Improving the Learning of Superheterodyne Demodulation of Frequency-Division Multiplexing Signals via the Educational Software Tool DOSHER

Abstract

The knowledge of signal demodulation processes using superheterodyne receivers is of great importance in the field of telecommunications. Superheterodyne receivers receive the current focus of many scientists in a wide variety of applications. This topic is part of the syllabus of the Communication Theory course at the School of Telecommunications of the Polytechnic University of Cartagena, Spain. The authors found that the academic performance of the students had not been entirely satisfactory in recent years. This situation was aggravated during the COVID-19 lockdown. Students had to reinforce their knowledge independently at home, despite the support provided by teachers. To the best of the authors’ knowledge, there is a noticeable lack of educational tools in this area; of those that are available, they exhibit a mismatch with the specific needs of this subject. This manuscript shows how the design of the educational software tool DOSHER, tailored to enhance the understanding of superheterodyne receivers, successfully alleviated the aforementioned drawbacks. DOSHER was designed, developed, and applied during the 2020–2021 academic year (during the COVID-19 lockdown). The results show that students were not only very satisfied with its use, but they also improved their marks. Analysis of students’ academic performance in the year of DOSHER implementation showed an average improvement in their marks of between 9–12% compared with previous years. In 2021–2022, when DOSHER was fully operational from the start, the improvement in terms of pass rate (31%) at the first mid-term was significant compared to previous years (<20%).

1. Introduction

The pandemic caused by COVID-19, in March 2020, forced Spanish educational institutions to suspend face-to-face teaching activities and their consequent virtual development. This fact required a redesign of the usual teaching and learning contexts at all levels of the Spanish educational system, specially in university teaching. This situation has therefore highlighted the need to strengthen the digitalization of education and the use of software tools to support teaching [1,2,3]. Not only is it essential for teachers and students to acquire greater digital competence, but the need for tools and simulators that can complement or reinforce face-to-face practical activities in the laboratories of many universities has also become apparent. In this sense, in the world of engineering, having software applications to support students in their learning is a fundamental requirement. In addition, the development of what is known as “scientific competence” among students should be facilitated so that they can also carry out experiments and simulations autonomously, thus making it possible to learn theoretical concepts through their application in more realistic environments. Unfortunately, in many cases, the applications and tools available for this purpose are for professional use with expensive licenses or do not even exist for certain fields of interest. Traditionally, the MATLAB and Simulink environments from Mathworks have been used for learning various scientific or engineering disciplines, as well as tools such as PSpice, COMSOL, or Mathcad, and ad hoc applications have been developed in high-level languages such as C++ or Fortran [4,5,6,7].

In particular, knowledge of the signal demodulation processes using superheterodyne receivers is of great importance in the field of telecommunications. Superheterodyne receivers play a central role for multiple scientists in a wide variety of applications, involving high-frequency communications [8,9,10,11,12], lasers [13,14], acoustics [15,16], diagnostic purposes [17,18], etc. This topic is part of the curriculum for bachelor’s degrees in telecommunications and electrical engineering subjects, such as Communication Theory, taught at the Higher Technical School of Telecommunications Engineering (ETSIT) of the Polytechnic University of Cartagena (UPCT), in Murcia, Spain.

To the best of the authors’ knowledge, there is a noticeable lack of educational applications and tools in this area, as well as a mismatch with the specific needs of this subject by those that are available [19,20,21,22,23,24]. There are a few fee-based hardware learning platforms designed to study and understand the working principle and basic operation of superheterodyne radio receivers, such as Scientech TechBooks [25], and some more generic fee-based software simulation applications for communications, such as TIMS (Telecommunication Instructional Modeling System) [26], and kits combining a hardware and software framework [27]. Therefore, the need to develop a graphical, intuitive, flexible, and free tool to reinforce the theoretical knowledge acquired in this branch of telecommunications engineering in an autonomous, practical, fast, and visual way for students, without the need to program code, generate new algorithms, create modules, develop complex setups, and pay a fee, is key. DOSHER, therefore, represents a contribution to the application of educational software in the field of communication systems and the demodulation process, aimed at predicting and analyzing the demodulation of frequency-division multiplex (FDM) signals using superheterodyne receivers. Furthermore, the tool invented by the authors and presented here includes an embedded tutorial with three guided practice exercises for the students to perform, which could be part of syllabuses focusing on the theory of communication.

