It is my understanding that the problem is one of a
fixed gain amp, and variable insertion loss for the
filters. When the gain and loss are summed it gives
you variable gain between filter choices. But in
general there is a several dB increase in total stage
gain when you switch in the cascaded filters. More
stage gain=reduced dynamic range.
It seems the worse case is the 500hz filter which has
the smallest insertion loss. The reduction in IMD was
less of a problem for the 250 filter which has a
higher insertion loss. I notice a very slight
decrease in stage gain when I turn on my 250hz filter,
and I'm not sure I detect any improvement in S/N as I
would expect. I run my Orion almost all the time with
the 250 off. I've never really found a case where it
was of any significant benefit.
73 W9OY
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