Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: What do you think about using a Reducing EMI Spread Spectrum Clock Oscillator, like the ones manufactured by Mercury Electronic?
Answer: Well, this isn’t really a question related to the subject of my webinar, but I have some time to kill on a long flight and I’ve seen all the films on offer.
I am not familiar with the spread-spectrum clocks made by that company, but the technology has been around and commonly used to reduce the emissions – as measured by many of the standard EMC tests – for well over a decade.
Spread-spectrum clocking of synchronous digital circuits is a powerful technique that I generally recommend, where asynchronous processing devices are not used for whatever reason.
It is important to remember that it only works where the EMC emissions test standard that will be applied uses Quasi-Peak, Average, RMS, or other integrating-type detectors. Some military and telecommunication emissions test standards use Peak detectors, which have a 1µs response time and so are not “fooled” by spread-spectrum clocking.
Using asynchronous logic devices (sometimes called ‘naturally clocked’, or ‘handshaked’) is very much better than spread-spectrum for reducing emissions.
The total amount of radiated energy (all else remaining the same) is usually about one-tenth of the same amount of digital processing in a synchronous-switching (i.e. clocked) circuit, giving a 20dB reduction straight away, but the most significant issue is that the emissions from an asynchronous logic circuit are spread all over the spectrum instead of being confined to the clock frequency and its harmonics, so the measured emissions levels are truly very low indeed, regardless of the type of detector used in the EMC receiver or spectrum analyzer.
(If you are not aware of asynchronous digital processors, they are used in hundreds of millions of smartcards, in most (if not all) cellphones, and are used for the core processing in powerful ICs from the Pentium IV on.
-Keith Armstrong
