DA9081.006
1 August, 2005
FREQUENCY SELECTION
The frequency selection and power down control is
accomplished via two digital control pins PDN1 and
PDN2. The control logic is presented in table 1.
Table 1
Frequency selection and power down control
PDN1
PDN2
RFI2 Switch
Selected Crystal
Output
High
High
Low
Low
High
Low
High
Low
Open
Open
Closed
Closed
-
QO1
QO2
QO2
Description
Power down
Frequency 1
Frequency 2, RFI2 capacitor connected in
parallel with antenna
Frequency 2, RFI2 capacitor connected in
parallel with antenna
If frequency 2 is selected then RFI2 switch is closed
to connect C
ANT2
in parallel with ferrite antenna and
tune it to frequency 2. Then only crystal output QO2
is active. Frequency 2 is lower frequency of the two
selected frequencies.
It is recommended to switch the device to power
down for 50ms before switching to another
frequency. This guarantees fast startup in switching
to another frequency. The 50ms power down period
is used to discharge AGC capacitor and to initialize
fast startup conditions.
The internal antenna tuning capacitor switch (RFI2)
and crystal filter output switches (QO1, QO2) are
controlled according table 1. See switch in block
diagram on page 1.
If frequency 1 is selected the RFI2 switch is open
and only crystal output QO1 is active. Antenna
frequency is determined by antenna inductor L
ANT
(see Typical Application on page 5), antenna
capacitor C
ANT1
and parasitic capacitances related
to antenna inputs RFI1, RFI2 and RFI3 (see
Antenna Tuning Considerations below). Frequency
1 is the higher frequency of two selected
frequencies.
ANTENNA TUNING CONSIDERATIONS
The ferrite bar antenna having inductance L
ANT
and
parasitic coil capacitance C
COIL
is tuned to two
reception frequencies f
1
and f
2
by parallel capacitors
C
ANT1
and C
ANT2
. The receiver input stage and
internal antenna capacitor switch have capacitances
(C
RFI1
, C
OFF2
) which affect the resonance
Frequency f
1
(higher frequency):
C
TOT1
=C
COIL
+C
ANT1
+C
RFI1
+C
OFF2
=C
COIL
+C
ANT1
+6.5pF+37pF=C
COIL
+C
ANT1
+ 43.5pF,
f
1
=
Frequency f
2
(lower frequency):
C
TOT2
=C
COIL
+C
ANT1
+C
ANT2
+C
RFI1
=C
COIL
+C
ANT1
+C
ANT2
+ 6.5pF,
f
2
=
frequencies. C
OFF2
is switch capacitance when
switch is open. When the switch is closed this
capacitance is shorted by on resistance of the
switch and is effectively eliminated. Following
relationships can be written into two tuning
frequencies.
1
2
π
L
ANT
⋅
C
TOT
1
1
2
π
L
ANT
⋅
C
TOT
2
3 (12)
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