Neven, Your suspicion regarding the modulating frequency is on the right track. Normally, the mod freq is set just above the audio range. There's something very wrong if the specs only allow 3 to 100 Hz. Is that a misprint perhaps?
You should be able to see the "spread" harmonics easily at 120 kHz RBW. If you don't see that, it's just not being modulated correctly. If you provide the specific SSC chip used, the group could probably help you better. Cheers, Ken _______________________ Kenneth Wyatt Wyatt Technical Services LLC Woodland Park, CO Email Me! | Web Site | Blog Subscribe to Newsletter Connect with me on LinkedIn On Feb 9, 2012, at 5:32 PM, Neven Pischl wrote: > Hello, > > I wonder if anyone can help with a question I have on spread spectrum clock > (SSC). > > I am trying to validate the effectiveness of a SSC chip to reduce emission. I > measure with the peak detector. With the SSC enabled (up to 2.5% down-spread) > I expect the level measured with a spectrum analyzer to go down at the > fundamental and even more at the harmonics, when compared with the peak > levels without the SSC enabled. The problem is that I only see that the > frequency gets spread but the peak value stays exactly the same with and > without the modulation, using 100kHz or 120kHz RBW of the analyzer. This > happens at the fundamental as well as at the harmonics, so there is > absolutely no reduction in the measured peak amplitude, it just looks wider. > > What is interesting is that if I reduce the RBW on the analyzer down to > 1-3kHz, I see the expected result on the analyzer. The frequency is spread > and the peak level with the spread is lower when compared with the peak level > measured without the SSC enabled, the same difference as one can see in > various papers and material on SSC, and which I also measured many times in > the past. > > However, that is not the required RBW that I must use between 30MHz and > 1GHzs, so we have a problem. > > I believe there is something in the modulation scheme of the particular IC > that must be changed to make it work when measured with 120 MHz RBW but I am > not sure what. > > Some of the modulation parameters: > Triangular waveform, quasi-linear ramp up and linear ramp down. > Linear is actually represented with “small” discrete steps. > Frequency range of the modulating waveform is very low, it can be adjusted > roughly between 3Hz and about 100 Hz. > The SSC devices on the market normally use about 30-40 kHz for the modulating > waveform > > I wonder if the modulating frequency has something to do with the observed > lack of amplitude reduction. The equations that describe the emi-reduction do > not contain the modulating frequency as a factor so it "should" not be a > facotr - BUT - maybe I am not taking into account the relation with the > measurement settings and non-ideal world. > > Any sugetions or comments, links to reference article, App Notes, etc?? > > Regards, Neven > - > ---------------------------------------------------------------- > 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://www.ieee-pses.org/emc-pstc.html > Attachments are not permitted but the IEEE PSES Online Communities site at > http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used > formats), large files, etc. > 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://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. 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>
