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 > > > On Thu, 2011-04-14 at 16:54 -0400, Jack Smith wrote: >> Alan: >> >> One are where a square wave RF source can get you into trouble is 2nd >> and 3rd order intermodulation measurements. >> >> Mini-Circuits Ap Note 00008 discusses the reason why. >> http://www.minicircuits.com/pages/pdfs/AN00008.pdf >> >> I usually use an outboard low pass filter on my HP 8657A signal >> generators when making IMD measurements of high performance amplifiers, >> and it's not a bad idea even with HP 8640B generators which are 60 dB+ >> down on the harmonics. >> >> As far as a low pass filter goes, CoilCraft has some very inexpensive >> ($3 range) 7th order elliptical low pass filter modules, the P7LP >> series, with cutoff frequencies from 300 KHz to 500 MHz. >> http://www.coilcraft.com/pdfs/lcfilt.pdf These are perfectly fine for >> output filtering of a single generator, but should not be used with >> multiple signals as they can themselves be intermod generators. I use >> them here for various tasks including signal generator clean up. >> >> If anyone is interested, I have a few extra printed circuit boards with >> provisions for PCB-mount BNC connectors and a P7LP filter and can make >> them available at a reasonable price (PCB only, you supply the >> connectors and filter modules). Usual double sided, silk screen, solder >> masked boards. I have installed a socket strip in one as an experiment >> so that one might just plug in the appropriate filter, but that's not a >> great idea as the filter modules are not all that mechanically robust >> and the extra lead inductance from the socket degrades stop band >> performance at higher frequencies. I can send a photo of the board to >> anyone interested. >> >> Jack K8ZOA ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
