I think Dana's explanation is a much clearer way to think of what is going on in an I-Q receiver.
Until you are really far down the signal chain, at the demodulator, where you might process the I and Q signals differently, there is no 'splitting' or division of the signal into I and Q. The signal in the I and Q channels is the same, just shifted in time / phase in one of the channels, relative to the other. --- Graham == On Thu, Aug 26, 2021 at 4:43 PM Dana Whitlow <k8yumdoo...@gmail.com> wrote: > Hi Jim, > > I think the best way is to view the signal as a phasor, with any > noise present adding a > random trajectory (a fuzzball) to the tip of the signal vector. > Conceptually speaking, > this eliminates needing to worry about the distribution of power between I > & Q, etc. > It lets you view the whole thing without regard for choice of axes, > coordinate system, > and all that. > > If the S/N is good, the fuzzball is small in size compared to the length of > the phasor, > and you can immediately see that neither the length nor the angle of the > sum vector > is much affected. > > But as the SNR is reduced, you eventually reach the point where some of the > noise > peaks almost reach down to the origin, and as the vector tip swings near > the origin > the phase angle changes very rapidly by nearly 180 deg, but the effect is a > temporary > glitch of zero area. > > But if the noise peak is a little bit bigger, the vector tip swings all the > way around the > origin, yielding an eventual effect of an added 360 deg (a whole extra > cycle) in phase > shift. This tends to have a far more deleterious effect on a signal. I > had a text given > to me by a friend in which the author used this kind of explanation to > explain, for > example, the "threshold effect" of noise in FM demodulation. I just > looked, but could > not find the book, else I'd have given you the title and author > information. > > This mode of thought also leads towards an understanding of the "FM capture > effect", > which spec was always highlighted in datasheets of HiFi FM tuners. But one > hears > little of it nowadays, I suspect because the advent of fast ICs has made it > so easy > to very- closely approach the "theoretical limit" that everybody is about > the same. > BTW, said "theoretical limit" is not fixed until one specifies other > parameters, and > at one time there was a standard test definition so that such a limit could > be defined > and measured against. > > In IQ demodulation I find the ATAN2 function a good deal more useful than > the old > arctan function, which needs a lot of help in order to work usefully. The > ATAN2 > function takes two arguments (I & Q values) and automatically places the > angular > result in the correct quadrant and is not bothered by either of the > arguments being > zero. The only place it gets in trouble is if *both* arguments are zero, > which is an > infinitely-tough nut to crack in any case. > > As with all the inverse trig functions, ATAN2 has a limited angular range, > from > -180 deg through zero to +180 deg, then snaps back to -180 deg again. > But it's not too difficult to "unwrap" the results so that a continuous > rotation of a > phasor leads to a nice smooth phase ramp with no discontinuities at all. > In many > cases this presentation makes the picture much clearer, although overly > high > noise peaks can create what some will call a false transition. If you're > really > interested in the signal alone, yes you have a problem then. But if you > consider > the "signal" to be the composite vector sum of some signal and added noise, > the unwrap process works correctly. > > Whew! > > Dana K8YUM > > > > On Thu, Aug 26, 2021 at 3:52 PM Poul-Henning Kamp <p...@phk.freebsd.dk> > wrote: > > > -------- > > Lux, Jim writes: > > > > >I'm looking for a simplified treatment of the uncertainty of I/Q > > >measurements. Say you've got some input signal with a given SNR and you > > >run it into a I/Q demodulator - you get a series of I and Q measurements > > >(which might, later, be turned into mag and phase). > > > > > >[...] > > > > > >I'm looking for a sort of not super quantitative and analytical > > >treatment that I can point folks to. > > > > Good luck with that :-) > > > > Some of the noise processes will be along the "vector" and distributed > > between I & Q components depending on the phase, while other noise > > processes affect the components individually. > > > > To make matters worse, both kinds of noise processes may depend on the > > phase, usually because of cross-talk and/or insufficient isolation. > > > > Low-resolution ADC's are a particular nasty problem, because they add > > +/-1 count jitter independent of the phase, and that causes very > > large arctangent errors. > > > > Counterintuitive as it may sound, it is easier to process the bits from > > ADC's where the low two bits are pure noise, than ADC's where all bits > > are good... > > > > -- > > Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 > > p...@freebsd.org | TCP/IP since RFC 956 > > FreeBSD committer | BSD since 4.3-tahoe > > Never attribute to malice what can adequately be explained by > incompetence. > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe > send > > an email to time-nuts-le...@lists.febo.com > > To unsubscribe, go to and follow the instructions there. > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send > an email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.