[Elecraft] roofing filter questions

[aj9c at indy.rr.com]

Tell him he should buy a K-3.


---- Dick Green WC1M <wc1m at msn.com> wrote: 
> 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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