I wasn't as clear as I could have been (a not too uncommon occurrence :-) 1st, note that I have said "MAY" several times here - if you were wanting to try clock spreading with a vengeance then you would plan it a little better than this. It's not just (or mainly) the clock spectral lines that get spread per se that get spread but EVERYTHING that depends on the clock signal. The vast majority of the chip's internal signals are not sine waves but are more or less square waves. Some of these have rather repetitive or even steady signal sequences. There are also external signals but as these normally occur less often and less regularly and are usually loaded more heavily they will contribute less to clock frequency square wave EMI. If you dither these signals randomly you will reduce the energy at a single frequency. If the method that you use to do it causes more radiation you may lose overall. The mechanism that I noted is quite probably not going to greatly increase radiation if the supply is properly decoupled fairly near the Vdd pin. Would I recommend the method for EMI reduction? - No. Do I return oscillator caps to Vdd? - sometimes, yes (usually in development prototypes). I haven't measured the resultant EMI changes but will try to do so sometime. Note that "real people" (eg HP) do dither their clocks purposefully in some designs to reduce EMI peaks. regards Russell McMahon -----Original Message----- From: Sean Breheny To: PICLIST@MITVMA.MIT.EDU Date: Sunday, June 13, 1999 6:07 PM Subject: Re: Osc cap return I'm afraid I don't understand the premise behind this at all. It seems to me that you are talking about creating a tiny FM transmitter with very little deviation,and I don't see how that will decrease harmonic content. The signal with no harmonics is a sine wave at a constant frequency. Doing ANYTHING to alter this INTRODUCES harmonics. It seems to me that shifting the osc. frequency will mainly shift the rest of the harmonics with it. I don't want to take the time to actually try to figure out the result in full detail,but even if it DOES spread the signal out wider, it won't be by much, and any unintentional receiver which received the original signal would probably have enough bandwidth to receive the slightly spread signal, too. Sean At 01:56 AM 6/13/99 +1200, you wrote: >This MAY be an even better idea than you realise. > >There will be some extra noise on Vcc right at the PIC even with good >decoupling and this will have the result of inducing some noise into >your oscillator. As a consequence you can expect some random jitter >on the clock - this in turn will decrease its spectral purity and >DECREASE the magnitude of harmonics, instead spreading the noise onto >other frequencies. The end result MAY be to reduce your EMC peak >emissions slightly. > >HP design systems where the clock is deliberately "spread" this way >for just this reason. Presumably adding varicaps returned to Vcc to >the PIC clock lines may help this effect :-) > > >regards > > > Russell McMahon > >-----Original Message----- >From: Ernie Murphy >To: PICLIST@MITVMA.MIT.EDU >Date: Sunday, June 13, 1999 1:43 AM >Subject: Osc cap return > > >With the PICs I've been using, Vcc not RTN is nearest the oscilator >pins. I >don't know if this is true the the whole PIC line, but may well be. > >So when I have to ground my osc caps (actually I use a resonator, >same >thing applies), I could either go the long way around to the real >ground >pin, or "Ground" these caps to Vcc. Guess what I do? Nail em right to >Vcc. > >Vcc is an AC short to ground (what do you think all those bypass caps >do?), >and is a fine return point for the caps. My application is an >accurate >timer, so I implicitly test start-up time. I've tested these things >from >-60 to +150¡C and they start up and start pronto every time. > | | Sean Breheny | Amateur Radio Callsign: KA3YXM | Electrical Engineering Student \--------------=---------------- Save lives, please look at http://www.all.org Personal page: http://www.people.cornell.edu/pages/shb7 mailto:shb7@cornell.edu ICQ #: 3329174 ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html