A-LEVEL ELECTRONICS. ELEC6 Practical System Synthesis Report on the Examination June Version: 1.0

Transcription

1 A-LEVEL ELECTRONICS ELEC6 Practical System Synthesis Report on the Examination 2430 June 2016 Version: 1.0

2 Further copies of this Report are available from aqa.org.uk Copyright 2016 AQA and its licensors. All rights reserved. AQA retains the copyright on all its publications. However, registered schools/colleges for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to schools/colleges to photocopy any material that is acknowledged to a third party even for internal use within the centre.

3 Since the revised coursework specification for A2 electronics came into use, it is pleasing to note that many of the established centres continue to be experiencing fewer problems with the specification. Requirements, on the whole, are being interpreted correctly and candidates work being generally consistently and accurately assessed. As reported before, the assessment for ELEC6 is more demanding than that for ELEC3 with several of the criteria requiring higher skills to justify marks offered. Administration issues are similar to those found for ELEC3 and will not be repeated in this report. To emphasise previous reports, it is important to note that the A2 subject specification states that coursework should be based on at least three active devices and be commensurate with the content of ELEC4 and ELEC5 and be of a more demanding nature than the AS coursework. The ELEC3 report indicates the basic concept of an active device, so reference should be made to that document. Once again, it should be noted that the assessment scheme does not offer additional credit for very complex projects, as it is the process of producing, testing and evaluating a system that is being assessed. Supervisors are expected to approve candidates projects and at the time, consider whether the project has the potential to meet the standards required and that the candidate has the ability to successfully complete the project. No candidate should be allowed to proceed without the approval of the centre supervisor. Many of the issues noted in previous reports have continued and need to be brought to the attention of all centres. It is essential that all centres ensure that they can provide clear photographic evidence of the existence of all candidates coursework hardware. It is very difficult to justify any marks for a candidate where there is no actual evidence of project hardware despite the quality and content of the accompanying report. It is also vital that any software devised and used by the candidate is fully annotated. As previously reported, candidates can use anything from machine code to Python for control systems so long as the code is fully documented and explained. It is worth noting, however, that a microcontroller is considered as one active device, however much functionality may be programmed into it. This applies whether it is a single IC, programmed separately, and then included in the system, or where it is located on a separate circuit board. It is also not appropriate for candidates to build their systems from pre-built subsystem boards, such is commonly found with some practical experimentation platforms such as the LJ D3000 'Digiac'. The systems on these boards are commonly more complex than the attached diagram indicates, so it is impossible to present an accurate circuit diagram, and there are other criteria the candidate would not be able to access. Supervisors should ensure candidates are aware of this, and if there is any doubt, seek advice from the coursework advisory service offered by AQA. The following sections describe the coursework process and how the criteria link, together with some common issues noted. They should be read and noted in conjunction with the subject specification and the coursework Support Booklet. Aa This criterion expects that candidates will independently select and then give a clear description of the problem they intend to solve using an electronic system. The description should be a short paragraph and will not contain details of either the circuitry or components as this may pre-empt the research has seen a continued improvement in the descriptions, although some are still rather too brief to justify the two marks on offer. As for ELEC3, group projects are prohibited. 3 of 9

