It might be useful to remember: Roofing filters at the front end of the
K3's 8.1 MHz 1st IF protect the ADC from overload by [very] strong
signals adjacent to your QRG. If you are not plagued by such very
strong signals ... usually from local-ish stations ... roofers are a lot
like snowflakes, "seen one, you've seen them all." So long as the
filter is wide enough for the signal bandwidth and the integrated
spectral power centered at 15 kHz does not overload the ADC, the
bandwidth limiting/positioning provided by the DSP will do it all for
you.
I have two roofers in my very aged K3, narrow and wide. They're the
ones that came with it [500 Hz and 2.7 kHz? Don't remember]. I don't
seem to have any locals here that overload the ADC, and on CW, there's
no discernible difference between them. My K3 hasn't seen a phone
signal in a long time. 😉
If you are plagued by a signal with lots of in-band phase noise [I was
in a previous QTH], filtering will improve the SNR at the ADC some, but
your only real recourse is to convince the op to buy a new phase-free
radio.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
------ Original Message ------
From "Henry Pollock - K4TMC via Elecraft" <[email protected]>
To [email protected]
Cc [email protected]
Date 12/24/2025 1:21:56 PM
Subject Re: [Elecraft] Advice on Additional Roofing Filters for K3
Tom,
I have the 1.8 filter in my old K3 #98 and have found over the years that I
almost never use it. The DSP Width and Shift controls with the stock 2.7
filter have been adequate for my needs in all SSB contest conditions when
using fast AGC. So, if I wanted a narrower filter for SSB, then the 2.1
would be the only one I would consider.
73,
Henry - K4TMC
On Wed, Dec 24, 2025 at 2:10 PM Jim Brown via Elecraft <
[email protected]> wrote:
On 12/24/2025 8:43 AM, Tom Bowles via Elecraft wrote:
> I'm considering adding a 1.8 kHz for SSB and a 250 Hz for CW, but I've
read that the 250 Hz filter often measures wider in practice (around
330-350 Hz) and may not be as sharp as expected. Has anyone experienced
this, and would you recommend it, or suggest alternatives like the 200 Hz
or 400 Hz instead?
First, these are ROOFING filters to protect the DSP IF from overload
from very strong nearby stations. It is that DSP that sets the IF
bandwidth. I'm a serious contester, and have used K3s since 2008. The
250 Hz 8-pole filter is great for CW. I found the 1.8 kHz filter too
narrow for SSB, and prefer the 2.1 kHz filter. If you work RTTY, the 2.8
kHz filter has much better response than the stock 2.7 kHz filter, which
greatly reduces the incidental AM on transmit with RTTY. If you don't do
RTTY, it doesn't matter.
73, Jim K9YC
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