[Elecraft] The Sherwood report

[Wes Stewart]

I haven't (yet) seen Rob's comments, cited by Paul.  The designers across the pond are well-aware of the IMD effects in crystal filters, for example here:

http://www.xs4all.nl/~martein/pa3ake/hmode/roofer_intro.html

I have searched for, but have not found, remarks that I recall being made by Wayne that said that Elecraft screens its filters for IMD.

(I wish they also screened for bandpass symmetry, my 2.8 KHz is very disappointing in that regard)

So considering the potential effects of asymmetry in the build of the filter(s) it doesn't surprise that reversing the filter might make a difference, *if* filter IMD is the dominate factor.

--- On Thu, 12/2/10, Paul Christensen <w9ac at arrl.net> wrote:



On the Ten Tec Eagle list, Rob Sherwood just posted a very interesting 
response to the 2 kHz DR rankings being discussed here.  Apparently, there's 
a measurable amount of IMD difference not only between roofing filter 
production samples, but the manner in which they're inserted.  For example, 
on the Orion II, he gets variations of several dB on close-spaced DR just by 
unplugging a filter and reversing it -- something not possible with the K3 
since the pin-out isn't symmetrical.  He goes on to discuss why.

Sounds like we could probably take handful of INRAD filters and choose the 
best ones that may likely be equal to the FTdx-5K under most, if not all 
narrow filter positions in the K3.

On multiple occasions, Rob has stated that once you get up anywhere near 
these numbers, a prospective purchaser should be looking at other 
performance factors and features.  Probably if you're within the top ten or 
so on his list, you would be pretty hard-pressed to notice any differences 
when other factors (e.g., the other guy's bandwidth) will mask incremental 
improvements to close-spaced DR.

For me, those "other factors" that Rob discusses include low distortion, low 
noise AF sections, low TX IMD, and super fast, click-free T/R times.  Right 
now, those are the most important factors to me now that basic receiver 
performance has advanced so much in the past decade.

Paul, W9AC