Designing Waveguide Filters for Communication Devices

RF/Microwave filters find wide application in communication systems, such as satellite links or wireless base stations. Microwave filters are passive devices employed to select a specific band of the frequency spectrum. Depending on the spectral region that is selected or rejected, they are classified in low-pass filters, high-pass filters, band-pass filters or band-stop filters. Passive devices at the output stage of the communication system must be able to deal with very high power signals. Because of that, waveguide technology is the ideal choice to implement these devices. This work presents design of different kind of waveguide based filter.

Working Principle of Filter?

So, what is magic behind filter? How does it reject signals and pass others? In order to understand this, let us first go through concept of mismatch. When there is perfect impedance match between input impedance of system with output load impedance, maximum energy transferred from input to output otherwise there are always some energy loss. A measure for this transmission loss is the reflection coefficient and the related return loss. A frequency dependent mismatch exists in RF/uW devices, due to which signals at those frequencies where the mismatch exists will experience reflection caused by the mismatch. Extreme mismatches are caused by open and short circuits. Filters approach open or short circuit impedances in their stop bands – implying near total reflection. Passive non-resistive filters work by reflection caused by a mismatch condition introduced by the frequency dependent nature of the input impedance. In the band pass filter (BPF), the resonators and the couplings are arranged in such a way, that the filter is transparent for pass band signals. In the stop bands of the filter the mismatch will cause reflection and thereby attenuation/rejection.

1. 8-Pole Interdigital Bandpass Filter

Interdigital  filters are coupled-line structures to implement bandpass filter. Interdigital filter have compact size compared to other coupled line filter hence more popular.  Below figure shows one type of 8-pole waveguide based with center frequency at 1.5 GHz. Each resonator element is a quarter-wavelength long at the midband frequency and is short-circuited at one end and open-circuited at the other end. Coupling is achieved by way of the fields fringing between adjacent resonator elements.

8-Pole Interdigital Bandpass Filter

In inter digital filter, the second pass band is centered at three times the center frequent y of the first pass band, and there is no possibility of spurious responses in between. The rates of cutoff and the strength of the stop bands are enhanced by multiple-order poles of attenuation at dc and at even multiples of the center frequency of the first pass band.

8-Pole Interdigital Bandpass Filter Result

8-Pole Interdigital Bandpass Filter meshThe simulated frequency response of the filter determined using FEM solver is shows the variations of S-parameters with frequency for the  L-band interdigital BPF.  The unwanted harmonics are suppressed with stop band attenuation better than -14 dB everywhere. FEM mesh and simulated data is are shown in figures.

8-Pole Interdigital Bandpass Filter filed

2. Dielectric filled Co-axial line Filter

Coaxial Dielectric filter filedThis is dielectric-filled coaxial cable low pass filter that is tuned with five annular rings (irises) that are added to the outer conductor wall in this design. To address the wideband frequency response with a fine frequency resolution, the model is simulated using fullwave 3D electromagnetic solver. The computed S-parameters show a low-pass frequency response with a cutoff frequency around 770 MHz.

coaxial line filter resultThis stepped-impedance low pass filter includes electrically conductive coaxial transmission line, at least one inductive element and at least one capacitive element. The capacitive elements and the inductive elements are disposed in an alternating manner along a length of the transmission line.

Coaxial Dielectric filter meshCoaxial Dielectric filter

3. Four-Resonator Comb Line Bandpass Filter

EM simulation is the key design tool for filter design and has reduced experimental comb Line Filterdesign work for distributed-element and waveguide resonator filters to a minimum or made it completely redundant. The EM simulation involves the calculation of the electromagnetic fields inside the filter structure. 3D EM simulation uses full wave analysis that is what actually exists in nature. 4-resonator combline filter fields at fc and port S-parameter response.

combline filter resultcombline filter filed

The design and dimensions of the model have been optimized to a point where great performance was significantly shown alongside with good matching around 535 MHz.



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