Ed- Given your scenario, you are right. However, in my experience of measuring radiated emissions of spread spectrum clocks, I have always noticed a decrease in not only the quasi-peak and average measurements, but the peak measurement as well. I think this may be due to the bandwidth of the spreading signal -- if it is wider bandwidth than the receiver bandwidth (120 kHz CISPR in my case), then there will be reduction in the peak as well. With a high bandwidth spreading signal, the RF will not spend enough time within the bandwidth of the receiver for the receiver to respond to the full amplitude of the signal.
Donald Borowski EMC Compliance Engineer Schweitzer Engineering Labs Pullman, WA, USA From: "Price, Edward" <ed.pr...@cubic.com> To: <don_borow...@selinc.com>, <EMC-PSTC@LISTSERV.IEEE.ORG> Date: 12/01/2011 08:06 AM Subject: RE: [PSES] Quasi-peak Don: I think that the ?spread spectrum clock? works because of both the receiver bandwidth and the detector function. For instance, imagine a pure CW clock signal, and it is being hopped around in 1 kHz steps, all in the range of 10 kHz. Now imagine that a receiver with a 1 MHz resolution bandwidth is watching that signal. The indicated amplitude will be the same with Peak, QP & Average detectors. Because the hopping is always within the receiver bandwidth, the hopping has no effect. As the hopping stays within the receiver BW, each detector has plenty of time to reach the full amplitude of the signal. Now imagine that a hop starts well outside the RBW; the receiver sees nothing. Then the clock hops into the RBW, and each detector starts charging. Fifty microseconds later, the clock hops out of the RBW. You look at the three detectors, and the Peak reads, say 1.0. The QP might read 0.1, and the Average might read 0.0. The difference was all about how long the receiver had to observe the signal; all detectors ?saw? the same amplitude signal, but they could only report what their time constants allowed. Ed Price 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 > -----Original Message----- > From: don_borow...@selinc.com [mailto:don_borow...@selinc.com] > Sent: Thursday, December 01, 2011 7:22 AM > To: EMC-PSTC@LISTSERV.IEEE.ORG > Subject: Re: [PSES] Quasi-peak > > Spread spectrum clocks "work" due to the measurement bandwidth of the > receiver, so this effect holds for peak, quasi-peak, and average. > > > Donald Borowski > Schweitzer Engineering Labs > Pullman, Washington, USA > > - ---------------------------------------------------------------- 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>
