[Elecraft] K3 2.7 k Filter vs. 2.8 k

[Larry Phipps]

For my general operating needs, I think either the 2.7 kHz or 2.8 kHz 
filter combined with the 500 Hz represent a reasonable approach. I 
talked with Eric at W9DXCC this past weekend, and I got the impression 
that 2.8 kHz / 500 Hz would be a very good overall combo for all my 
needs, but I am waiting to hear some feedback from the first run. Even 
though I ordered the first weekend, I specified the second run, partly 
because I wanted to see same data and hear some feedback on the filters 
before deciding. The variable CW filter might also be a good choice once 
it's available, depending on IMD performance. Eric indicated that Wayne 
likes the variable filter approach, and it would be a flexible one... 
but I want to see performance numbers.

73,
Larry N8LP



Bill Tippett wrote:
>
>
>         This topic has drifted from "2.7k versus 2.8k" to "2.7k
> versus 400 Hz".  *Of course* a 4-500 Hz filter is needed for CW
> and will have vastly better IMD/BDR than a 2.7k/2.8k.  But I
> repeat that there will be very little if any difference between a
> 400 (actually 435 Hz) 8-pole and 500 ( 565 Hz) 5-pole.  In my
> opinion, you are really wasting money to buy 8-pole filters for
> improved RX performance, unless the 130 Hz BW difference
> is important. In practice, this implies the difference in a signal
> spaced 435/2 = 218 Hz versus 565/2 = 283 Hz...65 Hz is not
> much difference, and I actually prefer the wider BW to catch
> more off-frequency callers.  From Eric's posted data:
>
> Filter            20kHz  10kHz  5kHz  2kHz
>
> 400 Hz, 8 pole    100+   100+   100+   95
> 500 Hz, 5 pole    100+   100+   100+   94
>
> Again in my opinion the 250/200 Hz filters are
> redundant and unnecessary if you have a 400/500.
> They do not improve 2 kHz IMD significantly as
> seen below (1 dB difference is meaningless as
> that is well within measurement error):
>
> Filter            20kHz  10kHz  5kHz  2kHz
>
> 200 Hz, 5 pole    100+   100+   100+   95
> 250 Hz, 8 pole    100+   100+   100+   95
>
> You could argue that the 250/200 would be better
> for IMD fom extremely close-spaced signals (e.g.
> <200 Hz spacing from your TX frequency), but at
> that spacing other factors such as the transmitted
> signal's phase noise, key clicks, etc. will override
> any theoretical IMD advantage (i.e. the IMD becomes
> "noise limited" in ARRL terminology).
>
>         Remember also that Passband Tuning can be used
> to shift a 400/500 Hz filter if you actually do need
> to eliminate a signal spaced at 200-250 Hz from your
> TX frequency...not that it would actually do any
> good to eliminate phase noise or key clicks.
>
>                                 73,  Bill  W4ZV
>
>
>
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