[Elecraft] CW Filter and Roofing Filter

[Fred Jensen]

Robert, I don't think you can make the comparison as you suggest, very 
different radios.  I don't remember when the term "roofing filter" 
entered the ham vocabulary but it has caused a lot of confusion, some 
angst, and possibly some over-buying of filters.

In the K3 and K3S, the bandwidth that you actually hear in your 
headphones or speaker is set by the DSP with the WIDTH knob.  It's 
essentially a brick-wall filter, built with binary arithmetic.  If the 
signal is not inside the passband, you don't hear it.

Our bands are full of signals, some very strong which will activate the 
hardware AGC and reduce the level of *all* signals at the input to the 
ADC ... including the one weak one you're listening to.  All the roofing 
filter does is restrict the BW of the energy applied to the ADC to 
exclude most of the other signals on the band.  I'm not at all surprised 
that you don't notice much effect from the 1st IF filter, you won't 
unless there are very strong signals that will activate the AGC.

Matching the roofer BW and DSP BW offers a small benefit since two 
filters are cascaded.  But, since the roofers are not adjustable, you 
have to find a happy compromise for your operating style.  My K3 came 
with a 2.7 KHz 5-pole filter for SSB and I added a 500 Hz for CW. 
That's all I have.  I contest some [not anywhere near hard core], nearly 
all CW, and I operate casually, nearly always at 250 or 200 Hz DSP BW. 
For SSB, I usually run the DSP BW at 2.1 KHz, that's what the Collins 
mech filter was in my S-Line and I'm used to the sound.  I've never 
noticed a strong signal, outside the DSP BW but inside the roofer 
activate the K3's hardware AGC.  Now, you asked questions ...

"When I read that a 500 Hz roofing filter is an option for the Elecraft 
K3S, what am I actually reading?"

Elecraft offers a 500 Hz filter [and maybe a 250 Hz] for narrow-band 
modes which will restrict the BW of the energy presented to the ADC.  If 
you operate CW or other narrow-band modes a lot, I think it's a good 
buy.  If you're mainly a phone guy, you might not ever encounter a 
situation where it would make a difference.  I think the filters are 
really Inrads, I had a 250 Hz Inrad in a TS-850, and it was pretty "ringey."

"Could there be a 500Hz roofing filter and a 500Hz CW filter
or are they the same?"

K3?  If so, no, they are not the same if "500Hz CW filter" means what 
you set in the DSP.  You can have a 500 Hz roofer, I do.  You can set 
the DSP BW on CW to 500 Hz [you can set it much wider than that in 
fact].  The K3 will select the 500 Hz roofer, and you'll have two 
cascaded filters.  Keep in mind, the roofers are analog crystal filters, 
not adjustable.   They are nominally xxxx Hz wide, but the skirts are 
not vertical.

"In several Youtube videos, I've seen operators using the variable width 
the filter signals. What am I seeing?"

If they're using a K3, they're adjusting the DSP WIDTH control.

"is the cw filter variable (like my FT-950) or fixed (like the Century 21)?"

There is no "CW" filter in a K3/K3S.  There is a roofing filter that 
restricts the BW going into the ADC, and there is a DSP filter 
controlled by knobs on the front panel which can be set to anything 
within its range, regardless of operating mode.  The roofer isn't 
adjustable.  The knobs do the same thing whether your listening to CW, 
RTTY, PSK31, SSB, or anything else.

Small exception to the above:  Separate from the roofing and DSP 
filters, the K3/K3S has a switchable [on/off] audio peaking filter for 
CW.  It too is done in binary arithmetic, is totally separate and only 
good for CW, and under some conditions it can render an unreadable 
signal quite readable.

Hope this helps,

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org


> On 6/23/2015 2:42 PM, rclark01 at comcast.net wrote:
>> Hi all:
>> I currently own a Yaesu FT-950 and am considering selling it to move
>> the the Elecraft K3S line. My question is about filters. My FT-950 has
>> a roofing filter and a variable CW filter down to 200hz. Quite
>> frankly, I can't tell much different when I switch in the roofing
>> filter but the variable CW filter makes a big difference. I also have
>> a 1970's TenTen Century 21 which has a 500Hz filter that, to me, seems
>> to do a better job than the FT-950. So..when I read that a 500 Hz
>> roofing filter is an option for the Elecraft K3S, what am I actually
>> reading? Could there be a 500Hz roofing filter and a 500Hz CW filter
>> or are they the same? In several Youtube videos, I've seen operators
>> using the variable width the filter signals. What am I seeing? Are
>> they adjusting the roofing filter or the cw filter? And...is the cw
>> filter variable (like my FT-950) or fixed (like the Century 21)?