On Thu, Oct 18, 2012 at 9:30 AM, Martin K wrote: > On Wed, Oct 17, 2012 at 10:16 PM, Sean Breheny wrote: > > OK, but my point is this: what aspects of the sum do you care about? > > Do you care if the result is a*x+b*y where x and y are your two > > signals and a,b are constants that are not exactly 1? If they are not > > exactly one, do you care more about the ratio a/b or simply that the > > result has roughly some particular total power and both signals are > > present? > > > > For example: if you have a point-to-point RF link and there are two > > transmitters (one at 100MHz and the other at 120MHz) and they share > > the same antenna, you don't really care if the two signals experience > > exactly the same gain and phase-shift, as long as neither of them is > > greatly attenuated or distorted. On the other hand, if you are doing > > an experiment where you are feeding two signals into a non-linear > > device and you expect to see sum and difference frequencies as well as > > higher-order products from the output, then you may care greatly about > > the _accuracy_ of the sum and the difference in the phase shift seen > > by the two signals. > > > > Sean > > > > I hit send without realizing I didn't answer the question, sorry. > A small fixed phase shift is probably inevitable and that's fine. The > input frequencies are fixed and not modulated (essentially) > The phase response shouldn't drift though. I can't imagine why it > would, can you? > > The amplitudes should be matched to within 10% or less. > Fixed offsets aren't a big deal because it can be accounted for in the > test results. > > The point of adding the signals in time domain is so the load can see > two frequencies. There shouldn't be significant difference and higher > order components. That would be mixing, right? > > A real mixer is a multiplier whereas a combiner is an adder. So, the circuit commonly referred to as a "mixer" (e.g. op-amp with two or more signals added together through resistors to the virtual ground on the inverting input) is really a combiner/adder. Carey Fisher --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .