[Elecraft] K3 2.7 k Filter vs. 2.8 k

[Bob Cunnings]

I took the reasoning a step further since the data for the 1 kHz 8
pole filter is:

Filter            20kHz  10kHz  5kHz  2kHz
---
1 kHz, 8 pole     100+   100+   100    94
---

So I ordered the 1 kHz filter since I'm just a ragchew type cw
operator. It's nice to be able to run as wide as 800 Hz or so when
tuning around, something I couldn't do with the narrower filters - but
with numbers like this I don't think I have much to worry about!

Bob NW8L

On 9/18/07, Bill Tippett <btippett at alum.mit.edu> 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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