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Micrel
considerably reduced. It will have more or less than
shown in the figure below depending on the outside
band noise. The penalty for using squelch is a delay in
getting a good signal in the DO pin, that is, it takes
longer for the data to show up. The delay is
dependent upon many factors such as RF signal
intensity, data profile, data rate, C TH and C AGC
capacitor values, and outside band noise. See Figure
8 and Figure 9 below.
MICRF211
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.
Three other pins are worthy of comment. They are the
DO, RSSI, and shut down pins. The DO pin has a
driving capability of 0.4mA. This 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 determine the
signal to noise ratio of the RF link, crude range
estimate
from the transmitter
source
and AM
demodulation, which requires a low C AGC capacitor
value.
The shut down pin (SHDN) is useful to save energy.
When its level close to V DD (SHDN = 1), the device is
not in operation. Its DC current consumption is less
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
dependent upon many things such as temperature,
crystal used, and if the there is an external oscillator
with faster startup time. Normally, with the crystal
Figure 8. Data Out Pin with No Squelch (SQ = 1).
vendors suggested, the data will show up in the DO
pin around 1msec time, and 2msec over the
temperature range of the device. When using an
external oscillator or reference oscillator signal, the
time is reduced considerably and can be around
140μsec. See Figures Figure 10 and 11.
Figure 9. Data Out Pin with Squelch (SQ = 0).
Other components used are 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, R1 and R2
Figure 10: Time-to-Good Data After Shut Down Cycle,
Room Temperature.
March 2007
12
M9999-030107
(408) 944-0800
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