4 Ab Once the problem to be solved has been identified, candidates are expected to undertake research to help them determine the parameters for their electronic system. Criterion Ab proved a harder criterion to gain full marks on if only because the sources are often not shown in this area of the report and because the details of the source(s) are not shown well. The criterion is looking for what products/systems are available to aid the solution, and simply finding the pin-out of a particular integrated circuit does not meet the criterion. It is also worth noting that presenting course content from ELEC1, 2, 4, and 5 will not attract any marks. Ac For criterion Ac, candidates are required to undertake practical research to help determine the parameters of their system. At least two practical investigations or experiments are needed together with the conclusions. Photographs can be a valuable record of evidence. Trying out a circuit for the first time, either practically or virtually, is not considered adequate for an investigation, as this would normally fit into the B-criteria. Again, in 2016 the results offered by a few candidates for Ac appeared very similar to that offered for Bc and were thus not credited. Ad Marks for Ad are credited where candidates provide specific operating parameters for the system they are creating. Both marks can be offered when at least three numerical and realistic parameters are given. The supply voltage range is likely to be one, as could be a realistic estimate of operational current. To reiterate from earlier reports, any project designed to operate at specific light levels or temperature conditions should include those parameters in the specification. This year most candidates responded to this criterion well. Ae Once again, it is sensible to repeat information from former reports concerning criterion Ae, as it is one of the more demanding A2 criteria. The justification should be in the form of a statement for each of the parameters defined. Something like I have chosen an operating supply voltage of 7 10 volts so that my system will work reliably from a PP3 battery. A new PP3 battery was found to have a terminal voltage of 9.6V with the voltage decreasing as the battery became discharged. I chose the lower limit of 7V because the 7805 voltage regulator I intend using needs at least 2V across it to function correctly. This year, many justifications continued to be weak. Af It is very unlikely that there is a unique solution for any electronic system design and so candidates are required to consider and give details of realistic alternatives within their system concept for Af criterion. An example may be a timing period of, say, 10 minutes. This could be constructed from a microcontroller, a monostable or a counter circuit. Candidates would be required to give circuit details of both possibilities for both marks. Again in 2016, some candidates failed to achieve upper grades in this criterion, often because the alternatives offered were not real alternatives or, more often, because the advantages and disadvantages were not fully discussed. Ag With the Ag criterion, candidates are expected to make a reasoned decision as to which of the alternatives they will use. These decisions should, where possible, be on the basis of functionality. Reasons such as it is cheaper, or I couldn t get the parts are not considered to be realistic reasons, nor would inadequate knowledge. 4 of 9

5 Ba Once the system has been defined and designed, construction can begin and candidates must be aware that systems must contain a minimum of three active devices. Most candidates should achieve at least one mark from Ba if details of at least one subsystem are provided and built. Protoboards are the recommended means of building this, as too often hard-wire circuits (pcb or stripboard types) do not work at this stage. The vast majority of projects were built this way in 2016, and most candidates scored well here. Bb The Bb criterion often allows for some areas of confusion. It is expected that candidates will build (not use simulator software) one of the subsystems identified earlier and carry out tests on that subsystem. Measuring the supply voltage is not an appropriate test, but measuring the current consumed is a suitable test. The specification does not prohibit the building of two or more subsystems, but the two marks must be awarded only if two relevant tests are carried out on any one subsystem. Candidates who gave only one measurement forfeited a mark. The use of a template can cause problems, since once a candidate uses the Bb space inappropriately there is no opportunity to record measurements from other subsystems. Bc Once all subsystems have been constructed and tested (lots of opportunities for Bb marks), candidates will now need to connect them to form a complete system. This is likely to involve solving incompatibility issues between subsystems. Bc marks are gained where candidates describe how these issues are solved, and may well include the addition of buffer stages and/or blocking capacitors or a range of other interfacing features. Simply stating that the output from one subsystem is connected to the input of the next is not sufficient to gain any marks, unless it has been justified in terms of the parameters matching. Generally candidates responses here were weak in Bd The Bd marks are given for providing a full and detailed description of how the system works. This criterion is one of the most difficult to fulfil, but is often completed quite successfully. However, there are often shortcomings that either reduce grades or cause moderators to reconsider decisions. A full circuit diagram should contain functional circuits (eg what the IC or its part does), not just a pin-out. That needs to be assigned to the layout plan, whereas the functions are part of the product design. Also, any bipolar or FET devices used need to have both their functionality defined (npn, pnp for bipolar types) and the device type, since that can often give a clue as to its suitability for a circuit. Circuit diagrams also need to contain details of passive component values and, where appropriate, types. Programs for microcontrollers should also be explained; full annotation on program listing is vital here, as the reader may not be familiar with the precise language used, though it is better to include this as an appendix to the report rather than in the main body. The functions of the microcontroller must be described in the main body. 5 of 9

