[Elecraft] K3 IF/AF Dsp questions (comparisons)

[Bill W5WVO]

Right, Greg. We're talking about VERY strong signals here.

For example: This is why dynamic range is so important to 6m contesters. 
Signals on 6m during intense sporadic-E (Es) openings can be incredibly 
strong, due both to the shorter ionospheric path for this kind of propagation 
and to the high efficiency of Es clouds as radio wave reflectors. (Es clouds 
tend to be both intensely ionized and very planar, like a reflecting mirror as 
compared to a refracting atmospheric layer as in F2 propagation.) Signals of 
40 dB to 50 dB over S9 are fairly common on 6m during a hot contest Es 
opening.

But readable signals on 6m can also be very weak, as the atmospheric noise 
floor is significantly lower on this band than on the HF bands, and copying a 
signal producing only a half-microvolt at the receiver front end is not 
unrealistic in the absence of man-made RFI or thunderstorms.

When you're talking about trying to copy this 0.5 uv S1 signal in the presence 
of 40-over-9 signals 5 and 10 kHz away (again, a common real-world scenario on 
6m during a contest), you're looking at a range of 40 dB + ( 8 S-units x 5 
dB/S-unit ) = 80 dB.

Just as a reality factor, my Kenwood TS-2000 has been measured by ARRL Labs to 
have a 5 kHz 3rd-order IMD dynamic range at 50.1 MHz of a scintillating 66 dB. 
OUCH! This isn't even close. A 25-over-9 signal would still push the envelope 
of the receiver's IMD dynamic range, and signals of that strength are all over 
the band on 6m during the June contest. I can tell you from experience that 
using a receiver with 66 dB @ 5 kHz DR3 is painful. You really can't hear weak 
signals if strong signals are within 10 kHz or more of your operating 
frequency.

Which is why I'm awaiting my K3 with bated breath. Too bad it won't be here in 
time for the ARRL June VHF, but I'm still holding out some faint hope for the 
July CQWW VHF. I'm in the first-production-run queue.  ;-)

Bill / W5WVO


FISCHER,GREG wrote:
> Don,
>
> I don't think the concern here is with S9 signals, but
> rather much stronger ones.  Here is a clip from Erics
> article talking about roofing filters......
>
> If you only use the 2.7 kHz stock filter for CW or data
> operation you will be significantly desensed once signals
> within that filter's bandwidth exceed about S9+25.
>
> Hope that helps...
>
> 73
> Greg
> AB7R
>
>
>
> On Wed, 16 May 2007 14:23:54 -0700 (PDT)
>  Don Rasmussen <wb8yqj at yahoo.com> wrote:
>> The 756pro has no roofing filters, yet there is little
>> or no AGC pumping when listening to an S2 signal with
>> the DSP filter set to 500hz and a S9 signal is within
>> the 15khz first hardware filter. I'm hoping the K3 can
>> do at least this well without switching in a roofing
>> filter of less than 2.7khz.
>>
>> The TenTec Argo V will pump somewhat in the same
>> scenario and pump very hard in a contest, at times
>> swamping the receiver. It's a great radio for a quiet
>> afternoon or contesting with modest antennas.
>>
>> The TS480Sat will pump dramatically and you need to
>> leave the 500hz filter in line anytime there is an S9
>> signal within the 2.8khz ssb filter. The 500hz filter
>> is superb for selectivity but adds a very high level
>> of background noise when the bands are not quiet.
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