Matlab Based Tool For Microwave Amplifier Analysis Computer Science Essay

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The high frequency microwave components such as an amplifier, oscillator etcetera have a wide range of applications in todays world of digital communication. These are mainly used in transponders and other mobiles and radar applications which are inaccessible if faults or error occur in them. So a better solution is to perform a complete analysis of the device under all possible conditions before practically implementing it. This analysis is difficult and time consuming as it is done manually. A better approach to perform analysis is with the help of computers using CAD tools. A GUI designed on MATLAB serves the purpose of mathematical computation and analysis of the device under consideration and provides the required information on device performance.

Index Terms-Microwave Amplifier, MATLAB®

Introduction

The present communication techniques like digital communication require a high efficiency of performance. This can be provided by the Microwave amplifiers used for the corresponding range of frequencies. As such designing of high frequency amplifiers is not an easy task. There are various factors which degrade the performance of the amplifiers.

The main factors that tend to affect the system can arise due to positive feedback, mismatch in the line of operation and the noise which can be both internal and external to the system. Hence it is always better to analyze the system before practical implementation. The existing techniques for such analysis are by CAD tools such as Microwave Office, ADS (Advanced Design Suite) or simply by performing mathematical analysis manually using formulae. Though the calculations of parameters gives an idea about the device but is a time consuming process and the trial and error method is tedious to be performed for a wide range of frequencies under which the device is expected to operate practically. Whereas to the best of knowledge of authors, this CAD tools are often costly and the ones' available for free lack many important features.

The tool tries to eliminate the drawbacks of conventional methods and outputs the result in a more efficient way. It is normally found that a GUI gives a better appeal for many applications. And hence the tool 'AMPLIFIER ANALYSIS' uses GUI which includes various panels along with the important tool for Microwave i.e. THE SMITH CHART. Inputs from user are accepted by the GUI which performs mathematical calculations and the results are provided instantly. Stability, Gain and Noise circles can be plotted in the tools which are important in the analysis of a Microwave Amplifier along with certain other features which make it better than the existing analysis tools.

THEORY

It is usually seen that designing of the low frequency amplifiers like BJT, FET consume few hours to provide the expected output as analyzed on paper. It also requires proper device selection, appropriate biasing and various other factors. Similarly the high frequency amplifiers have to be analyzed and then implemented. This involves the step of transistor selection such as HEMT, HBT and MESFET which are done on the basis of application for which one needs to design. The S-parameters of the device selected are then measured by Network Analyzer. These devices are biased properly for efficiency.

Analysis of the amplifier can be done with the help of certain parameters. The results are better understood graphically than analytically. So it is better preferred to plot these values on the smith chart. On plotting it is found that the locus of such points is constant and gives a circle as a two dimensional figure.

The most important consideration in any electronic circuit is that it should not oscillate at high frequencies resulting into instability. The stability consideration of an amplifier provides the engineer a complete knowledge of the terminations which can be used at various high frequencies. The stability circle can be plotted by considering the equations for centre and radius as in Eq. (i) and Eq. (ii) for source and in Eq. (iii) and Eq. (iv) for load.

Center, Cs= ……. Eq. (i)

Radius, rs= || ……. Eq. (ii)

Center, CL= ……. Eq. (iii)

Radius, r L= | ……. Eq. (iv)

It is normally seen that the system does not provide the expected amplification. This occurs due to the losses (mismatch) in the circuit. The Gain consideration provides the Engineer, the maximum gain the system is able to generate and how it can be improved if the system does not provide the maximum gain.

mitra1.jpg

Fig. 1. Snapshot of Amplifier Analysis Tool

The gain circle can be plotted by considering the equations for centre and radius as in Eq. (v) and Eq. (vi)

Center, Cgi = ……. Eq. (v)

Radius,rgi = ……. Eq. (vi)

if i=L, ii=11 and if i=S, ii=22

Noise in electronic circuits hampers the performance of a system. Hence the devices have to be analyzed considering the Noise the system can add and also the surrounding environment can contribute. The noise circle can be plotted by considering the equations for centre and radius as in Eq. (vii) and Eq. (viii)

Center, ……. Eq. (vii)

Radius, ……. Eq. (viii)

Where,

The engineer can analyze the regions on the smith chart and decide the performance of the transistor.

