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XE1205
Figure 23 illustrates the switching time between two carrier frequencies in receive mode. TS_RFSW is defined as the
switching time of the frequency synthesizer and the time needed by the base band filter to reach the steady-state when
the boost process is used (Init_filter set to “11”).
Actual mode
receive
Programmed frequency
NSS_CONFIG
A ctual frequency
f1
f1
f2
f2
TS_RFSW
Figure 23. Switching between two carrier frequencies in receive mode.
Figure 24 shows the switch ing time between two carrier frequencies in transmit mode. TS _TFSW is defined as the
switching time of the frequency synthesizer and the time required by the power amplifier to reach the steady-state
operating conditions.
Actual mode
transmit
Programmed frequency
NSS_CONFIG
A ctual frequency
f1
f1
f2
f2
TS_TFSW
Figure 24. Switching between two carrier frequencies in transmit mode.
7.3.2
XE1205 switching time using SW(1:0) pins.
If MCParam_Select_mode
is high then the trans ceiver
mode is fixed by SW(1:0)
pins
and
register
MCParam_Chip_mode can be used in read mode but has no effect on the transceiver mode.
7.4
SELECTION OF THE REFERENCE FREQUENCY
The refer ence clock used for the frequency synthesizer and t he internal digital circuitry may be generated by either an
external 39 MHz quartz crystal or a TCXO or an external clock. If an external clock is used, the register OSCParam_Osc
has to be set high, and the 39 MHz clock signal should to be supplied to the pin “XTA” and the pin “XTB” should be left
open. If a TCXO is used, its output signal should be connected to the pin “XTA” and the pin “XTB” should be left.
T he transceiver can be used with a 39 MHz fundamental mode quartz crystal or with a 3 rd overtone crystal. Third
overtone operation requires an internal resistor to be connected in parallel with the crystal. This resistor can be
connected by programming the register OSCParam_Resxosc(3:0). The required value depends on the crystal
? Semtech 2008
38
www.semtech.com
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