625
3250
6500
13000
Cth
(C6)
100nF
47nF
22nF
10nF
Cagc
(Cagc)
4.7μF
2.2μF
1μF
0.47μF
Table 10. Suggested Cth and Cagc Values.
Other components used include C5, which is a
decoupling capacitor for the Vdd line; R4 reserved for
future use and not needed for the evaluation board;
R3 for the shutdown pin (SHDN = 0, device is
operation), which can be removed if that pin is
connected to a microcontroller or an external switch,
and R1 and R2 which form a voltage divider for the
AGC pin. One can force a voltage in this AGC pin to
purposely decrease the device sensitivity. Special
care is needed when doing this operation, as an
external control of the AGC voltage may vary from lot
to lot and may not work the same for several devices.
DO, RSSI and Shutdown Functions
Three other pins are worthy of comment. They are the
DO, RSSI, and shut down pins. The DO pin has a
driving capability of 0.6mA. This drive current is good
enough for most of the logic family ICs in the market
today. The RSSI pin provides a transfer function of the
RF signal intensity vs. voltage. It is very useful to
Figure 12. Time-to-Good Data After Shut Down Cycle,
Room Temperature
November 2011
18
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