Chuck, I am quite familiar with how to calculate a voltage or power ratio in dB, but refering to the first issue, when you combine two oscillators, does the noise improve by 3dB?
Didier > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Chuck Harris > Sent: Wednesday, December 24, 2008 10:02 AM > To: Discussion of precise time and frequency measurement > Subject: Re: [time-nuts] New topics (was Re: He is aTime-Nut > Troublemaker....) > > Didier wrote: > > >> Square root of 2 is about 1,414 or about 3,01 dB. > > > > I am always confused when considering noise, is it 10*log(p1/p0) or > > 20*log(p1/p0)? > > A moment's reflection on why the 10 log vs. 20 log, might help. > > The conversion from a power ratio to dB is: > > dB = 10 log (P1/P2) > > Remember that Power = VxV/R, so: > > P1/P2 = (V1xV1)/(V2xV2), the R's cancelling. > > So, > > dB = 10 log [(V1^2)/V2^2)] or, 10 log[(V1/V2)^2] > > If we want to express this as a ratio of voltages, rather > than a ratio of powers (there's a pun in there somewhere ;-), > we need to take the square root of (V1/V2)^2 outside of the log. > > To do this, we need to remember that log[X^2] = 2 log X, so: > > dB = 10 log[(V1/V2)^2] = 20 log[V1/V2] > > A couple of things to note: > > 1) dB's are dB's. 3dB represents the doubling of a power ratio, > 6dB represents the doubling of a voltage ratio. > 2) Convention says that if -dB's are loss, and +dB's are > gain, but that > is just convention. > > -Chuck Harris > > _______________________________________________ > time-nuts mailing list -- [email protected] To unsubscribe, > go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
