Microwave Discrete and Microstrip Filter Design - Chapter 6

Microwave Discrete and Microstrip Filter Design

PathWave Advanced Design System (ADS)

Background

Microwave filters play an important role in any RF front end for the suppression of out of band signals. In the lumped and distributed form, they are extensively used for both commercial and military applications. A filter is a reactive network that passes a desired band of frequencies while almost stopping all other bands of frequencies. The frequency that separates the transmission band from the attenuation band is called the cutoff frequency and denoted as fc. The attenuation of the filter is denoted in decibels or nepers. A filter in general can have any number of pass bands separated by stop bands. They are mainly classified into four common types – namely low-pass, high-pass, bandpass, and band stop filters.

An ideal filter should have zero insertion loss in the pass band, infinite attenuation in the stop band, and a linear phase response in the pass band. An ideal filter cannot be realizable as the response of an ideal lowpass or band pass filter is a rectangular pulse in the frequency domain. The art of filter design necessitates compromises with respect to cutoff and roll off. There are basically three methods for filter synthesis. They are the image parameter method, insertion loss method, and numerical synthesis. The image parameter method is an old and crude method, whereas the numerical method of synthesis is newer but cumbersome. The insertion loss method of filter design on the other hand is the optimum and more popular method for higher frequency applications.

Since the characteristics of an ideal filter cannot be obtained, the goal of filter design is to approximate the ideal requirements within an acceptable tolerance. There are four types of approximations – namely Butterworth or maximally flat, Chebyshev, Bessel, and Elliptic approximations. For the prototype filters, maximally flat or Butterworth provides the flattest pass band response for a given filter order. In the Chebyshev method, sharper cutoff is achieved and the pass band response will have ripples of amplitude 1+k2. Bessel approximations are based on the Bessel function, which provides sharper cutoff, and Elliptic approximations results in pass band and stop band ripples. Depending on the application and the cost, the approximations can be chosen. The optimum filter is the Chebyshev filter with respect to response and the bill of materials. Filters can be designed both in the lumped and distributed form using the above approximations.

Results and Observations

The simulation of the lumped element model shows that the lowpass filter has a cutoff frequency of about 2 GHz and has a gentle roll off, which is expected for a Butterworth filter.

Layout Simulation Steps for Distributed Low Pass Filter

Calculate the physical parameters of the distributed lowpass filter using the design procedure given above. Since the length has already been calculated, the only parameter left to calculate is the width of the Zl and Zh transmission lines for the stepped impedance low pass filter. As a reminder, Zl = 10 Ω and Zh = 100 Ω, which means the low impedance line width is 24.7 mm and the high impedance line width is 0.66 mm. Both calculations were done with a dielectric constant of 4.6 and a thickness of 1.6 mm.

Now that the parameters have been determined, create a model of the distributed bandpass filter in a new layout window. The easiest way to do this is to use the library components. Select the TLines-Microstrip library. Use the MCFIL components for the coupled line sections and the MLIN components for the 50 Ω lines. Connect Pin 1 to the input and Pin 2 to the output.

Setup the EM simulation using the procedure defined earlier for 1.6 mm FR4 dielectric. Define the frequency plan as 1 GHz to 3 GHz with 101 number of points. Don’t forget to turn on Edge Mesh under the Options > Mesh tab of the EM Setup window.

Once the simulation finishes, plot S(1,1) and S(2,1). Add markers to the -3dB bandwidth points to determine the lower and upper cutoff frequencies. The plot is shown in Figure 23. Note that the markers are not at -3 dB. That is because the passband ripple varies between approximately -1.0 dB and -1.5 dB, which will slightly impact the definition of the lower and upper cutoff frequencies.