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Thursday 19 July 2018

The Media Technology programme took the Outside Broadcast truck on a bit of a strange mission this week - we were asked whether we could help an equipment manufacturer to characterise the power supply on a typical OB vehicle. They'd noticed that some of their equipment occasionally behaved differently when installed on similar vehicles and were powered from generators. Of course professional grade equipment goes through a comprehensive test procedure before the designs are accepted. This includes monitoring the device's performance at a range of temperatures, humidities and supply voltages as well as with electromagnetic noise and noisy power supplies. Their lab is perfectly capable of altering the supply voltage, adding spikes, simulating ‘brown-outs’, changing the frequency and lifting the earth reference but there appeared to be something ‘different’ about production vehicles.

On permanent sites, the power utility provider installs a fixed Earth connection that serves as a safety feature and as a ground reference for all the signals within and between facilities. A lot of effort goes into designing grounding topologies to meet safety regulations as well as to minimise noise. When vehicles are powered from a fixed distribution point, they are connected into the grounding system to address these issues. However generators often do not have an Earth connection and are instead often allowed to ‘float’. Whilst this may increase the noise floor of the system, it is generally better than connecting systems that have slightly different Earth potentials. This in turn impacts the neutral as it would normally be bonded to Earth at the service entrance or local substation. The live is then often poorly regulated with the petrol generator producing 180 V in our setup rather than the specified 230 V and with the cycle frequency varying considerably around the expected 50 Hz. Add a large battery in parallel that provides an Uninterruptible Power Supply and it's no-wonder that the power supply units on the equipment struggle to provide stable power rails to the sensitive circuits on board.

floating-earth-1

The engineers connected a range of scopes and analysers to monitor how the devices behave under such circumstances and discussed the potential impact of stray capacitance or divergent neutral and Earth connections. They were able to disprove some theories and identified some areas to focus on within the lab environment. It can be challenging, frustrating and extremely rewarding to work on such corner cases and we hope our facilities and input helped to some degree. The outing reiterates the importance of the science behind media technologies and provided some excellent context for the electronics and systems units that our students study.