Hello Amund, Just to clarify one point, a spread spectrum clock is very different to a spread spectrum transmission scheme. Bluetooth is just one example of a spread spectrum transmission scheme where the modulated carrier ‘hops’ between frequency channels within a defined band. The receiver has to synchronously tune itself to the transmission frequency. Keeping with the Bluetooth example, simplistically there are 79 x 1MHz spaced bands between 2402 and 2480 MHz. During transmission, the carrier hops between these carrier frequencies, it connat stay at any one frequency for more than 400ms. Relevant to your question…. Spread spectrum clocks are used in digital systems to reduce emissions. It’s a little bit of a ‘cheat’ because the energy of the overall emissions is generally the same, but the narrowband levels measured by an averaging and/or integrating detectors (e.g. CISPR) are greatly reduced. Say you have a digital system clocking at 100MHz, you have potential narrowband emissions problems at 100MHz, harmonics thereof and any other frequencies divided down or synthesised up. If you ‘modulate’ your 100MHz clock by e.g. +/- 0.5% (99.5 – 100.5 MHz) you spread your emissions across a proportionate band. This band is generally much greater than the measurement bandwidth of measuring receivers.
This is basically what happens to the emissions. | Narrowband Clock | || |- - - - - / \ - - - -- Limit |/ \ || | |/\ +------------------------ | Spread Spectrum Clock | | |- - - - - - - - - -- Limit ||~~~~~~~| |/\ ||| +------------------------ Hope this helps with spread spectrum clocks. ----- Original Message ----- From: Amund Westin Sent: 11/30/11 07:11 PM To: [email protected] Subject: [PSES] Quasi-peak I tried to find some information on how the Quasi-Peak detetor works. How long time does it measure at each frequency, why does a spread spectrum clock solve emission problem, etc .... Anybody who knows where I can find it? B.r Amund - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> Da! vid Heald: <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://product-compliance.oc.ieee.org/ Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
