DA9179.002
19 May, 2005
ANTENNA TUNING CONSIDERATIONS
The ferrite bar antenna having inductance L
ANT
and
parasitic coil capacitance C
COIL
is tuned to three
reception frequencies f
1
, f
2
and f
3
by parallel
capacitors C
ANT1
, C
ANT2
and C
ANT3
. The receiver
input stage and internal antenna capacitor switches
have capacitances C
RFIP
, C
OFF2
, C
OFF3
which affect
the resonance frequencies. C
OFF2
and C
OFF3
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 into three tuning frequencies.
Frequency f
1
(highest frequency):
C
TOT1
=C
COIL
+C
ANT1
+C
RFIP
+C
OFF2
+C
OFF3
=C
COIL
+C
ANT1
+6pF+37pF+119pF=C
COIL
+C
ANT1
+162pF,
1
f
1
=
2
π
L
ANT
⋅
C
TOT
1
Frequency f
2
(middle frequency):
C
TOT2
=C
COIL
+C
ANT1
+C
ANT2
+C
RFIP
+C
OFF3
=C
COIL
+C
ANT1
+C
ANT2
+ 6pF+119pF=C
COIL
+C
ANT1
+C
ANT2
+ 125pF,
1
f
2
=
2
π
L
ANT
⋅
C
TOT
2
Frequency
f
3
(lowest frequency):
C
TOT3
=C
COIL
+C
ANT1
+ C
ANT2
+ C
ANT3
+C
RFIP
=C
COIL
+C
ANT1
+ C
ANT2
+ C
ANT3
+6pF,
1
f
3
=
2
π
L
ANT
⋅
C
TOT
3
4 (9)
MAS [ MICRO ANALOG SYSTEMS ]
