CW2: Signal Chain and Measurement Systems Technical Report Assessment

Assessmen Coursework 2

Objectives and Tasks

The design process to implement the task will take place around a set of objectives:

• To use example building blocks to become familiar with both VHDL and the computer-based tools.
• To add progressive functionality and complexity towards the goal of achieving a completely working system on target FPGA hardware.

There are three tasks The first task contributes 20% to the overall coursework. The second task consists of two milestones which contribute 40% to the overall coursework and the third task contributes the remaining 40% (More detail as to the tasks, design process and assessment will be covered during the weekly sessions).

Questions

1. Discuss one example of a signal source (typically a sensor). Choose one which might be relevant to you due to working experience or passion. Describe it from the physical point of view, briefly presenting materials, relevant design aspects, basic working principles. Use images, block diagrams, and/or analytical/mathematical formulation to support the description, and make it more interesting to read. References are certainly important here. 

Expand on the previous answer: use an electrical equivalent model (also known as lumped element model) to demonstrate your capability to recognise the main characteristics of a signal source generating an analogue signal, and model them with electrical components, such as capacitors, inductors, etc. (An example could be the stiffness of a membrane which is electrically modelled as a capacitance, if an “impedance model” is used). Expand on the assumptions used to produce the model, on its accuracy (e.g. in the frequency domain), and its validity. 

Once the lumped element model is finalised (there might be more than one depending on the level of accuracy or depending on the frequency range which needs analysing, for example), use a simple simulation tool (e.g. MultiSim or LT_Spice), and expand on the capability of the model to represent different variations of the device by altering some electrical parameters.

2. What is the main purpose of the devices (sensors and ancillary electronics) located at the beginning of a signal chain? Are they meant (and designed) to deal with information, or with power? Expand on the topic. 

Describe the simplest electrical circuit used to model the interaction between a sensor and the first stage of amplification in a signal chain. Choose two values for the input impedance of an amplifier connected to a sensor, and show how they affect the input voltage read by the first stage of an amplifier (suggestion: assume a fixed, large value for the output impedance of the sensor). 

Explain in writing, using the thermal noise generation model equation for resistors, what is the trade-off between accuracy and noise when interfacing a signal source to an amplification circuit. Typically, by using the previous answer, two different noise voltage contributions can be calculated, and then compared.

3. How do we define the open-loop gain of an ideal operational amplifier? 

• What is the formula of the closed-loop gain in the case of a voltage amplifier? 
• Draw a clear block diagram to support your explanation and the calculations that will give you a formula as a result.

4. How do we define an ideal differential amplifier? (Please note, a differential amplifier is NOT an operational amplifier…).

• What is the formula that describes the gain of a differential amplifier implemented with a single operational amplifier?
• What is the biggest limitation/problem when implementing a differential amplifier with one single single Op-Amp?

5. Give in your own words a definition of an electronic filter, and list at least 4 typical parameters used to specify one.

• List the main limitations of passive filters with respect to active filters and comment on them (suggestion: use a table)
• Is there a difference between the slope of a second-order passive low-pass filter and the slope of a second-order active low-pass filter? Explain your answer and provide the magnitude of the slope both in dB/oct and dB/dec

Brief Summary of the Assessment Requirements

Coursework 2 focuses on developing the student’s understanding of signal sources, analogue modelling, signal chain fundamentals, operational amplifiers, differential amplifiers, and electronic filters, using both theoretical concepts and simulation tools. The assessment is divided into three tasks, weighted at 20%, 40%, and 40%.

Key Pointers to be Covered in the Assessment

Task 1

1. Choose and describe a sensor (signal source)
   • Physical structure, materials, working principles
   • Use of diagrams, images, equations, and references
2. Build an electrical equivalent (lumped element) model
   • Recognise characteristics like capacitance, inductance, stiffness, impedance
   • Justify assumptions, limitations, frequency-range validity
3. Simulate the model
   • Use MultiSim, LTSpice, or similar
   • Modify electrical parameters to observe behaviour variations

Task 2

1. Explain the role of sensors and early-stage electronics in a signal chain
   • Clarify whether they deal with information or power
2. Model the sensor–amplifier interaction
   • Show effect of two different amplifier input impedances
   • Analyse changes in input voltage
3. Discuss noise vs accuracy trade-off
   • Use resistor thermal noise equation
   • Compare two noise contributions

Task 3

1. Define open-loop gain of an ideal operational amplifier
2. Derive closed-loop gain of a voltage amplifier
   • Support with block diagram
3. Define an ideal differential amplifier
4. Provide gain formula using a single op-amp
   • Discuss its main limitation
5. Explain electronic filters
   • Give 4 specification parameters
   • Compare passive vs active filters
   • Explain slope differences for second-order filters (in dB/oct and dB/dec)

How the Academic Mentor Guided the Student (Step-by-Step Approach)

The mentor adopted a structured, progressive guidance method to help the student understand each requirement and build the final solution logically.

