Tim Wescott wrote on 6/17/2017 4:36 PM:> On Sat, 17 Jun 2017 14:03:58 -0400, rickman wrote: > >> Tauno Voipio wrote on 6/17/2017 12:44 PM: >>> On 17.6.17 09:30, rickman wrote: >>>> Why is I/Q mixing done in receivers prior to the ADC rather than using >>>> a single mixer and converting to complex from a real data stream. Is >>>> there some aspect of the signal that is lost if the signal is not down >>>> converted with an I/Q local oscillator? >>> >>> >>> It is a simple question of sample rate. The lower IF is easier to >>> sample and convert. >>> >>> There are digital receivers that sample the raw RF and do the >>> quadrature mixing numerically and then decimate the sample rate for >>> further processing. >> >> I guess I didn't ask the question correctly. The analog mixing could be >> done singly rather than as an I/Q pair, then in the digital domain >> convert to complex sampling. The sample rate of one ADC would need to >> be twice the sample rate of using separate ADCs, but otherwise would be >> the same. > > It's more complicated than that. > > Consider a signal "out there" that consists of two sine waves at F1 and > F2, and an undesired signal "out there" that is a sine wave at some > frequency F3 which we don't know and don't control, except that it's not > F1 or F2. > > With I/Q mixing you can choose a carrier frequency F0 = (F1+F2)/2, and > you end up with a pair of complex signals at +(F1+F2)/2 and -(F1+F2)/2. > This is called "direct conversion", and it can work great.Either I don't follow you or you made a mistake in the numbers. After mixing the signals would be at (F1-F2)/2 and (F2-F1)/2, no? So how is this different from real mixing followed by real to complex conversion? You don't mention how F3 is involved.> With simple signals there's no concept of "negative frequencies" (or, > depending on how you hold your mouth, there's no way to distinguish a > negative frequency from a positive one). So if you try to use the same F0 > as above, you end up with two signals, at the same frequency, added > together and indistinguishable from one another.For real mixing you wouldn't use F0 - (F1+F2)/2. You would use a frequency just above the highest or just below the lowest and then pick the frequency of the final mixing to center the frequencies of interest.> So, you can't use direct conversion. You can, instead, use a > superheterodyne process: choose F0 to be lower than the lower of F1 and > F2, or higher than the higher of F1 and F2. Then you get a signal out > that has two signals at frequencies F1 - F0 and F2 - F0, or F0 - F1 and F0 > - F2. However, if F0 < F1, and F3 = 2F0 - F1, then F1 - F0 = F0 - F3, > and the received signal has the desired signal and the undesired signal > sitting right on top of each other. The second, undesired, signal is > called an "image". > > To fix this, you have to use a filter at RF that passes the band > including F1 and F2, but sharply attenuates energy at F3 -- actually, at > any F3 that might be an issue. Now add to this that reasonably low-noise > mixers and low distortion mixers don't mix with sine waves -- they > generally switch, which effectively means that they're mixing the > incoming signal with a square wave, which has content at F0, 2F0, 3F0, > etc. And yes, there's not _much_ content at 2*F0 or 4*F0, etc., but it's > there, and it can cause issues. And there's certainly energy at 3F0, > 5F0, etc. > > So RF filter design, and frequency downconversion schemes (in multi-stage > superheterodyne receivers) is not at all trivial. It's often MUCH easier > to just take the signal in at the antenna, thump it down to baseband with > an I/Q mixer, and proceed from there. > > But not always, so there will still be superheterodyne stages in radios, > probably for years to come.Ok, I get it. Thanks -- Rick C
I/Q Mixing
Started by ●June 17, 2017
Reply by ●June 21, 20172017-06-21
Reply by ●June 24, 20172017-06-24
