[Elecraft] K3 Roofing filters

[Don Wilhelm]

Ivan,

You are right to ask, the filters can drive the cost of the K3 spiraling 
upward.
I will not answer simply, but rather give you some guidelines for selection.

Your choice of roofing filters will depend a lot on your operating 
conditions and preferences.
Remember that the roofing filters are present NOT to achieve the final 
bandwidth - that is done in the DSP processing.
The roofing filters are used to protect the input of the DAC from strong 
adjacent signals - ones that you would not hear because they are outside 
the DSP passband.
There is another mechanism - Hardware AGC - that also protects the DAC 
from overload.  Its response is what you will hear when there are strong 
signals within the passband of the roofing filter but outside the 
passband you have set in the DSP - the strong unwanted signal will cause 
"pumping" of the AGC and constantly change the receivers sensitivity.  
The Hardware AGC will begin to operate when the signal strength is 
greater than S-9+30 dB (If I recall correctly).  For weaker signals, it 
will not activate.

So -- if you are an SSB ragchewer, the 2.8 kHz filter will likely be 
sufficient, you probably seek to operate in a clear area of the band anyway.
But -- if you are operating in a crowded band with lots of adjacent 
strong signals (heavy DXing or serious contesting), you will likely want 
to add roofing filters.  How much tolerance you have to those nearby 
strong signals will influence your choice of filters.

Of course, if you want to operate FM, you will need the 13 kHz filter, 
and for AM transmit, the 6 kHz filter.

5 pole or 8 pole? - if you have the subRX, *and* want to use diversity 
receive, the filters in the main and the sub must be matched for the 
filter offset.  The choice of 8 pole filters makes it easy, they have 
zero offset.  If you choose the 5 pole filters, matched offset  filters 
are available.

If you decide to purchase without the subRX initially, but plan to add 
it later, order the 8 pole filters unless diversity receive is not a 
consideration.

If you cannot decide by order time, I would suggest you go with the 2.8 
kHz 8 pole filter only, then operate using only  the DSP filtering for 
some period of time to allow you to find out where you are experiencing 
difficulty, then purchase whatever additional filters you need for your 
operation.  The filters are not difficult to add later (unless the subRX 
is installed, because it must be removed to get to the main filter area).

73,
Don W3FPR

On 8/6/2013 5:37 AM, g3izd at sky.com wrote:
> I am about to purchase a K3 (k). My main interest is certainly CW with occasional forays into SSB and even data, and I note the selection of roofing filters available, no doubt the 8-pole are somewhat better but what is the general feeling regarding the bandwidth(s)  to be included.
> Ivan G3IZD
>