6 Be Most candidates are able to achieve a reasonable grade for Be, but this can be a difficult criterion to meet. It is at this point that the IC pin-out becomes an important factor, for instance, and it can often be more convenient to see a plan for the layout before the system is actually set into prototype board or whatever other medium is being used. Often the photographic evidence of the final product does not allow the reader to see how the planning has taken place, but clearly labelled diagrams or photographs of the build are good sources of evidence to support the criterion. Effective component layout is usually key to a successful system and supervisors may choose to guide weaker candidates but award a single mark for this criterion. Many candidates responded well, with efficient and clear layouts, evidenced by good photographs showing each subsystem being built and tested. Bf In continuation from previous reports, Bf can still cause problems for assessors and moderators if not carried out appropriately. Candidates are expected to consider and document the risks they are likely to encounter when undertaking practical work and identify ways of reducing these risks to a safe level. They should do this for all the components and the equipment they use as well as the risks to their colleagues. It should take the form a risk assessment, identifying the hazards, who might be affected, their likelihood and severity, and the control measures used to mitigate the risk. Electric shock from batteries cannot in normal circumstances be considered a hazard; however, where a system is powered by a vehicle battery (for example), which can deliver large currents, the significant fire or burn risks should be identified. Risks associated with soldering if no soldering was used to construct the circuit cannot be considered. Such documentation and the construction of at least two subsystems are required for the criterion. Failure to document safety considerations will reduce the marks available and it should be noted that a supervisor s witness statement is not an adequate substitute. Bg In order to find and resolve faults in a system, it is vital that it is neatly constructed with wires being no longer than needed to connect the various parts of the circuit. Wires should not loop over components, and always be straight. Colour coding can be used to improve clarity. The Bg criterion is the next step from Be, not Be itself, and so the construction of the project should rely upon Be being completed satisfactorily. The criterion is very clear here, and photographic evidence is vital. However, the photographs need to be clear and in focus enabling the reader to see what components are where (labelling is vital to assist in this) and how the components are interfaced to each other and how power is applied. Colour photographs or good labelling is of vital importance for this criterion. It is clear this year that the successful centres with strong candidates are setting high standards for this criterion. Bh At A2 it is expected that candidates will construct their complete systems and that the systems will work to a large extent. Two marks for Bh can be awarded only if the system fully works and the candidate received only minimal guidance. Once again, there have been examples where supervisors have awarded two marks despite candidates giving details of help and assistance provided for them in their reports, though this continues to be a diminishing problem. 6 of 9

7 Ca Once a system is built, it is important to ensure that it works and thus a test plan needs to be created. For criterion Ca, it is the plan that is important, measurements are not, and it is hard to see how a completed test template without adequate descriptions of what is being tested, why and how could be used to justify any marks for this criterion. It is important that the plan tests the system to show that it both works and achieves the desired specification parameters, and will thus need equipment to carry out the tests. This criterion is used to make an assessment of the plan and still seems to cause difficulties for both centres and for candidates. Marks are sometimes awarded even when there is no evidence of a plan being devised prior to the start of testing or where it refers to subsystem tests, which gain no marks in this section. There is evidence in 2016 that this is diminishing. Cb At this level, it is expected that candidates will have carried out basic testing and thus the Cb criterion is used when the testing involves the recording of all numerical system parameters. Recording of all conceivable voltages is not assessing a system parametrically and is unlikely to gain two marks. This section continues to be marked generously by some assessors. Computerbased test equipment can provide candidates with the opportunity to paste items such as oscilloscope traces into reports. Whilst this is a useful attribute, its convenience can be reduced unless the pictures are interpreted for the reader to understand fully. Photos of multimeter displays cannot replace detailed recording. Cc Many candidates continue to list the tolerances specified for the instruments used. To gain the marks for criterion Cc, candidates need to justify the accuracy to which their measurements are made: in digital circuits 'high' or 'low' may be sufficient for voltages (though candidates should consider the ranges 'high' and 'low' cover for the logic series), but when comparing the voltage inputs from sensors, two decimal places might be more appropriate. This continues to be a weak area for candidates, and centres should consider this in their teaching. Cd With the system tested and measurements carefully recorded, criterion Cd allows candidates to assess the built system to see how well it meets the original specification. This must involve reference to both the design parameters and the recorded measurements, and thus may mean that marks can be lost if the original parameters are weak or there is little evidence of adequate testing of the system. Reference to the specification in Ad is vital here. If the system does not meet the original specification, full marks can still be available if this is explained fully. Candidates should not go back and adjust their previous specification, but if it can be justified, the specification can be altered here. Ce & Cf Any kind of prototype is likely to perform differently from the original design parameters, and practical work at this level will also show deviations. The Ce criterion should not be allocated two marks if the candidate shows how they made the system work or improve it by changing the original specification. This criterion is intended to allow candidates to identify clear limitations and possible solutions to those limitations. There will be some occasions, usually where the candidate has produced a thoughtful project with a carefully considered specification, when the complete system meets or exceeds all of the design parameters. This may mean that there are no limitations and supervisors can award both marks for Ce and Cf, so long as full marks have been justifiably awarded for Ae, Cb and Cc. 7 of 9

