that would filter out the harmonics of a square-wave test signal. I
> 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
>
>
>
>