[Elecraft] XG3 device...

[Geoffrey Mackenzie-Kennedy]

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