As to whether or not TT fixes the degradation of dynamic range performance
at the 2 kHz bandwidth with the 500Hz/250Hz filters is something they could
overcome in a short period of time if they wanted too. If they don't George
at INRAD will surely make quite a few dollars off an improved design
500/250Hz roofing filters for the Orion. The numbers as Rob Sherwood was
pointing out on Saturday are quite impressive on the Orion's Dynamic Range
from 20 kHz to 2 kHz with only a 4db difference in such a huge change in
bandwidth that is fantastic, congrads TT engineers, now if the 500Hz roofing
filter could be somewhere close, say the upper 80's that would be the icing
on the cake.
Dave, N4IN
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of Bill Tippett
Sent: Wednesday, May 19, 2004 11:43 AM
To: tentec@contesting.com
Subject: [TenTec] Sherwood Presentation
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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