SI4734 Просмотр технического описания (PDF) - Silicon Laboratories
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SI4734 Datasheet PDF : 69 Pages
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AN383
APPENDIX C—AM FERRITE LOOP STICK ANTENNA INTERFACE MODEL
This appendix describes how to interface a ferrite loop stick antenna to the AM receiver input. The application note
begins with an overview of AM ferrite loop antennas followed by the interface to a ferrite loop stick antenna. The
last section of the application note presents designers with guidelines for designing ferrite loop stick antennas.
AM Ferrite Loop Stick Antenna Overview
An AM antenna works on the basis of Faraday's Law. Faraday's law dictates that a varying magnetic field through
a wire loop induces an EMF (Electro-Motive Force) in the loop and is expressed as:
EMF = –d-d----t-
Where = magnetic flux.
The negative sign in Equation indicates that the current generated in the loop is in a direction which generates a
magnetic field that opposes the magnetic field causing the induced EMF. An AM loop antenna is made of a single
loop or many loops of a conducting material wrapped around an air core or a ferrite core. In the case of radio
transmissions, the induced voltage represents the AM signal being transmitted by a radio station as
electromagnetic waves.
A ferrite loop stick antenna is a coil wrapped around a ferrite core. Ferrite is a ferromagnetic material which does
not display any magnetic properties till it is excited by a magnetic field. A ferrite multiplies the applied magnetic field
by a factor that is known as the effective permeability of the ferrite material. Since the permeability of a ferrite
material is orders of magnitude higher than air, the voltage induced in a loop antenna wound around a ferrite core
is also orders of magnitude greater than the voltage that would be induced in an air loop antenna of the same size.
All ferrite loop stick antennas have an inductance associated with them and this can be expressed as:
Lant = k--------r----l-o-r--N-----2--A---
Equation 3. Trap Frequency Calculation
Where:
Lant = Antenna inductance
k = Permeability modifier constant
µr = Relative permeability of ferrite rod
µo = Permeability of air/free space
N = Number of turns in coil
A = Cross-sectional area of ferrite rod
lr = Length of ferrite rod
In Equation 3, µr is the relative permeability of the ferrite rod. The rod dimensions play an important role in
determining the relative permeability of the rod. The permeability modifier constant is based on the ratio of ferrite
rod length to the coil length. The relative permeability of the rod and the permeability modifier constant are
combined together to yield the effective permeability of the antenna and are used to reduce Equation 3 to the
following:
Rev. 0.8
63
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