This research has shown that the development and use of software applications tailored to the needs of students, such as DOSHER, is necessary in the engineering field to improve student satisfaction and academic performance, particularly in situations where face-to-face attendance is not possible.

The paper is outlined as follows: Section 2 explains the methodology followed, introduces the terminology and basics, and gives an insight into the graphical user interface (GUI); Section 3 presents the results obtained thanks to DOSHER; finally, Section 4 presents the authors’ conclusions.

2. Materials and Methods

The methodology used to design DOSHER involved first compiling the marks achieved by students in the first part of their Communication Theory studies over the years 2017–2018, 2018–2019, and 2019–2020. The data collected showed that the learning about the superheterodyne detection of modulated signals was significantly poor, with a pass rate of less than 20% of those who took the exams. Among the possible reasons for this was the fact that the teaching of this part of the subject requires significant graphical support, which is usually provided by the teacher by solving exercises using the traditional blackboard. In addition, this part of the subject requires extensive practice in order to master the different situations that can arise depending on the tuning parameters chosen and the type of modulated signals. In addition, a decline in academic performance was observed in the course from 2019–2020, when COVID-19 imposed university closures and classes had to be conducted remotely.

Based on this evidence, it was considered necessary to reinforce the teaching–learning process by using free, intuitive software tools adapted to the content of this subject, but nothing with these characteristics was found on the market.

2.1. Intermediate Frequency Demodulation

A superheterodyne receiver is a device present in all modern TV and radio receivers, in which the incoming radio frequency (RF) signal is converted to an intermediate frequency (IF) signal through the intervention of a mixer and a local oscillator [28]. It is also used for the superheterodyne detection of high-frequency acoustic waves [29], or can even involve both acoustic and electromagnetic signals, using a mixer-first acoustic-filtering front end [16].

The block diagram in Figure 1 presents a tunable bandpass filter of B_RF bandwidth, a mixer with an also adjustable local oscillator (LO), a fixed bandpass filter centered to IF (f_IF) with B_IF bandwidth, and a baseband demodulator (either made up of another mixer, LO, and low-pass filter, if they are linearly modulated signals, or if they are Frequency Modulated (FM) or Phase Modulated (PM) signals, by an angle demodulator, for example):

We can estimate the output frequency of the first mixer as:

$$f_{out}=\left|f_{in}\pm f_{LO}\right|$$

(1)

where $f_{LO}$ is the frequency of the RF local oscillator, and $f_{in}$ is the input frequency to the mixer. In a receiver, when $f_{LO}$ is less than the RF ($f_{in}$), it is referred to as low-side injection, and the mixer is usually called a low-side down-converter. Alternatively, when the LO frequency is above the RF, it is defined as high-side injection, and the mixer is named a high-side down-converter.

If a high-side tuning is chosen and $f_c$ is the carrier frequency of the channel to be demodulated, we find that, for $f_{in}=f_c$, and $f_{LO}=f_{IF}+f_c$,

$$f_{out}=\left|f_{in}\pm (f_{IF}+f_c)\right|.$$

(2)

Therefore, the original signal will be transferred to:

$$f_{out}=\left|f_c+(f_{IF}+f_c)\right|=\left|2f_c+f_{IF}\right|$$

(3)

Another replica of the signal will fall on the spectrum at:

$$f_{out}=\left|f_c-(f_{IF}+f_c)\right|=\left|-f_{IF}\right|=f_{IF}$$

(4)

On the other hand, if a low-side injection is considered, with $f_{LO}=f_c-f_{IF}$, the output frequency of the RF mixer will be $f_{out}=\left|f_{in}\pm \left(f_c-f_{IF}\right)\right|$. One copy of the signal will be carried at:

$$f_{out}=\left|f_c+\left(f_c-f_{IF}\right)\right|=\left|2f_c-f_{IF}\right|$$

(5)

and the other one will be moved to:

$$f_{out}=\left|f_c-\left(f_c-f_{IF}\right)\right|=\left|f_{IF}\right|=f_{IF}$$

(6)

After applying the IF band-pass filter, only the selected signal of interest will go to $f_{IF},$ assuming no interferences at the RF (Figure 2).

