Geoff,
While all the situations that you pose are valid measurements, I believe
that the 'figure of merit' that should be applied is with both signals
within the receiver passband. That alone will indicate the ability to
copy a weak signal in the presence of a strong one - that is basic BDR
(IMHO).
Third order blocking IMD is quite another thing - there are many
variables to consider. The best that we can hope for is that the
'standardized' tests will provide a good indicator of the performance on
the real world - a receiver with narrow 'roofing filters' should excel
in that test - although the test reports should specify the filter
widths. The close spacing tests go a long way toward simulating the
'real world', but are really only a set of conditions that are defined
for lab tests - the real world operating conditions may present an
entirely different set of parameters.
Current tests do consider MDS to be the minimum signal that can be
demodulated with no other considerations. Right or wrong, that is the
way it is for now - something must be defined for lab measurements to be
valid - we just trust that they are adequate to be useful in actual
operating conditions.
73,
Don W3FPR
Geoffrey Mackenzie-Kennedy wrote:
Curt,
Would you not agree that a 3rd Order dynamic range test performed with
both test tones placed outside of the IF filter's passband ,with a
product in the passband, does not provide a complete measure of a
receiver's odd order IMD performance but only that of its front end? A
receiver with a "weak" cascade following the IF filter might appear to
be be a good performer based on tests done this way, but will probably
fall apart when two or more strong signals enter the IF filter's
passband. During the development of high performance receivers common
practice in my experience was to employ multiple signal tests, two tones
in the IF passband and one or two placed on the skirts. I am aware of
suggestions that have been made during the past years regarding these
test procedures used to evaluate amateur receivers, likewise to restore
proper useage of the term MDS which is another story :-)
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