[Elecraft] kx3 receiver versus K3s receiver

[K9MA]

The I/Q decoding happens after digitization. If a strong signal 
overloads the ADC, all bets are off.  Without the audio filters, that 
strong signal could be quite far away.  (The other low pass filters seem 
to cut off around 14 kHz.)  In any case, it could be well outside your 
DSP passband. The optional audio filters are two pole active low pass 
filters, with bandwidths of something like 700 Hz and 1.6 kHz.  There 
are two:  I and Q.  But, because the IF is baseband, where both sides of 
zero beat are present, these filters can reject signals on the "wrong" 
side.  Indeed, they couldn't, or the I/Q decoding couldn't remove one of 
them.


73,

Scott  K9MA

On 1/9/2017 17:23, Brendon Whateley wrote:
> I'm not sure why you would think this is a problem, the "other side" 
> signal is removed by the I/Q decoding technique, not by filtering. The 
> filters do the job of roofing filters, reducing and removing near in 
> strong signals -- from the IF signal that just happens now to be at 
> baseband frequencies.
>
> If I'm missing something, please educate me on what I'm missing?
>
> - Brendon
> KK6AYI
>
> On Mon, Jan 9, 2017 at 10:24 AM, K9MA <k9ma at sdellington.us 
> <mailto:k9ma at sdellington.us>> wrote:
>
>     The KX3 "roofing filters" are audio filters.  (I wouldn't even
>     call them "roofing" filters.)  They are analog filters, which can
>     improve rejection of adjacent signals which might overload the
>     ADC.  They will not, however, reject the "audio image", on the
>     other side of zero beat.  I'd expect, especially during contests,
>     the the effect would show up once in a while, probably making a
>     weak signal totally unintelligible.  Has anyone noticed this
>     effect? Especially on CW, it would be fairly unlikely, as the
>     interfering signal would have to fall within the narrow passband
>     on the other side of zero beat.
>
>     73,
>
>     Scott  K9MA
>
>
>     On 1/9/2017 11:54, Ignacy wrote:
>
>         Perhaps as it should be. A bigger and more expensive radio is
>         a better radio
>         but one cannot take the bigger radio everywhere. I assume you
>         had roofing
>         filters in KX3.
>
>         I used KX3 directly driving Expert 1.3k for at Easter Island
>         during CQ WW
>         SSB and a few days later in CW. KX3 was reliable and worked
>         pretty well. But
>         I had a feeling that discerning call signs in pileups was a
>         bit worse in
>         KX3. Also, a better speech processor makes K3 about 1-2 S
>         stronger than KX3
>         at same peak power so no one can win a SSB contest with KX3.
>         But KX3 is
>         about 20% weight and 10% of volume.
>
>         Ignacy, NO9E
>
>
>


-- 
Scott  K9MA

k9ma at sdellington.us