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Further work into the potential effect into the use of dithered clock oscillators on wideband digital radio services

A report for the Radiocommunications Agency by York EMC Services Ltd, University of York

Authors: D A J Pearce, Prof A G Burr, T Whitehouse, I D Flintoft

Summary

Recent years have seen the manufacturers of digital electronic products increasingly utilising dithered clock oscillators (DCO) in their equipment, primarily as a means of reducing the peak levels of emissions seen by the detectors used in EMC compliance tests. The study described in this report modelled a digital broadcast receiver to determine its immunity to DCO generated interference. The modelled levels were compared with practical test results obtained from typical digital receivers.

It has been concluded that DCO enabled equipment has a smaller margin of Electromagnetic Compatibility between it and broadcast services (analogue or digital) than non-DCO enabled equipment. Furthermore it was found that the digital receivers investigated were less immune than might be expected from the modelling: their immunity to both DCO and CW interference is considered to be dependent on the tuner design. Two possible approaches to improving the EMC margin have been suggested.

  1. The existing EMC limits could be reduced for DCO enabled equipment. This is highly practical in terms of being able to perform the test with existing methods and equipment. The proposed reductions would give a significant improvement to the EMC of DVB-T and DAB receivers. However, it is not known how practical it is to build IT equipment with these more stringent limits. This would have to be investigated before the proposed limits could be implemented.
  2. 2. An alternative approach to improving the EMC of both DVB-T and DAB receivers is to investigate improving the immunity of the receivers to DCO interference. The immunity levels of the receivers tested were not as good as the modelled value. This suggests that the tuner design might be improved to give superior immunity to DCO noise. If this approach proved practical then an in-band DCO immunity test would have to be developed and standardised so that receivers can be tested to ensure maximum immunity.

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You can download this report as a PDF file (761 kB).


Last Updated: 2006-Feb-01

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