Topband: Minimum discernible signal ?

[Tony AB2CJ]

Greetings all

There is much talk about the ICOM 756 Pro's low noise floor and weak signal
capabilities when compared to analog or AF DSP receivers. The shape factor
of the DSP filters are on the order of 1.2:1 and can be narrowed down to
50hz ! The multiple AGC loops are said to do a good job and blocking dynamic
range is in the 125~130db class. Third order IMD dynamic range is around 90
~95db.  With that said, those who test this rig say that there is more to
the story than what these numbers reflect. In terms of weak signal
performance, the IF DSP in combination with an extreamly low noise floor is
said to be a cut above analog or crystal filtered rigs.

Question - Forgeting about desense from QRM for a momet, if two rigs are
said to have identical minimum discernable signal figures by ARRL Lab
standards, how is it possible that a receiver with DSP IF's is said to hear
better near the noise floor than conventional crystal filtered receivers? I
always thought that if two rigs show a MDS of -140dbm for instance, that
they would both hear the same? Or is there more to IF DSP then the MDS
figure represents?

Question 2 - Obviously atmospheric and man made noise limits weak signal
performance on HF but, what difference can one expect on a quiet near noise
free band ( 15 meters in mid winter ) from a receiver that's said to
ave  -135dbm and one with -141dbm? Under these conditions, and in terms of
weak signal performance, is 6dbm a substantial gain? Again, forgetting about
desense from QRM for a moment.

I enjoy working CW-QRP and digital mode QRP ( PSK31) and if these claims
about the IC756 Pro are true, then it should be a real winner for weak
signal work. Since I'm not a contester, I'm not worried about the less than
perfect blocking dynamic range of the Pro. With that said, the ARRL Lab
stated that they were able to pull out individual CW siganls in the middle
of pile-ups using the 50hz filter setting. They also said that they were
able to pull QSO's 200 and 300hz apart during the CQ World Wide 160 Meter CW
Contest. How bad can the dynamic range be?

Thanks, es 73
Tony
AB2CJ


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