Step 1: Interpreting the Assessment Brief

The mentor first explained the coursework objectives:

• To familiarise the student with VHDL concepts and FPGA-related design thinking
• To gradually increase complexity through modelling, analysis, and simulation
• To link sensor behaviour with electronic systems and signal processing concepts

This ensured the student clearly understood why each task exists and what skills it aims to develop.

Step 2: Selecting and Describing the Sensor (Task 1.1)

The mentor guided the student to:

• Choose a sensor aligned with their interests/experience (e.g., pressure, temperature, vibration)
• Break down the physical working principles
• Use block diagrams, structural sketches, and equations to strengthen technical explanation
• Use references from textbooks, datasheets, and research papers

This step established a strong foundation for modelling.

Step 3: Building the Lumped Element Model (Task 1.2)

The mentor helped the student:

• Identify mechanical/electrical analogies (e.g., stiffness → capacitance, mass → inductance)
• Convert physical behaviour into electrical components
• Justify frequency-domain assumptions
• Create multiple models if accuracy requirements change

Mentor emphasised writing clearly about:

• Model validity
• Ideal vs real conditions
• Parameter sensitivity

Step 4: Simulation and Parameter Variation (Task 1.3)

The mentor walked the student through:

• Setting up the model in LTSpice/MultiSim
• Running AC and transient analyses
• Modifying R, L, or C values to observe different system behaviours

This helped the student connect theory to practical circuit behaviour.

Step 5: Understanding Sensor Amplifier Interaction (Task 2.1–2.2)

The mentor explained:

• Why sensors and front-end circuits primarily handle information, not power
• How signal degradation occurs if impedance matching is poor
• How to choose two amplifier input impedance values and calculate resulting input voltages

This improved the student's understanding of analogue interface design.

Step 6: Noise vs Accuracy Trade-Off (Task 2.3)

Using the resistor thermal noise formula, the mentor showed:

• How to calculate noise for two cases
• How noise increases with resistance
• Why higher impedance improves voltage accuracy but increases noise

This helped the student present a balanced technical discussion.

Step 7: Op-Amp Theory (Task 3.1–3.2)

The mentor ensured the student:

• Clearly defined open-loop gain
• Derived closed-loop gain using the standard feedback formula
• Included a neat block diagram for clarity

This strengthened the student’s understanding of linear circuit theory.

Step 8: Differential Amplifier Concepts (Task 3.3–3.4)

Guidance focused on:

• Differentiating between differential amplifiers and operational amplifiers
• Writing the correct gain formula for single-op-amp implementation
• Explaining key limitations like poor common-mode rejection

Step 9: Filters and Their Specifications (Task 3.5)

The mentor helped the student:

• Define filters in simple terms
• List key parameters (cut-off frequency, Q-factor, roll-off, passband ripple)
• Create a comparison table for passive vs active filters
• Explain slopes for second-order filters in both units:
  • 12 dB/oct
  • 40 dB/dec

Final Outcome Achieved

By following the mentor’s structured guidance:

• The student produced a comprehensive solution addressing all tasks, formulas, diagrams, simulations, comparisons, and theoretical explanations.
• The submission aligned fully with the assessment brief.
• Each question was answered with clarity, technical correctness, and proper academic reasoning.
• Simulation-based evidence strengthened the applied-learning component.
• The final coursework demonstrated understanding of sensor behaviour, analogue modelling, noise concepts, impedance matching, operational amplifiers, differential circuits, and filter design.

Learning Objectives Successfully Covered

The mentoring process helped the student achieve:

1. Understanding of physical-to-electrical modelling of sensors
2. Ability to design and interpret lumped element models
3. Skills in using simulation software for circuit analysis
4. Knowledge of signal chain fundamentals and front-end interface design
5. Competence in analysing noise, accuracy, and impedance effects
6. Proficiency in op-amp theory and differential amplifier limitations
7. Strong understanding of filter characteristics and performance parameters
8. Ability to link theory, diagrams, and simulations into a coherent technical report

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