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LW1DSE > TECH 13.11.11 19:39l 138 Lines 5806 Bytes #999 (0) @ WW
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LC filter circuits: overview and information about the basics of the design
of an RF LC filter circuits including the design considerations and formulae
(formulas) and construction tips.
Filters using LC components, i.e. inductors and capacitors are used in a
number of radio frequency applications. These filters enable a band of
frequencies to be passed through the filter, while those in the stop band of
the band pass filter are rejected.
Filters using LC components, are arranged in ether a γ or T network.
For the γ section filter, each section has one series component and either
side a component to ground. The T network low pass filter has one component
to ground and either side there is a series in line component.
LC "γ" and "T" section low pass filters:
---------------------------------------
Low pass filters using LC components, i.e. inductors and capacitors are
arranged in ether a γ or T network. In the case of a low pass filter the
series component or components are inductors whereas the components to ground
are capacitors. In this way these filters pass the low frequency signals, and
reject the high frequency signals. These filters may be used in applications
where there are unwanted signals in a band of frequencies above the cut-off
frequency and it is necessary to pass the wanted signals in a band below the
cut-off frequency of the filter.
There is a variety of different filter variants that can be used dependent
upon the requirements in terms of in band ripple, rate at which final roll
off is achieved, etc. The type used here is the constant-k and this produces
some manageable equations:
L = Zo / (γ Fc) Henries
C = 1 / (Zo γ Fc) Farads
Fc = 1 / (γ ϋ(L x C)) Hz
Where:
Zo = characteristic impedance in ohms;
C = Capacitance in Farads;
L = Inductance in Henries;
Fc = Cutoff frequency in Hertz.
LC "γ" and "T" section high pass filters:
----------------------------------------
High pass filters using LC components, i.e. inductors and capacitors are
arranged in ether a γ or T network. In the case of a high pass filter the
series component or components are capacitors whereas the components to ground
are inductors. In this way these filters pass the high frequency signals, and
reject the low frequency signals. These filters may be used in applications
where there are unwanted signals in a band of frequencies below the cut-off
frequency and it is necessary to pass the wanted signals in a band above the
cut-off frequency of the filter.
There is a variety of different filter variants that can be used dependent
upon the requirements in terms of in band ripple, rate at which final roll
off is achieved, etc. The type used here is the constant-k and this produces
some manageable equations:
L = Zo / (4 γ Fc) Henries
C = 1 / (4 Zo γ Fc) Farads
Fc = 1 / (4 γ ϋ(L x C)) Hz
Where:
Zo = characteristic impedance in ohms;
C = Capacitance in Farads;
L = Inductance in Henries;
Fc = Cutoff frequency in Hertz.
LC "γ" and "T" section band pass filters:
----------------------------------------
Band pass filters are typically used where a small band of frequencies need
to be passed through the filter and all others rejected by the filter.
Like the high pass filters and the low pass filters, there are two topologies
that are used for these filters, namely the γ and the T configurations.
Rather than having a single element in each leg of the filter as in the case
of the low pass and high pass filters, the band pass filter has a resonant
circuit in each leg. These resonant circuits are either series or parallel
tuned LC circuits.
The equations below provide the values for the capacitors and inductors for a
constant-k filter. As the filter is a band pass filter there are two cut off
frequencies. One at the low edge of the pass band and the other at the top
edge of the pass band.
L1 = Zo / (γ (f2 - f1)) Henries
L2 = Zo (f2 - f1) / (4γ f2 f1) Henries
C1 = (f2 - f1) / (4γ f2 f1 Zo) Farads
C2 = 1 / (γ Zo (f2 - f1)) Farads
Where:
Zo = characteristic impedance in ohms;
C = Capacitance in Farads;
L = Inductance in Henries;
Fc = Cutoff frequency in Hertz.
Further details:
---------------
The choice of components for any RF filter including a bandpass filter can be
crucial to its performance. In the case of a band pass filter it is even more
important as the circuit comprises six components rather than just three. As
a result of this, close tolerance components should be used to ensure that
the required performance is obtained. It is also necessary to check on the
temperature stability to ensure that the RF filter components don't vary
significantly with temperature, thereby altering the performance.
Care must be taken with the layout of the RF filter, especially when the RF
filter is used for high frequencies. Capacitive and inductive coupling are
the main elements that cause the filter performance to be degraded.
Accordingly the input and output of the filter should be kept apart. Short
leads and tracks should be used, Components from adjacent filter sections
should be spaced apart. Screens used where required, and good quality
connectors and coaxial cable used at the input and output if applicable.
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