Tool

The tool provides the user an easy way to perform analysis of microwave amplifier. The methodology used is a graphical approach with the help of Smith Chart. The tool is made more user friendly by the use of Graphical User Interface (GUI) provided by MATLAB. A GUI provides a platform that the user can interact with, for the tasks of gathering and producing information. It is easier for people with few computer skills to work with and use the GUI. Various elements of the GUI like push-button, edit boxes, axes windows make the GUI more lively and interactive. The snapshot of amplifier analysis tool is shown in Fig.1.

The device characteristics which are represented by the S-parameters are taken as input along with the frequency at which they are measured. The noise parameters of the device can also be added if required for analysis. Input values can be directly entered through the edit-boxes or uploaded as a Touchstone file by browsing through user files. The stability factors such as Delta (Δ), Rollet factor(k), B1, B2, µ1 and µ2 are displayed to the user to determine if the given device is stable or not. If required, the corresponding stability circles are plotted. The gain, based on the device parameters, has a maximum value. Any number of gain circles based on the user's requirement may be plotted on the smith chart.

mitra2.jpg

Fig 2. Snapshot of Results and Output

In a similar way noise circles (greater than the minimum value) may be plotted. The respective centers and radii for stability, gain and noise circles are also displayed to the user.

Once these circles are plotted, matching network is required for the amplifier to attain certain gain and noise in the stable region of operation. For the matching network, user selects a point on the display, which reflects the Γ plane. Based on the selected point, different matching configurations are provided as per the user requirement and possibilities. This includes networks based on both transmission lines and lumped elements like inductors and capacitors.

Flexibility is provided by the means of optimization. In this optimization, the user selects a new point for matching based on the gain and noise to be designed for the amplifier. Optimization serves as an important part of the tool as this allows the user to get accurate results.

Additional functionality is added by means of various tools like zoom, print, etcetera for the user to get a real time results from the tool.

Results And Discussions

The Implementation of GUI for such an important analysis has made it easier for an R.F engineer to determine the various parameters of the device even before implementing it practically. This tool helps the user to obtain the following parameters.

Stability factors(Δ,k,B1,B2,µ1,µ2)

Stability circles with Centre and Radius

Maximum value of gain for the S-parameters of the device

Gain circles with centre and radius

Noise circles with respective noise parameters, centre and radius

Matching Network

The most important feature is that it provides the user the possible matching network with the respective values of elements. The tool also has a feature of Optimization i.e. the user can alter the points and acquire the required result. Various circles are represented by different color combinations which help the user to identify them easily. Each circle holds the respective frequency for which it is plotted which makes it better for user. The user has a facility of matching the point using two possible techniques

Micro strip Matching

Matching using passive elements

Though these techniques of matching provide considerable effects on the device, the user can change the source and load terminations and hence see the effect of various different parameters like stability, gain and noise. The results obtained from the tool are shown in Fig.2.

It is found that MATLAB, on which the tool is built, provides the advantage to convert the code in a standalone format which has helped the authors to create the exe file of the code. This file consists of the MATLAB compiler which is freeware, so it is never required for an RF engineer to have MATLAB on his computer to use this tool. The tool is independent of MATLAB and can run on any 32-bit, 64-bit Operating System, which makes it easy for the user to utilize the tool irrespective of the configuration of his system.

Conclusion

The discussions made and the results obtained lead to the conclusion that this tool is versatile and yet simple. The tool provides a platform for fast and accurate calculations and graphical analysis as compared against the time consuming and less accurate manual method. The use of Touchstone file format enables use at multiple frequencies and hence makes analysis simpler. This works as a standalone application and the user need not purchase expensive licenses. The end software is easy to install and occupies less memory space.

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