[TenTec] Sherwood Presentation

[Bill Tippett <btippett@alum.mit.edu>]

Hi Buck,

K4IA:
>The roofing filter may make the IMD figure worse but it also cuts down on 
the
QRM and, very importantly, keeps the AGC circuit from pumping or generating
clicks on its own.  This is the primary difference I note with the Orion vs my
Mark V Field.  That Field collapses when there is a signal nearby and I don't
think it is because of IMD.  You can narrow the filter on the Field to
eliminate the QRM from the nearby signal but the AGC pumps so badly (or is 
the front
end overloading?) you can't hear the weaker signal.

        What you are describing is typically called desense or blocking and
is simulated by the Blocking Dynamic Range (BDR) measurement.  This is
another important metric for receivers which is measured by ARRL, Sherwood,
and others.  Here is a summary of ARRL's measurements:

http://www.elecraft.com/K2_perf.htm#5%20kHz%20numbers

Note the BDR measurements in the 3rd (@ 5 kHz) and 4th (@ 20 kHz) columns from
the left.  Sherwood also lists it as "100 kHz Blocking" in the 5th column
from the left here:

http://www.sherweng.com/table.html

        IMD is typically the most critical measurement, which is why many 
tables
like the above rank RX's by close-spaced IMD performance, but BDR, Phase Noise,
Sensitivity and other metrics are also important and should be considered as
part of the total picture of performance.  Sherwood just happened to focus on
IMD, but that is by no means the only performance metric.  If you happened to
have a very close ham neighbor, indeed BDR might be much more important to you.
Also, as you start looking at REALLY close-in performance, Phase Noise of the
internal synthesizer begins to come into play since it can override IMD
performance.  This is why you sometimes see terms like "noise limited" for
close-in measurements.  For example, note the following footnote at the bottom
of Sherwood's table above:

^ = Measurement was phase-noise limited rather than intermodulation limited.

                                        73,  Bill  W4ZV

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