2.2. Image Frequency

Image frequency interference is a negative phenomenon that occurs when portions of other signals that are at a spectral distance $f_{IF}$ from the local oscillator, and specular to the tuned carrier, that is, $f_{in}=f_c+2f_{IF}$ (when up-side injection) or $f_{in}=f_c-2f_{IF}$ (when low-side injection), are not properly filtered by the RF filter and cause IF interference. When high-side injection is set:

$$f_{out}=\left|f_c+2f_{IF}\pm (f_{IF}+f_c)\right|$$

(7)

So, the interference will fall on:

$$\left|f_c+2f_{IF}+(f_{IF}+f_c)\right|=2f_c+3f_{IF},$$

(8)

which would properly be filtered by the IF band-pass filter, or

$$\left|f_c+2f_{IF}-(f_{IF}+f_c)\right|=f_{FI},$$

(9)

where the interferent signal would overlap with the desired one. Alternatively, if low-side tunning is configured:

$$f_{out}=\left|f_c-2f_{IF}\pm (f_c-f_{IF})\right|,$$

(10)

Then, the interference will fall at:

$$\left|f_c-2f_{IF}+(f_c-f_{IF})\right|=2f_c-3f_{IF},$$

(11)

which would again be removed by the IF band-pass filter, or

$$\left|f_c-2f_{IF}-(f_c-f_{IF})\right|=f_{FI},$$

(12)

where the interferent RF signal would drop again within the IF band.

2.3. DOSHER Software Tool

The authors were responsible for all the stages of the DOSHER development process, including planning, design, and coding, in order to create, within a few months, a computer program capable of helping students to master this block of the subject, both in class and autonomously at home, and which provides them with guided practical exercises and a complete user manual.

DOSHER (acronym for DOwn Super HEterodyne Receiver) version 1.2 is an application developed in the MATLAB environment, compiled so as to be able to be run autonomously through an executable (.exe) file, which allows the analysis, in frequency and level, of FDM signals modulated in Double Sideband (DSB), Low Single Sideband (LSSB), or Upper Single Sideband (USSB).

2.3.1. Graphical User Interface

The capabilities and tools available in the application are briefly described below. Figure 3 provides a general view of the DOSHER graphical user interface (GUI).

Step-by-step menus for the demodulation process with DOSHER are detailed in Appendix A.

The following is a summary of the procedure for DOSHER users to analyze and adjust the parameters of a superheterodyne receiver. First, the student must manually load or create the desired RF multiplex signals. The user then enters the parameters of the filters and local oscillators of each receiver stage in “Parameters”, selects the type of tuning (either up-side or low-side) and the intermediate frequency, and selects the channel to be demodulated from the numbered list. Once the receiver is fully defined, the user proceeds to “Demodulate”. The result of this process is presented graphically in the 6 figures embedded in the DOSHER HMI, showing the power spectral density of the signal before and after each receiver stage. The DOSHER user can check whether there is any interference by image frequency and, if so, in which spectral band it occurs, as well as the intermediate frequency ranges that avoid this negative phenomenon. Finally, once the receiver has been properly designed, the user can generate a report with all the graphs and data by pressing the “Report” button.

2.3.2. DOSHER Flowchart

The flowchart in Figure 4 indicates the step-by-step process to be followed to obtain the available results with DOSHER.

2.3.3. Guided Practices Using DOSHER

As part of its Help/Tutorial option, DOSHER includes a set of three laboratory sessions (Appendix B) with corresponding instructions to guide students in properly achieving their objectives during their eventual laboratory sessions. In this sense, such exercises are designed to last less than 1 h and 30 min—as discussed in the following section.

