Yes, it will be strange. A lot of it is caused by the reduction in reverse biased diode junction capacitance as VDD is raised. Most ICs are riddled with such junctions because that is how the parts are isolated from each other (exception: dielectric isolation). This means that risetimes get shorter and internal crosstalk goes down. So, the broad spectral peak will shift upward, perhaps the width of individual spurs will reduce slightly. The spectral peak should also rise in amplitude, because the logic swings increase with VDD for most logic families. The overall complexity of the spectrum and the details of individual features cannot reasonably be predicted, since the result at the antenna for any specific frequency is the sum of what is happening in the whole system in both amplitude and phase. Any specific line could go up, or it could go down! Accurate prediction would require models of such fine detail and such huge amounts of computing power that the fastest method of computation is to forget digital modeling altogether and use the faster and more credible results of an analog computer. In this instance, the model is the actual system, and the computation is the taking of actual measurements, by you in the lab. Orin On Fri, 29 Jun 2007 10:43:08 +0200 "jun feng" <junfeng.zhou@xxxxxxxxx> writes: how about the peak emission in frequency domain as the function of VDD, from my measurements the relationship is strange ! regards, Junfeng On 6/29/07, olaney@xxxxxxxx <olaney@xxxxxxxx> wrote: It's proportional to peak or RMS dI/dt, which is a combination of logic swing, load impedance (and linearity), and rise/fall times. Orin On Fri, 29 Jun 2007 01:04:02 +0200 "jun feng" < junfeng.zhou@xxxxxxxxx> writes: > Hi, > Generally speaking , what is the relationship between the digital > circuits radiated emission( peak emission or maybe RMS ?) and the > supply > voltage ? I could not find an exact answer for this. Maybe you guys > here can > educate me ! Thanks, > > > > regards, > > Junfeng > ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: http://www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu