Having used three different K3's equipped with 4 different sets of 250 & 400 hz
8 poles in overload city major contests, the usefulness of these filters is
belied by the apparent closeness of numbers.
In practice we have found that associating the 400 with a DSP width of 450, and
the "250" with a DSP width of 350 has been extremely useful, however "too close
by the numbers" that may look on paper.
We use 450 for running for as long as it may last, and when the inevitable 30
over 9 crowder squeezes down on us, I reduce to 350 and if that isn't enough,
*ADDITIONALLY* shift the center away 50 hz more. This combination plus the
noise blanker for key clicks has worked extremely well. (Having 50 Hz
granularity on the CW shift/widths would be *SO* useful here...)
I have measured the combined (roofing+DSP) drop on the steepest part of the
skirts at ~12 db per 10 Hz with these two 8 pole filters. So bringing in the
skirt "only" 30 or 40 Hz is enough to push down the crowder quite a bit,
usually well out of hardware AGC, without narrowing the listening window to the
point of missing QSO points and multipliers from all the inevitable
off-frequency callers.
Anyone who asks us what filters to buy for CW contesting, we tell them 400/250
8 pole set to 450/350. Tried and true, not theory.
W4TV and I will just have to agree to disagree.
73, Guy.
----- Original Message -----
From: ni0c
To: elecraft@mailman.qth.net
Sent: Thursday, February 05, 2009 4:47 PM
Subject: [Elecraft] [K3] Elecraft roofing filters
W4TV wrote:
"The difference between the 250 and 400 Hz filters is not
enough to be worthwhile (approximately 375 Hz vs. 430 Hz). "
Joe, what is your source for these numbers? I've seen numbers
like this mentioned previously on the reflector.
I recently tested my K3 CW roofing filters using the XG-2 generator
at 7040 KHz (50 microvolts input) and KS7D's nice software package,
"K3 Filter Tools."
Here's what I came up with, with AGC off and with the DSP
bandwidth set at least as wide as 900 Hz for all tests (to isolate
the effects of just the crystal roofing filter):
200 Hz, 5-pole: -6 dB BW = 210 Hz; -30 dB BW = 430 Hz
250 Hz, 8-pole: -6 dB BW = 260 Hz; -30 dB BW = 500 Hz
400 Hz, 8-pole: -6 dB BW = 380 Hz; -30 dB BW = 580 Hz
Note the uncertainty in each of the bandwidths above is
plus or minus 20 Hz, because I ran these sweeps in 10 Hz
increments to save time. (I'll repeat these tests using a
1 or 2 Hz increment, when I have some spare time.)
These tests on my filters (as well as my experience by ear)
indicate there is an appreciable difference in the 250 and
400 Hz 8-pole filters. I've always wondered, too, about
manufacturing tolerances on narrow crystal filters.
73,
Chuck Guenther NI0C
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