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
