Hi Toshi
The Orion 565 has an extra IF amplifier for the narrow 250 Hz Roofing filter,
which adds noise.
Testing reported in QST showed that the dynamic range was worse with the 250 Hz
filter than with the wider filters because of this extra amplifier.
A common solution used in the USA is to substitute a 600Hz roofing filter from
International Radio (INRAD) in place of the standard 1,000 Hz roofing filter
and use that instead of the 500 and 250 Hz roofing filters. Most of us have
found that that 600 Hz filter is adequate for even the most demanding contest
operation.
Terry N4TZ
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