[Elecraft] K3 filter/DSP selection?

[Don Wilhelm]

Chuck,

The K3 filtering (both IF and DSP) is quite different than the K2 - in 
fact, there is no comparison.  Bear with me and I will try to explain.

The K3 will control the filters with 2 knobs - the SHIFT/LO-CUT and the 
WIDTH/HI-CUT.
The K3 does all its ultimate filtering in DSP (at a 15 kHz DSP IF), and 
so DSP must be active all the time (it is doing all the work of both 
filtering and demodulation as well as a number of other tasks).

The 8MHz IF filters in the K3 are "roofing filters" whose main task is 
to keep very strong signals out of the ADC at the front end of the DSP.  
The DSP itself can handle signals well above S-9 + 30, and has its own 
AGC (normal or DSP AGC) for signals of normal strength, but the ADC has 
its limits and would cause bad signal distortion if overloaded.  
However, the K3 has an additional hardware AGC at the 8MHz IF that will 
activate only when the signals *inside* the roofing filter passband are 
stronger than S-9+30.  The result is that *if* that  hardware AGC is 
activated, it will cause signal "pumping" (following that strong 
signal).  The solution is to use a more narrow roofing filter to narrow 
the passband that the ADC input to the DSP is exposed to.

The additional roofing filters (if installed) are automatically switched 
in when you narrow the DSP bandwidth.  No button-pushing is necessary.  
Just move the WIDTH knob and it will happen.

What all this roofing filter stuff means for you is that the stock basic 
K3 with only the 2.7 kHz roofing filter will do a great job unless you 
have an extremely strong undesired signal within that 2.7 kHz bandpass.  
For normal operation, that one roofing filter is likely to be 
sufficient, but if you are operating in contest times or chasing DX 
where the possibility that there will be several strong stations near 
the frequency of the weak signal you are trying to copy, you will want 
additional roofing filters installed to keep the hardware AGC from 
activating.

If initial cost is a consideration, my recommendation is to first use 
the K3 with only the stock roofing filter, then after a bit of 
operating, you will know whether and when you need additional roofing 
filters.

Considering wider bandwidth roofing filters:
If you are considering ESSB or AM transmitting with the K3, then you 
need the 6 kHz roofing filter
If you are considering FM operation, you will need the 13 kHz roofing 
filter.

73,
Don W3FPR

chuck allen wrote:
> Hello,
>
> I have a K2 and am considering selling it to fund a bare bones K3. However, what I find awkward about the K2 is the way the cw filters (I only do cw) and the DSP filters are selected ... too much button pushing to get the bandwidth I need and if I go past the best setting I have to scroll all the way thru again. Plus no easy way to activate DSP noise reduction unless I have it pre-programmed.
>
> So I'm trying to understand how this is accomplished on the K3. I'd love to have a knob to control the filter and DSP bandwidth, a pushbutton to activate DSP, and another pushbutton to activate DSP noise reduction. I've looked thru the K3 operating manual but am not exactly following it.
>
> Could someone please shed a little light on how the K3 performs this and if they find it user friendly?
>
> thanks in advance.
> chuck
> af4xk
>
>
>
>       
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