Brian:
Wow, you want a practical answer!! Well, in my experience of doing susceptibility on a lot of military systems (where the comm links are almost always encrypted digital bit streams), I find that the default 1 kHz 50% duty cycle (square wave) is a good choice for modulation. One of our data links might be running in Ku-band, with a 20 MB stream. We might have frames of data that are a millisecond long, with each frame consisting of words which are in turn composed of bits. The time slot of each bit is used as a "place-holder" for either a 1 or a ) logic bit. If we have a word that is supposed to be say 1001100001110, and we drop our test signal onto this bit stream, the link receiver might see 1111111111111. This is not what was sent, and would cause an error to be declared for the word. If there is no provision for error correction or redundancy, then this bad data could do most anything, from causing one tiny anomaly to crashing the whole system. All you have to do is drop some energy at the moment that 0's are happening, and the receiver reads them as 1's. When you manage to create interference, the threshold is very sharp. The digital channel doesn't degrade gracefully, but rather collapses precipitously. Ed Price [email protected] <blocked::mailto:[email protected]> 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: Brian Oconnell [mailto:[email protected]] > Sent: Thursday, December 01, 2011 9:22 AM > To: [email protected] > Subject: Re: [PSES] Quasi-peak > > Good stuff, this empirical experience. > > But the question remains - does this spread-spectrum stuff, for a > comparative power level, increase or decrease interference with my > master-blaster 5000 remote toilet controller? One member said that it only > will affect stuff that is very close to the operating freq and that the most > digital receivers would not see it. But EMC amateurs such as me need MOAR > empirical experience from Don and Ed and et al. > > For my employer's products, I am more concerned about customer complaints > than demonstrated margin from some fantastical limit line in an EMC > standard. > > Brian > > -----Original Message----- > From: [email protected] [mailto:[email protected]]On Behalf Of > [email protected] > Sent: Thursday, December 01, 2011 8:58 AM > To: Price, Edward > Cc: [email protected] > Subject: RE: [PSES] Quasi-peak > > 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" <[email protected]> > To: <[email protected]>, <[email protected]> > 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 > [email protected] 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: [email protected] [mailto:[email protected]] > > Sent: Thursday, December 01, 2011 7:22 AM > > To: [email protected] > > 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- > [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]> - ---------------------------------------------------------------- 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]>
