Filter Design Part 1. Frequency Response of Low-Pass, High-Pass, Band-Pass & Band-Stop Filters.

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Filters Freq Response: Low-Pass, High-Pass, Band-Pass & Band-Stop Filters.

Filters play a critical role in many RF/microwave applications, where they are used to separate or combine different frequencies. Given the limited nature of the electromagnetic spectrum, which must be shared, filters are essential for selecting or confining RF/microwave signals within designated spectral limits. Emerging applications, such as wireless communications, impose increasingly stringent requirements on RF/microwave filters, demanding higher performance, smaller size, lighter weight, and lower cost. Depending on specific requirements and specifications, RF/microwave filters can be designed as lumped-element or distributed-element circuits. They may also be implemented using various transmission line structures, including waveguides, coaxial lines, and microstrips.

Radio frequency (RF) & microwave filters represent a class of electronic filters designed to operate on signals in the megahertz to gigahertz frequency ranges (medium frequency to extremely high frequency). These components are used in electronic systems to pass or reject specific frequencies and attenuate unwanted signals within the microwave and RF range. This frequency range is commonly used by most broadcast radio, television, and wireless communication systems (e.g., cellphones, Wi-Fi, etc.). As a result, most RF and microwave devices include some form of filtering for the signals they transmit or receive. Such filters are often used as building blocks for duplexers and diplexers, enabling the combination or separation of multiple frequency bands.

Type of Filter
Centre Frequency, fo
Cutoff Frequency, fS
Selectivity Factor, Q of BPF
Shape Factor, SF
Insertion Loss, IL
VSWR & Return Loss
Phase Linearity & Group Delay
Filter Impedance

Low Pass filter allow low-frequency signals without any attenuation (decrease in power) but it rejects any high-frequency signals

Allows high frequency signals to pass without any attenuation in its amplitude & blocks (rejects) any low-frequency signal is called high pass filter.

This type of filter allows a specific band of frequencies & blocks any other frequencies lower or higher than its passband frequencies.

This type of filter attenuates the signal whose frequencies lies in a fixed band of frequencies.

In general, most RF and microwave filters consist of one or more coupled resonators. Therefore, any technology used to create resonators can also be applied to fabricate filters. The unloaded quality factor of the resonators typically determines the filter's achievable selectivity. Generalized filter theory operates on the principles of resonant frequencies and coupling coefficients of coupled resonators in a microwave filter.

Lumped-element LC filters
Planar filters
Coaxial filters
Cavity filters
Dielectric filters
Electroacoustic filters
Energy tunneling-based filters

Lumped-Element LC Filters
The simplest resonator structure used in RF and microwave filters is the LC tank circuit, which consists of parallel or series inductors and capacitors. These circuits are very compact, but the low quality factor of the resonators results in relatively poor performance.
Lumped-element LC filters operate within upper and lower frequency limits. At very low frequencies (kHz to Hz), the size of the inductors required for the tank circuit becomes prohibitively large. To address this issue, very low-frequency filters are often designed using crystals. At higher frequencies (above 600 MHz), the inductors in the tank circuit become too small to be practical. This is because the electrical reactance of an inductor with a given inductance increases linearly with frequency. At higher frequencies, achieving the same reactance would require an impractically low inductance.

Planar Filters / Distributed Element Filter
Planar transmission lines, such as microstrip, coplanar waveguide and stripline, are effective in creating filters. The processes used to manufacture microstrip circuits are very similar to those employed in the production of printed circuit boards (PCBs). This similarity gives these filters the significant advantage of being largely planar.
Precision planar filters are typically manufactured using a thin-film process. Higher Q factors can be achieved by employing low-loss tangent dielectric materials for the substrate.

Coaxial Filters
Coaxial transmission lines provide a higher quality factor than planar transmission lines, making them ideal for applications that require higher performance.