Topband: Re: Minimum discernible signal?

[K3BU@aol.com]

Ditto  to W8JI comments.
Actually the old R4B with modifications looks even better than R4C when it 
comes to reception with strong QRN. Sharp first filter, plus low IF LC 
selectivity filtering allows to narrow the bandwidth without significant 
ringing and stretching the spikes. 

The 20 kHz or so spacing for intermod tests is useless. We need to know how 
close can 30 over 9 signal get to received frequency before it starts 
bothering the reception.

One potential problem looming with new radios using DSP for demodulation. 
OK2RZ compared TS870 and TS850 side by side and noticed difficulties with 
deciphering the pileups while using 870. No problems with 850, switching A/B 
and doing real life comparisons. I speculate that 870 using DSP for 
demodulation might have problem with "pileup signals" - not being able to 
pass them as they are, but trying to "clean" them up to the point when DSP 
QRMs itself. I got new version of 870 and 850 and will do some tests and 
comparisons. If DSP detection is the culprit, then there might another mod 
coming to "derate" 870 with product detector to be even better radio. 
Replacing stock filters in 870 with 2.1 INRAD filters makes a world of 
difference in selectivity and DSP function, but looks like there is still 
plenty room for improvement.
DSP helps, but it can't replace hard selectivity in the early RF/IF stages, 
too late to cleanup the intermod mess generated in the early stages, so don't 
bet on them wonderDSP radios.

Yuri, K3BU


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