Hi Alan, Due to visitors arriving tomorrow, I must make this my last post on the subject we have been discussing.
I believe that this discussion started after Jack had warned that a square wave source can get you into trouble if used to make 2nd and 3rd intermodulation measurements, and noted that MiniCircuits AN-00-008 offered an explanation. As a summary, In the last paragraph of my last post I suggested that an "unwanted" mixer mixer can exist near the input of the filter, and that it would be exposed to the full impact of the source's harmonics. I should stress that this is not speculation, but fact derived from many diagnostic tests on filters during design. In the case of two-tone IMD tests using sources whose harmonic content is high, any "unwanted" mixer near to the filter's input will generate a host of products, and some of these will pass through the filter and contaminate the test results. In the case of single-tone tests using a similar source, this "unwanted" mixer in the filter can still affect the accuracy of test results, by how much will depend, of course, on the characteristics of this "unwanted" mixer (which are related to the filter's design), and the level of the incoming test signal's fundamental and each harmonic.Please bear in mind that this "unwanted" mixer is NOT protected by the selectivity of the filter. If in this second case the fundamental test frequency is F1, then the harmonics of F1 should not get through the filter. However because the "unwanted" mixer is exposed to the incoming signal's fundamental and its harmonics, among the products generated will be 2F1- F1, 3F1 - 2F1, 4F1- 3F1 etc, all of which will get through the filter and contaminate the test results. However if the single-tone test signal is a true and symmetrical square wave, the amplitude of the incoming even order harmonics should be small which would help to reduce this problem, leaving the "unwanted" mixer's "self generated" products at F1. Lastly, with regard to front end filters and dynamic range, I can assure you that the design of the front end filters as used in a high performance HF receiver is not casual. What is causing some concern is that the gap between robust front end filters and top range mixers (H-Mode class), in terms of their IMD performance, is closing. Useful front end filters tend to be large in size! 73, Geoff GM4ESD On Friday, April 15, 2011, at 22:43 +0100, Alan Bloom wrote: > Hi Geoff, > > The case we're considering is where the test signal(s) are near the > receiver tuned frequency so they are well within the passband of the > input filters. > > If the receiver's in-band dynamic range is significantly affected by > non-linear filters in the front end, then that is a serious design > error. The distortion should be dominated by the mixer, not the filter. > If the filter is distorting then it doesn't matter if the generator has > harmonics - you're hosed anyway. > > For in-band receiver testing, generator harmonics should not matter. > The only exceptions would be for receivers that do not have filters > protecting the front end or perhaps the exceedingly rare case where a > generator harmonic happens to fall on a receiver image or spur frequency > (easily fixed by changing the test frequency slightly). But in almost > all cases the harmonics should not matter. > > Alan N1AL > > > > On Fri, 2011-04-15 at 21:30 +0100, Geoffrey Mackenzie-Kennedy wrote: >> Hi Alan, >> >> Sorry that I was not here to reply to your previous e-mail. >> >> With respect, filter non-linearity and the level of harmonics coming from >> the signal sources are related in terms of the amplitude of the IMD >> products >> seen at the output of the filter. >> >> If the receiver being tested is perfect and linear, obviously it would >> not >> matter if the IMD test sources contained harmonics because, for example, >> the >> fundamental of one source (F1) has nowhere to mix with the second >> harmonic >> of the second source (2F2) to produce the third order product 2F2-F1 - >> assuming there is no IMD contribution from the test equipment. >> >> On the other hand if the receiver's signal path passes through a >> non-linear >> element such as an input filter, then of course there is 'mixer' >> available >> inside the filter to produce the natural third order product 2F2 - F1 as >> would be expected when using clean sources. If the sources produce >> harmonics >> however, then a twin third order product also at 2F2 - F1 would also be >> generated, where 2F2 is the second harmonic generated by the F2 source. >> The >> composite level of these two third order product as seen at the filter's >> output could be significantly different from that of the natural product >> depending on how they add. >> >> Although the second harmonic of a test source should be greatly >> attenuated >> by a typical front end filter, an unwanted 'mixer' can exist near the >> input >> of the filter, for example in the form of an inductor's core, and would >> be >> exposed to the full impact of the source's harmonics. >> >> Geoff >> GM4ESD ______________________________________________________________ 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
