Signal Integrity & EMC
Assignment 2
Important: we suggest you start the technical exercises associated with assignment 2, in week 7- see the study plan in the module overview. Your tutor may check your progress but no assessment of work will be undertaken until the assignment has been submitted formally.
You can change your solutions at anytime you wish, prior to submitting the assignment formally and/or until the assignment deadline.
Note that the last week of the study plan have been reserved for assignment 2.
Assignment 2 consists of 6 technical exercises and a research exercise. The solutions to all the exercises and the research exercise will be presented in a single document produced in Microsoft Word. If you wish to use Microsoft Excel you may do so, however if you do use Excel make sure you submit the Excel file(s) with your assignment.
Where Comsol files have been produced to obtain solutions they should be stored on the Bolton server in a folder called Ass2 and given a file name that relates them to the relevant assignment section. For example:
ass2_1 is a Comsol file pertaining to section 1 of assignment 2.
If you require more than one file for a section use the convention:
ass2_1_1, ass2_1_2, etc.
Your tutor will need access to these files for assessment purposes.
Only files that are to be used for assessment purposes should be in your assignment directories.
Contents
Assignment Section |
Mark % |
| A2.1 Electrostatic Discharges | 15 |
| A2.2 Surge Protection | 15 |
| A2.3 Electromagnetic Radiation- Emissions | 15 |
| A2.4 Power Supplies | 15 |
| A2.5 Transmission Lines | 15 |
| A2.6 Pre-compliance Testing | 10 |
| A2.7 Electromagnetic Radiation- Immunity | 15 |
Total |
100 |
| A2.8 Report |
Assignment 2 is weighted at 70%
Important: we suggest you start the technical exercises associated with the assignment in week 7- see study plan in the module overview. Your tutor may check your progress but no assessment of your work will be undertaken until the assignment has been submitted formally.
You can change your solutions at anytime you wish, prior to submitting the assignment formally and/or until the assignment deadline.
Do not forget to reference your work.
A2.1 Electrostatic Discharges
A small electronic product consists of a plastic case with a PCB mounted internally as shown in Fig A2_1. A display window is inserted in the case with a small air gap between the window and the case of 0.5mm. The PCB may be considered as a ground plane for the exercise that you are to undertake.
Figure A2_1 Product Case Details
Determine, with the aid of field plots the vulnerable zones on the case for Electrostatic Discharges. Estimate the insulation strength of the both the window and case if direct electrical breakdown through the case or the window is to be avoided.
Assume tests are carried out using a discharge gun with a rounded tip at 4KV d.c.
Note the thickness of the PCB ground plane has been set to 1 mm to allow meshing not to be too dense and to reduce field plotting solution times.
Describe how the case/window design could be modified to eliminate the vulnerable zones.
Explain clearly how you have set up the test (simulation) and explain concisely how you have interpreted the results.
A2.2 Surge Protection
The circuit shown in Fig A2.2 is intended to protect a signal loop against fast transients. The resistor Rin represents the input impedance of the circuit being driven by the voltage source VS. If VS = 5V max (d.c) and Rin = 100K, determine all the surge protection components if the static (d.c) error produced by the surge protection circuits is to be no more than 1% of full scale.
Assume the surge peak value is 2KV.
Figure A2.2 Fast Transients Protection Circuit
You will need to specify component values, current ratings and voltage ratings and also estimate the energy dissipated in each component of the surge protection networks for:
- A single surge
- A burst 15 surges
Also estimate the average power dissipated by the surge protection components over a burst of 15 surges.
Clearly state any assumptions you make and show all the steps in your solution.
A2.3 Electromagnetic Radiation - Emissions
A half wave electric dipole is to be used to test a domestic electronic product for immunity at 1GHz. If the radius of the dipole is to be 10mm estimate the value of the dipole current to produce the correct electric field strength if the product is to be placed at 0.6m from the dipole.
If the dipole excitation current is kept at the same value obtain field plots for several frequencies over the test range. What would have to be done to keep the field strength in the test zone reasonably constant over the frequency test range?
Explain how radiated interference degrades the signal integrity of an electronic based product.
