> I've only spent a small amount of time in comparative listening sessions
> with my unmodified 75A4, 1000MP and a few other rigs in the shack hooked
> up to a Beverage. The listening that I did do indicated that weak cw
> signals on a noisy 160M often could be read on a 75A4 when the same signal
> could be "heard" but not "read" on the other receivers. I won't even
> begin to hypothesize why this may be so - it's just an observation. But
> it made me believe that in specific terms of weak signal reception on
> Topband in the presence of noise, current rigs on the market are missing
> something, as might be current testing methodologies.
The factor you are looking for is probably ringing and group delay
errors in filters, which can mess up some receivers when "rough"
noise is present with weak signals.
Since the tests indicated poor filter rolloff (where they were labelled
"tone") the 75A4 may have better than normal "empty band" weak
signal performance. Many mechanical filters are not as sharp-
skirted as crystal filters but have better group delay characteristics,
which would logically make them better on "empty bands".
>From the numbers posted the 75A4 is not bad at all for MDS. It
would not be my choice (nor would an R4C unless it had a 600Hz
Sherwood first IF filter and increased 1st mixer injection) on a
crowded band. That assumes the test was done correctly and the
receiver was in good shape.
When I get some time, I'll dump some rough noise into the MDS
test and use that as a noise test in the receivers on my web page.
The numbers posted for the 75A4 indicate it would not be hot
during contests, or on crowded bands. They do not reflect the
empty-band-with-rough-noise performance.
73, Tom W8JI
W8JI@contesting.com
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