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Abstract—
The aliasing signal interferes with the actual signal and resembles more-like the original intermediate signal. This is called image frequency. To eliminate this image frequency a filter is to be designed in the RF (Radio Front) stage. At S-band microwave frequency (2 GHz to 4 GHz), the lumped components cannot be used due to distributed effects. Also, for the compactness of the filter, the distributed components are being used in microwave frequencies. Hence the coupled stage of microstrip filter with good frequency response is adopted in this paper.
The filter design is carried out by using a simulation software namely, ADS (Advanced Design System). Only the coupled stages of a microstrip filter cannot reduce the aliasing effect, hence extra strict requests are made on the insert loss and standing wave ratio of the received signal in a satellite receiver. This filter is designed at the center frequency of 2.491 GHz. Only on optimization, the filter achieves minimum aliasing interference at the desired center frequency. This is done by the GOALs and OPTIM controller embedded with ADS.
Keywords: image frequency, insertion loss, reflection ratio, microstrip filter.
Introduction
Satellites receivers at microwave frequencies are being used for a large number of purposes. A satellite receiver without adequate filtering at its input is able to pick up signals at two different frequencies simultaneously; the desired frequency and the image frequency. Hence there is a need for the installation of the preselect filter at the RF stage before mixing. At high frequencies (GHz or higher) the wavelength is so short that only distributed elements are possible to practically realize, while at low frequencies lumped elements are used due to the fact that distributed elements become too large[2]. Also, for the compactness of the system, distributed elements are preferred. The existing systems focuses on the Chebyshev type of band pass filter to overcome this problem because this filter provides desired roll-off in the stop band. But the existing system requires more than seven stages to attain the attenuation of around 66 dB. This is evident from the plot of attenuation verses normalized frequency in fig.2, which demands more number of stages for the same attenuation in the alias signal frequency. These problems can be eliminated with the proposed system where, the microstrip filter is used which posses small-size, light weight and high frequency response[3]. This proposed system filter design is made feasible through the simulation software called ADS by Agilent Corporation. This software can make a thorough analysis on high frequency circuit design till realization with high accuracy. This is dealt with the rest of the sections in this paper.
The section II discusses about the design flow of microstrip filter in detail. The design is done through the calculation of the microstrip filter parameters using the LineCalc tool in ADS. The schematic design of the microstrip filter is simulated and the frequency response curve is obtained before optimization. Then the optimization of the filter is carried out using the OPTIM controller with ADS in section III. The optimized values are obtained after which the results are discussed in section IV.
filter design
The filter design steps:
Fig.1 Filter design steps
The project aims in the design of the filter type which can produce the maximum attenuation at the center frequency of 2.491 GHz. This band pass filter selects the band of lower
and upper cut-off frequencies as 2.441 GHz and 2.541 GHz respectively. The higher attenuation reduces the energy of the alias interference at this particular frequency. The length of each stage is considered to be λ/4 (