Note 1: Use the Axial Symmetry (2D)/Electromagnetics Module/Electromagnetic Waves/TE Waves/Harmonic Propagation application mode for your field plots.
Note 2: The distance 0.6m has been chosen to allow computational times to be reasonable. This is not the distance used in immunity tests.
A2.4 Power Supplies
A regulated power supply is required to supply 3.3V d.c. The a.c. supply voltage is 240V +/- 6%, 50Hz, which supplies a transformer, bridge rectifier and reservoir capacitor. The voltage regulator used has a drop out voltage range of 6.5V to 7.5V. The power supply is required to supply a circuit consisting of 100 high speed surface mount integrated circuits (ICs) each with a through current of 15mA/IC lasting for 100pS. The average self capacitance of signal tracks between ICs is 6pF and the total number of signal tracks is 1200. If 30% of ICs switch synchronously with the clock and in doing so drive 40% of the signal tracks design the power supply, specifying the transformer primary to secondary ratio, the size and voltage rating of the reservoir capacitor and the current rating of the bridge rectifier. The clock is a 1GHz square wave with rise and fall times of 100pS and a 1:1 mark/space ratio.
If the ICs are to be mounted on a PCB of dimensions 30cm by 30cm and the power supply planes are separated by an FR4 laminate determine the minimum thickness of laminate possible (you will have to do a little research for this).
Estimate the value and number of decoupling capacitors required to ensure the transient voltage change on the planes is no more than 5% of the supply voltage. Determine the self capacitance and self inductance of the planes stating any assumptions you make and explaining any simulations you undertake.
You will need to use some judgment in carrying out this design- which is the type of exercise we may carry out during the development of product at a fairly early stage to give us some idea of the power supply and decoupling requirements. When the circuit design (100 ICs) has been firmed up and the PCB is being laid out power supply design and PCB design, which includes decoupling, could be firmed up.
A2.5 Transmission Lines
A coaxial cable has an inner conductor of 1mm diameter. The dielectric between the inner and outer conductor has a relative permittivity of 2.5. If the characteristics impedance of the cable is to be 75ohms, determine the inner diameter of the outer conductor.
The cable is used to transmit a 3 volt digital signal with a repetition rate of 400MHz and a mark space ratio of 1:1. The rise and fall times of the signal are each 10% of the signal periodic time. The length of the cable is 1.5m and a logic gate with a spread in input threshold voltage of 0.9V to 2.1V is connected at the receiving end of the cable.
Calculate:
- The propagation delay along the cable.
- The value of terminating resistor to ensure no degradation of the signal
at the receiving end of the cable.
- The reduction in noise immunity of the gate if the terminating resistor
has a tolerance of +/- 1%
- The worst case reduction in noise immunity if the permittivity of the cable is 2.5 +/- 10% and the terminating resistor has a tolerance of +/- 1%.
Assume the cable is lossless and verify any cable parameters you use with static field plots.
A2.6 Pre-compliance Testing
Imagine your company has decided to consider introducing pre-compliance EMC
testing. Research the main items you would require in terms of specification
and cost. We suggest you identify suppliers of EMC test equipment on the Internet,
or otherwise, to obtain this information.
Briefly describe the benefits of pre-compliance testing.
A2.7 Electromagnetic Radiation - Immunity
You are to develop a procedure for determing the effect of a slot on screening effectiveness, using FEM software.
A suggested model geometry is shown in Fig A2.7.
The direction of current produced by source I is into and out of the screen. Using a source frequency of 1 GHz, investigate the effect of the slot dimensions on screening effectiveness at a point P, just inside the screened region.
Determine the screen effectiveness for several slot widths, expressed as a fraction x of the wavelength of source signal.
Figure A2.7 Model Geometry
Compare your answers for the screen effectiveness of a copper sheet without a slot.
A2.7 Report (Overall Assignment)
- Tidy up your solutions to the excises and present them in a report produced
in Microsoft Word. Make sure you reference your work and clearly explain
the steps in your calculations stating any assumptions you have made.
- State the relevance of your solutions to EMC/signal integrity, where appropriate, for each of your solutions.