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 <[email protected]
<mailto:[email protected]>> 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
[email protected]
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