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LC FILTERS DESIGN
THIS FILTERS TUTORIALS SITE
This chapter lc filters is the most interesting tutorial of all for newcomers to radio design. Without a thorough grasp of lc filters you will never fully understand the ramifications of receiver and transmitter design.
Here you should gain a full understanding of the selection and design basics for lc filters. These lc filters topics cover everything necessary for your continued understanding of radio design and transmitter design.
LOW PASS FILTERS
We will start with low pass filters (LPF) because they are the basis of other filter designs. High pass or band pass filters are often simply transformations from low pass filter designs. It is simply two L networks added together.
HARMONIC TRAP FILTERS
With the addition of one small component, a capacitor, we can transform a low pass filter into a filter with near infinite attenuation at a designed trap frequency. I call this an "harmonic trap filter".
BAND PASS FILTERS
LC Band pass filters are usually LC filters containing resonator combinations of inductance and capacitance which are designed mathematically to respond to design frequencies while rejecting all other out of band frequencies. Because LC bandpass filters have inherent limitations these statements should not be taken too literally.
CONVENIENT FILTER SOFTWARE PROGRAM
This LC and crystal filter software was written by Neil Heckt of Almost All Digital Electronics as an aid for filter designers to simply plug in various filter parameters and hey presto! there is your finished design. I will present you with the design example of a crystal filter of the crystal ladder filters variety as designed on my evaluation copy of the downloaded software.
HIGH PASS FILTERS
Assuming you have mastered the design of low pass LC filters we will now proceed to the design of a high pass filters. A high pass filter is simply the transformation of a low pass filter. Just as one high pass filter design example, we will say we need a five pole butterworth filter with a cut off frequency Fc at 2000 Khz. That is we want to pass all frequencies above 2000 Khz but attenuate those below 2000 Khz, that is the function of a high pass filter.
IF AMPLIFIER BAND PASS FILTERS
IF amplifier filters are LC resonator stages separated by individual stages of amplification. The principle is called synchronously tuned filters stages. In IF amplifier filters there can be any number of resonators per individual stage.
ACTIVE BAND PASS FILTERS
Active bandpass filters are simply filters constructed by using operational amplifiers as active devices configured to simulate inductors or what are known as "gyrators". Active bandpass filters are used largely at audio frequencies where otherwise the size of the inductor would become prohibitive. The are many different types of active filters including high pass, low pass, band reject and there are numerous responses including multiple feedback bandpass (MFBP), dual-amplifier bandpass (DABP) and, state variable bi-quad all pole circuits. Interestingly all known filter responses such as Butterworth and Chebyshev may be synthesised.
Buy Lancaster's Active Filter Cookbook - Don Lancaster
NARROW BAND PASS FILTERS
Narrow band filters are filters where the fractional bandwidth is extremely small in a relative sense. My personal definition of a narrow band filter is where the narrow band filter has a fractional bandwidth NOT exceeding 1%. A filter with a bandwidth of 200 Khz ( ± 100 Khz ) centred around 7100 Khz would represent a bandwidth of about 2.86% Here I present a very detailed design of a Butterworth narrow band filter.
ANTENNA PRE-SELECTOR FILTERS
I'm asked every so often to produce a page on antenna pre-selector filters. An antenna pre-selector filter is nothing more than a narrow band filter, a topic already covered at some length on another page. Some confusion results because the narrow band filter topic contains no provision for variable tuning. There are several reasons for this. Contrary to your immediate impressions I find no particular benefit in introducing variable tuning to an antenna pre-selector filter. I can see a number of drawbacks however.
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Updated 1st January, 2001