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
RFI2 Switch
PDN1
PDN2
Selected Crystal
Output
Description
High
High
Open
-
Power down
Frequency 1
Frequency 2, RFI2 capacitor connected in
parallel with antenna
High
Low
Low
High
Open
Closed
QO1
QO2
Low
Low
Closed
QO2
Frequency 2, RFI2 capacitor connected in
parallel with antenna
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 2 is selected then RFI2 switch is closed
to connect CANT2 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.
If frequency 1 is selected the RFI2 switch is open
and only crystal output QO1 is active. Antenna
frequency is determined by antenna inductor LANT
(see Typical Application on page 5), antenna
capacitor CANT1 and parasitic capacitances related
to antenna inputs RFI1, RFI2 and RFI3 (see
Antenna Tuning Considerations below). Frequency
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.
1 is the higher frequency of two selected
frequencies.
ANTENNA TUNING CONSIDERATIONS
The ferrite bar antenna having inductance LANT and
parasitic coil capacitance CCOIL is tuned to two
reception frequencies f1 and f2 by parallel capacitors
CANT1 and CANT2. The receiver input stage and
internal antenna capacitor switch have capacitances
frequencies. COFF2 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.
(CRFI1
,
COFF2
)
which affect the resonance
Frequency f1 (higher frequency):
1
CTOT1=CCOIL+CANT1+CRFI1+COFF2=CCOIL+CANT1+6.5pF+37pF=CCOIL+CANT1+ 43.5pF, f1 =
2π LANT ⋅CTOT1
Frequency f2 (lower frequency):
1
CTOT2=CCOIL+CANT1+CANT2+CRFI1=CCOIL+CANT1+CANT2+ 6.5pF, f2 =
2π LANT ⋅CTOT 2
3 (12)
MAS [ MICRO ANALOG SYSTEMS ]
