Sure, but if the filters are non-linear that's a whole different problem that has nothing to do with harmonics in the signal source.
The question was whether signal generator harmonics are normally a problem when measuring a receiver. I claim they are not because they are removed by the filters in the receiver front end. When I was working in the R&D lab of the Hewlett Packard Signal Sources division, one of our competitors came out with a new signal generator with digital modulation. The modulation was generated at an IF which was heterodyned up to microwave frequencies without any filtering to remove the image. So there was a zero-dB spur X MHz away from the desired signal. They got away with it because it was far enough away to be filtered out by the receiver under test. Alan N1AL On Fri, 2011-04-15 at 13:11 -0400, Jack Smith wrote: > Only to a first approximation are filters linear. > > Usually the major source of non-linearity is in the inductors. Even > powdered iron is non-linear to some degree. Ferrite is worse, of > course. Air is pretty close to perfect. > > Capacitors are also non-linear at some level, as C is not constant > with applied voltage. NP0/C0G caps are very good, but again not > perfect. > > Filter generated intermodulation problems are real and measurable. > Been there and done that and have the Tee-shirt. > > Jack K8ZOA > > > On 4/15/2011 1:01 PM, Alan Bloom wrote: > > Hi Geoff, > > > > I guess I don't understand. Filters are linear devices. How can they > > cause IMD? If the signal generator's harmonics are removed by the > > receiver filters then it is the same as if the sig gen were "clean", no? > > > > 73, > > > > Alan N1AL > > > > > > On Fri, 2011-04-15 at 11:54 +0100, Geoffrey Mackenzie-Kennedy wrote: > > > Hi Alan, > > > > > > Indeed, most receivers do have some form of filter "up front" in the > > > front > > > end, and therein lies one source of IMD measurement error if harmonics > > > are > > > present in the applied test signals. > > > > > > While the MiniCircuits application note AN-00-008 which Jack mentioned > > > points out that harmonics present in the applied test signals are a > > > potential source of error when measuring the IMD performance of a device > > > such as a mixer, for the same reasons error can be introduced by a > > > receiver's input filter(s). So it is essential that "clean" sources are > > > used > > > when testing a receiver. > > > > > > 73, > > > Geoff > > > GM4ESD > > > > > > > > > On April 15, 2011 at 04:26 +0100, Alan Bloom wrote: > > > > > > > Hi Geoff, > > > > > > > > Most receivers have bandpass and/or low-pass filters in the front end > > > > that would filter out the harmonics of a square-wave test signal. I > > > > could dream up some unusual situations where the harmonics could cause a > > > > problem, but normally it would not be an issue. > > > > > > > > Alan N1AL > > > > > > > > > > > > On Fri, 2011-04-15 at 00:37 +0100, Geoffrey Mackenzie-Kennedy wrote: > > > > > Hi Al, > > > > > > > > > > You did say "normally not an issue", but when making even or odd order > > > > > intermodulation measurements on a receiver the use of RF sources whose > > > > > harmonic content is significant can and usually will lead to invalid > > > > > results. > > > > > > > > > > 73, > > > > > Geoff > > > > > GM4ESD > > > > > > > > > > > > > > > On April 14, 2011 at 22:58 +0100, Alan Bloom wrote: > > > > > > > > > > > Hi Jack, > > > > > > > > > > > > You're right that when testing a wide-band device like an amplifier > > > > > > you > > > > > > have to filter out the generator harmonics in order to get a valid > > > > > > third-order IMD test. But when testing a narrow-band device like a > > > > > > receiver the harmonics are normally not an issue. > > > > > > > > > > > > Those Coilcraft low-pass filters look very nice, especially for he > > > > > > price. Where can you buy them? > > > > > > > > > > > > Alan N1AL > > > > > > > > > > ______________________________________________________________ > > Elecraft mailing list > > Home: http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/mmfaq.htm > > Post: mailto:Elecraft@mailman.qth.net > > > > This list hosted by: http://www.qsl.net > > Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