The competencies and skills that are expected to be acquired throughout the activities are:

• Understand the basics of demodulation using superheterodyne receivers.
• Review the influence of different linearly modulated signals (DSB, LSSB, USSB) when demodulating with superheterodyne receivers.
• Adjust filters and local oscillator parameters to improve the demodulation of the desired signal with both up-side and down-side tuning.
• Understand the image frequency phenomenon when demodulating signals.
• Design and tune superheterodyne receivers to avoid image frequency interference.
• Increase the motivation to face new challenges and problems in this field of theory of communication and signal processing, and find solutions.

With the practice exercises proposed, the students will be able to:

• Reinforce their knowledge of the demodulation of linearly modulated signals with superheterodyne receivers in a comprehensive and simple way, developing competencies 1 to 5 throughout the 3 practices.
• Analyze, design, and find solutions for superheterodyne receivers, carried out in the practical sessions covering competencies 3 to 6.
• Be aware of the influence of the image frequency phenomenon and how to overcome it, by means of reinforcing skills 4 and 5 in practice 3.

3. Results

The School of Telecommunications at the Universidad Politécnica de Cartagena, Spain used this software in one subject, “Theory of Communication”, throughout the 2020–2021 academic year. This subject is included in two bachelor’s degree programs, with a duration of four academic courses. Theory of Communication is a core subject offered in both the Engineering Telematics and Engineering Telecommunication Systems degree programs in the second quarter of the second year, with 183 students enrolled during the study period. The subject is divided into two main blocks, the first of which focuses on the content of superheterodyne receivers. The tool was made available at the discretion of any enrolled student via remote access to the UPCT Virtual Classroom platform (supported by Moodle) after the first term test until the final exam of the 2020–2021 term, once it was fully debugged.

In addition, a voluntary and anonymous survey based on different authors, [30,31] was administered to the students using DOSHER. Both questions and results are shown in

Table 1. Evaluation survey of DOHSER administrated to rate DOSHER features, using scale of 1 (strongly disagree) to 5 (strongly agree).

Question	M	D
I value my previous knowledge of designing and tuning superheterodyne receivers for demodulation of FDM signals with a score of:	3.44	1.15
The use of DOSHER enhances my motivation and interest in the subject	3.83	0.92
The application allows the acquisition of new theoretical concepts	3.67	1.19
The application allows the consolidation of theoretical concepts	3.89	0.76
Theoretical concepts could be better learnt without DOSHER	1.72	0.46
The application has a clear and intuitive structure	4.17	0.86
The level of interactivity was appropriate	4.00	0.77
Graphical simulation and results were useful	3.78	0.65
The look of the interface was attractive	4.17	0.79
The tool allows the design and optimization of superheterodyne receivers free from image frequency interference	3.94	0.94
My knowledge level of superheterodyne receivers has improved after using DOSHER.	3.83	0.79
The help tutorial, sample files and exercises included in DOSHER are helpful	4.06	0.94
DOSHER has helped me to solve or contrast the solution to the problems and exams presented in class	3.72	0.75
My overall assessment of the tool is positive	4.20	0.80

, where M is the mean and D is the standard deviation of the responses. Students were asked to rate the features, acceptability and usability of DOSHER choosing a scale of 1 (strongly disagree), 2 (disagree), 3 (neither agree nor disagree), 4 (agree), and 5 (strongly agree).

In view of the results of the survey, the general assessment of the students (43 replies) can be considered highly satisfactory (4.2 out of 5 for question 14). The analysis of the statistical significance obtained shows, based on the percentage of students who completed the survey, an uncertainty (1σ) of 3.17%, and a margin of error for a confidence interval of 2σ of approximately 6.3%.

In addition, teachers found it interesting to define voluntary tasks in the UPCT virtual platform so that students could upload the reports of the three guided practices included in DOSHER. When the lockdown was over and classroom attendance was allowed, these specific tasks were carried out in the classroom under the guidance and supervision of the teachers (and authors of this study). This provided the basis for a qualitative assessment of the extent to which this tool had achieved its objectives.

The preliminary analysis of the results after the exploitation of DOSHER by the students also shows an improvement that could be attributed to the use of the tool. The evolution of the marks obtained only by the 46 students who did not pass the first test (with the content of Block 1 only) and who took the final exam (covering the whole subject) in the 2020–2021 course is quite good: 74.8% improved their marks compared to the first test. The same analysis was undertaken in previous years. In the absence of DOSHER, 65.2% of the 36 students who failed the first test achieved better marks in the final test in 2017–2018; 63.1% of the 38 improved their marks in 2018–2019; and 61.8% of the 40 improved their marks in 2019–2020.

In the 2021–2022 academic year, the tool was fully implemented in the teaching of the first block of the subject. Although DOSHER is a tool designed to reinforce students’ concepts at home in an autonomous way, this does not prevent teachers from using it as a complementary and very useful tool in class, as was the case. The exercises given in class were quickly solved with DOSHER in a clear, fast, and precise way. The performance of the students in this course showed a significant improvement (31% pass rate among those presented) compared to previous years.

Nevertheless, outcomes should be observed with caution, due to the multiple biases that can occur when extrapolating population samples over time, as well as the variability of the questions in different exams and years.

The analysis of the evolution of the students’ academic performance in the years before and after the use of DOSHER, the satisfaction survey questionnaire, and the review of the reports of the three activities uploaded by the students provide the authors with sufficient data to ensure that the achievement of the objectives initially set out with the DOSHER tool has been largely attained.

Although the DOSHER tool was rated very highly by the students, and the academic results have improved, there is still room for improvement. On the basis of all the data in the study, the authors feel that competences 5 (ability to design and tune superheterodyne receivers) and 6 (increasing student motivation) are still halfway towards the target.

4. Conclusions

In this work, we have presented DOSHER as an intuitive educational tool to acquaint students with the demodulation of linearly modulated FDM signals using superheterodyne receivers. The DOSHER educational tool was developed throughout the academic year 2020–2021 (during the COVID-19 lockdown) with the main objective of supporting autonomous learning of the concepts of this subject by the students, thus reinforcing the classes led by the teacher.

DOSHER is a stand-alone application coded in the MATLAB environment, intuitive, and visual, developed with a friendly graphical interface, to be used as a complementary pedagogical tool to reinforce the teaching of telecommunications engineering subjects related to the processes of demodulation and superheterodyne reception.

Its incorporation into laboratory sessions, or its individual use at home by students, will allow them to improve their academic performance and mitigate future learning difficulties in this field caused by situations that impede presence.

With DOSHER, students can design and tune filters and oscillators to enhance the expected baseband signal at the output of the receiver, starting from FDM signals either predefined in a file or created by the user with the graphical tool included in DOSHER. Students can easily follow the evolution of the signal stage-by-stage from its origin in the RF band to its complete demodulation in the baseband, observing possible interferences, and tuning the intermediate frequency to avoid them.

In addition, DOSHER includes a set of three guided exercises to enable students to acquire knowledge of demodulation and superheterodyne reception.

The authors intend to extend the capabilities of DOSHER in the future by integrating successive stages of frequency conversion and multiple IFs of different values (e.g., dual or triple conversion superheterodyne). We will also include the demodulation of AM signals and further improve the graphical interface.

The analysis of the survey carried out and the results obtained by the students show the need to maintain the use of this tool in the present and the future. DOSHER has been designed by the authors, who teach this subject, so it is fully adapted to the scope of the subject of Communication Theory, it does not present superfluous functions that do not correspond to this subject, and it is intuitive and easy to use. Therefore, any engineering student wishing to consolidate or extend their knowledge of the demodulation processes using superheterodyne receivers should download the tool and start practicing its use.

The authors aim to Increase the pass rate to 50% of students taking this first block of the subject within a few years, and we will continue to strive to fully achieve the original goals.

In conclusion, due to the advantages of the educational tool presented, DOSHER represents an excellent option for enhancing and improving student learning about superheterodyne receivers in any course related to Communication Theory.