Microwave Filter design, Applications to BPF ,Application to LPF, Application to dual stopband filter, Band-pass filters

Microwave Filter design

              There is a vast amount of published literature on the design of microwave filters from which some are highly mathematical. Fortunately there are some relatively straight forward procedures which enable us to design certain useful classes of microwave filters. Filter designing using the image parameter method consist of a cascade of simpler two-port filter sections to provide the desired cutoff frequencies and attenuation characteristics but do not allow the specification of a frequency response over the complete operating range.
Thus, although the procedure is relatively simple, the design of filters by the image parameter method often must be iterated many times to achieve the desired results. A more modern procedure, called the insertion loss method, uses network synthesis techniques to design filter with a completely specified frequency response. The design is simplified by beginning with low-pass filter prototypes that are normalized in terms of impedance and frequency. Transformations are then applied to convert the prototype designs to the desired frequency range and impedance level. Both the image parameter and the insertion loss method of filter design provide lumped element circuits. For microwave applications such designs usually must be modified to use distributed elements consisting of transmission line sections.

  Applications to BPF

Conventional PBGSs or PBGSs with non-uniform distribution generate stopband at a particular frequency. Second harmonic suppression may be attained significantly with the non-uniform PBGSs as they provide better performance than conventional ones. Uniform circular PBGS and non-uniform circular PBGS with Binomial distribution will be investigated to see their capabilities of harmonic suppression. To suppress the third harmonic along with second harmonic, the stopband should be enormously large so that they can cover both the harmonics. As PBGSs follow Bragg’s condition to define the approximate center of the stopband frequency so they cannot generate enormous stopband suitable for third harmonic suppression. In this circumstance DGSs may be considered to be a vital candidate for both the second and third harmonic suppression.

    Application to LPF     

   All the available works on LPF are seen to use PBGSs/DGSs in the ground plane to improve the performance of conventional LPF. Such designs need to take care both in ground plane and in conductor plane. In the present research the non-uniform distribution of PBGSs and DGSs are considered to realize improved LPF performance without having any attention into conductor layer. Taking care of slots etched in the ground plane is only needed.

   Application to dual stopband filter

Attempt will be taken to design a dual stopband filter by PBGSs and DGSs. The dual stopband filter will be realized by conventional PBGSs designed at two frequencies. But this design is not compact as it consists of two sets of PBGSs. To make it more compact, conventional DGSs with proper dimensions will provide a dual stopband filter. Besides hybrid designs with modified DGSs having interleaved PBGSs will also yield a compact dual stopband filter. Chebyshev distribution in the hybrid DGS-PBGS design will further improve the performance. All these designs are novel indeed.

Band-pass filters

There are applications where a particular band, or spread, or frequencies need to be filtered from a wider range of mixed signals. Filter circuits can be designed to accomplish this task by combining the properties of low-pass and high-pass into a single filter. The result is called a band-pass filter. Creating a bandpass filter from a low-pass and high-pass filter can be illustrated using block diagrams: (Figure below)

                                       System level block diagram of a band-pass filter

What emerges from the series combination of these two filter circuits is a circuit that will only allow passage of those frequencies that are neither too high nor too low. Using real components, here is what a typical schematic might look like Figure below. The response of the band-pass filter is shown in (Figure below) 

 

Capacitive band-pass filter.

The fact that the high-pass section comes “first” in this design instead of the low-pass section makes no difference in its overall operation. It will still filter out all frequencies too high or too low.While the general idea of combining low-pass and high-pass filters together to make a bandpass filter is sound, it is not without certain limitations. Because this type of band-pass filter works by relying on either section to block unwanted frequencies, it can be difficult to design such a filter to allow unhindered passage within the desired frequency range. Both the low-pass and high-pass sections will always be blocking signals to some extent, and their combined effort makes for an attenuated (reduced amplitude) signal at best, even at the peak of the “pass-band” frequency range. Notice the curve peak on the previous SPICE analysis: the load voltage of this filter never rises above 0.59 volts, although the source voltage is a full volt. This signal attenuation becomes more pronounced if the filter is designed to be more selective (steeper curve, narrower band of passable frequencies).

There are other methods to achieve band-pass operation without sacrificing signal strength within the pass-band. We will discuss those methods a little later in this chapter.