8 Once suggested modifications have been made, the system needs to be re-assessed to ensure that the limitation(s) have been overcome for the Cf criterion. Marks for this criterion should not be awarded when a candidate makes a non-working system function. Assessors continue to mark this criterion generously. Da Once the system has been retested after modification, a formal evaluation of the performance of the complete system is required to meet criterion Da. The evaluation needs to be numeric and should be in the form of a table where design and measured parameters can be compared and evaluated. If the initial specification is weak or the system not fully tested, marks for this section are hard to justify. Db For Db, marks should only be awarded if the finished product meets or exceeds the original design specification and solves the original problem. Dc This criterion is often hard to meet as it covers the whole of the report and includes the quality of the written work, adequate photographic evidence, appropriately annotated program code and a complete circuit diagram including integrated circuit functions, not just pin-out, component values and types and correct identification of active devices such as bipolar and field effect transistors where these are used. Inadequacy in any of these areas will result in loss of at least one mark. Once again in 2016, instances of circuit diagrams were found where the circuit could not lead to a functioning project, or there were clearly links missing in the photographs (failing to tie unused CMOS inputs high or low is alarmingly common) but had been accepted by the supervisor. Dd This is the section where full details of all sources of information are expected. The intention is to reduce the likelihood of plagiarism and to assist in the understanding of how much help was provided to the candidate. The sources should be given full detail, including: the full URL of any websites used, the date when they were last accessed; title, author, publisher and page details from books and magazines investigated; and at least a one-line statement about the information gained from each source. The Record of Supervision (RoS) sheets should provide full details of consultation between candidates and supervisors and once again, the amount of detail seems to have reduced. It is important to ensure that such consultation reports are completed, as the quality of the finished reports can be correlated with the details recorded on the RoS sheets. In reflection of previous reports, the continued use of word processing makes for easier-to-read reports but still requires some effort from candidates to consider the layout and structure of the report and undertake a spellcheck prior to printing. It is helpful if all report pages are: numbered contain the name and number of the candidate in the header or footer stapled together or connected using treasury tags and not inserted into plastic wallet folders or large loose-leaf folders 8 of 9

9 Once again, several centres have attempted to use templates, but this can reduce the opportunity to gain Quality of Written Communication marks if adhered to too rigidly. Centres attempting to develop such template schemes need to ensure that they are based on the criteria, otherwise candidates can easily penalise themselves. Candidates can produce high quality reports by following the coursework marking criteria. Mark Ranges and Award of Grades Grade boundaries and cumulative percentage grades are available on the Results Statistics page of the AQA Website. Converting Marks into UMS marks Convert raw marks into Uniform Mark Scale (UMS) marks by using the link below. UMS conversion calculator 9 of 9