DA6179.000
22 January, 2007
ANTENNA TUNING CONSIDERATIONS
The ferrite bar antenna having inductance LANT and
parasitic coil capacitance CCOIL is tuned to three
reception frequencies f1, f2 and f3 by parallel
capacitors CANT1, CANT2 and CANT3. The receiver
input stage and internal antenna capacitor switches
have capacitances CRFI, COFF2, COFF3 which affect
the resonance frequencies. COFF2 and COFF3 are
switch capacitances when switches are open. When
switches are closed these capacitances are shorted
by on resistance of the switches and they are
effectively eliminated. Following relationships can
be written for the three tuning frequencies.
Frequency f1 (highest frequency):
CTOT1=CCOIL+CANT1+CRFI+COFF2+COFF3
1
f1 =
2π LANT ⋅CTOT1
Frequency f2 (middle frequency):
CTOT2=CCOIL+CANT1+CANT2+CRFI+COFF3
1
f2 =
2π LANT ⋅CTOT 2
Frequency f3 (lowest frequency):
CTOT3=CCOIL+CANT1+ CANT2+ CANT3+CRFI
1
f3 =
2π LANT ⋅CTOT 3
5 (10)
MAS [ MICRO ANALOG SYSTEMS ]
