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SI4785 Datasheet PDF : 69 Pages
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AN383
AM Ferrite Loop Stick Antenna Interface
The front-end of the AM RX is an LC tuned circuit, the purpose of which is to gain the received signal from any
tuned AM station. The LC tuned circuit comprises of an inductance which comes form the attached antenna and a
variable capacitance that is provided by a tunable capacitor inside the AM RX. Figure 41 shows the model circuit
for the front-end of a loop antenna attached to the AM RX.
Ferrite Loop Stick Antenna PCB
AM Front End
Vant
R ant
Lant
Cant
CPCB
R de-Q
C tune
R LN A
CLNA
R C -tun e
Figure 41. AM Front-End with Ferrite Loop Antenna
The purpose of the variable tuning capacitor (Ctune) is to resonate the front-end such that the resonance frequency
is the same as the frequency of the AM station one wants to listen to. In order to tune to a specific station, the Ctune
capacitor is adjusted such that the front-end achieves resonance and the voltage received from the antenna is
gained up before entering the AM front end. Ctune also has a stray resistance (RC-tune) associated with it which
affects the Q of the tuning capacitor. The Q of the on-chip tuning capacitor is given as follows:
QC – tune = -----o---R----C-----–---t-1-u--n---e---C----t--u---n---e
Equation 8. On-chip Capacitor Q Calculation
Where:
QC-tune = Quality factor of on-chip tuning capacitor
o = Resonance frequency
RC-tune = Stray resistance of on-chip tuning capacitor
Ctune = Capacitance of on-chip tuning capacitor
The reason the stray resistance is shown as a variable resistance in Figure 41 is because the on-chip capacitor is
implemented as a bank of capacitors and as capacitors are switched in or out, the stray resistance also changes.
The purpose of the variable resistor (RDe-Q) in the front-end is to reduce the gain of the front-end circuit. Being able
to reduce gain is helpful if the received signal is too strong and does not need to be gained by the front-end circuit.
It will be shown later that the gain of the circuit at resonance is equal to Q of the circuit and is directly proportional
to the total resistance between the AM input and ground. Since the variable resistor (RDe-Q) is part of the total
resistance between the AM input and ground and is the only variable resistor, it can be stated that the gain of the
front-end can be controlled by the De-Q resistor.
CPCB is the capacitance associated with the PCB. RLNA and CLNA are the impedance and the capacitance from the
Low-Noise Amplifier (LNA). The circuit in Figure 41 can be reduced to a simpler circuit by assuming that the circuit
is tuned to a certain AM station and the front-end is in resonance, which allows the conversion of Rant to a parallel
resistance and RC-tune to a parallel capacitance. The new circuit is shown in Figure 42:
Rev. 0.8
65
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