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 > > > > > > 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
