Spread spectrum clocks "work" due to the measurement bandwidth of the receiver, so this effect holds for peak, quasi-peak, and average.
Fundamentally, spread spectrum clocks do not solve emission problems. They just take the emitted energy and spread it over a wide bandwidth, wider than the measurement bandwidth, so that only a fraction of the energy falls within the measurement bandwidth. In effect, the measurement is "fooled". In the "real world", spread spectrum may or may not help reduce interference to communications systems. If the communications system is a fixed channel, narrow band system, then it may help. I say "may" because if the interference before spreading was in your desired channel, it will help. On the other hand, if you are using a near by channel, the interference without spreading would be rejected, but with spreading, some of the interference energy may now fall within your channel. Similarly for frequency hopping systems. Without spreading, the interference without spreading may make one channel unusable. With spreading, it will likely cause less severe problems on multiple channels. For non-hopping spread-spectrum communication systems (which are typically wide band), if all the interfering energy is contained within the channel, the fact that the interfering energy is spread or not makes no difference. Donald Borowski Schweitzer Engineering Labs Pullman, Washington, USA From: Jim Hulbert <[email protected]> To: "[email protected]" <[email protected]> Date: 12/01/2011 06:25 AM Subject: RE: [PSES] Quasi-peak Sent by: [email protected] It should be noted that quasi-peak is only specified up to 1 GHz. For emissions above 1 GHz, peak and average limits are defined and both must be met using peak and average detection receivers respectively. Don't think a spread spectrum clock helps much against a peak limit. Jim Hulbert From: [email protected] [mailto:[email protected]] On Behalf Of Anthony Thomson Sent: Thursday, December 01, 2011 5:36 AM To: [email protected] Subject: Re: [PSES] Quasi-peak 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]> David Heald: <[email protected]>
