[Elecraft] roofing filter questions
Dick Green WC1M
wc1m at msn.com
Tue May 13 13:11:02 EDT 2008
Hi,
I'm looking for articles on how the new roofing filter designs work. I've
seen the PowerPoint on Rob Sherwood's site, but it doesn't answer all my
questions.
Questions came up in a discussion with a friend of mine who is considering
the INRAD roofing filter mod for his Kenwood TS-950SDX. He operates almost
100% CW, and doesn't hear any IMD products. His beef is noise from stations
that are just outside the passband of his cascaded IF crystal filters. He's
hoping the roofing filter will increase the selectivity of the radio.
My sense is that it will help, but not by providing additional selectivity.
My understanding is that it will improve the dynamic range within the
passband by keeping the hardware AGC in the downstream IF stages from being
triggered by loud adjacent signals. Is this the correct way to think about
it?
My friend maintains that the roofing filter would provide selectivity as
well, but my sense is that unless it's narrower and/or has steeper skirts
than the IF filters, it won't. I believe the Kenwood roofing filter mod is
only on the order of 2K-3K wide, so the selectivity shouldn't improve.
Even if the roofing filter was the same or narrower than his IF filters,
would it improve selectivity? Is there an analogy with cascaded IF crystals?
Typically, a filter in the final IF stage (e.g., in the 450KHz range)
outperforms a filter in an earlier stage (e.g in the 8 MHz range.) Is this
because it's easier to make better filters at lower frequencies, or because
it's more effective to filter after any amplification by previous IF stages?
Obviously, I don't know squat about this.
73, Dick WC1M
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