The QP detector has 2 tasks:
1. Mimic the response of a standard AM receiver IF band filter (4 kHz or so) to short impulses (<1 mS) (while the meas rec. uses a 120 kHz filter) 2. Hold the response to increase the measurement result when short pulses repeat Gert Gremmen Van: emc-p...@ieee.org [mailto:emc-p...@ieee.org] Namens Price, Edward Verzonden: donderdag 1 december 2011 16:45 Aan: Anthony Thomson; EMC-PSTC@LISTSERV.IEEE.ORG Onderwerp: RE: [PSES] Quasi-peak Tony: I don’t want to appear to be overly picky, but your diagrams don’t show “what happens to the emissions,” but rather show how the QP detector receiver “sees” the emissions. In a dithered, or frequency hopping clock, the clock hops to a frequency, dwells there for a short moment and then hops to a new, relatively far away frequency. If your receiver happens to be sitting right at say, 100 MHz, and the clock hops to 100 MHz, the receiver only has a short time (before the clock hops again) to indicate the amplitude of the signal. A peak detector will quickly charge and show the signal level, but a QP detector has a slower time constant, so it can’t get up to the full signal amplitude before the clock hops away from the receiver’s “view.” The clock’s amplitude doesn’t change or spread or in any way decrease; all the dithered clock does is hop and jump all over a range of frequencies. Ed Price ed.pr...@cubic.com <blocked::mailto:ed.pr...@cubic.com> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty From: Anthony Thomson [mailto:ton...@europe.com] Sent: Thursday, December 01, 2011 2:36 AM To: EMC-PSTC@LISTSERV.IEEE.ORG 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 | |~~~~~~~| | / \ | | | +------------------------ - ---------------------------------------------------------------- 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 <emc-p...@ieee.org> 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 <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com> - ---------------------------------------------------------------- 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 <emc-p...@ieee.org> 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 <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
