Posted by
Geoffrey Mackenzie-Kennedy on
Jan 14, 2010; 6:33pm
URL: http://elecraft.85.s1.nabble.com/In-band-rx-IMD-tp4374684p4394485.html
On Thursday, January 14, 2010, at 4:53 PM, Guy Olinger K2AV
<
[hidden email]> wrote:
> This is another one of those subjects that seems to have advanced on
> rumor and inuendo. Actually measuring in-band IMD is not as simple as
> it looks. Basically (whether included on a single chassis or not) the
> apparatus must be able to generate a pair of signals which are
> combined in a way that precludes IMD in the test signal to start with.
> Then the frequencies must be set so that A, B, 2A-B, and 2B-A are all
> inside the passband, (inside the roofing filter for the K3 with dsp
> bandwidth set to match) .
Hi Guy,
I don't know about the subject being one that has advanced on rumour and
inuendo, certainly not in "non-amateur" circles, but I agree that measuring
in-passband IMD is not as simple as it looks. To obtain a clean pair of test
signals, crystal oscillators whose phase noise is suitably low are often
used instead of variable frequency signal generators whose phase noise can
be too high. Also all parts of the test jig, not only the method of
combining the test signals, must be designed and operated to ensure that
receiver IMD measurements are not polluted by test jig generated IMD - which
is, of course, a requirement placed on any IMD test jig.
<snip>
> I for one would like to see some hard lab-grade facts on the existence
> or non-existence of this "problem".
I can send you direct if you would like some data in the form of a Gain
Distribution Analysis for a receiver in use here. This includes both the
in-passband and the "out of passband" IMDDR3 stage by stage and overall. A
critical point often missed during receiver design is the level of IMD
generated by a roofing filter, which means that the receiver designer is
flying blind. Filter IMD measurement is one case where in-passband IMD
measurement is very useful.
73,
Geoff
GM4ESD
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