[Elecraft] RE: Difference between "real" filters (FL1 thru FL4) and DSPfilters (SF1 thru SF4)

[Fred Jensen]

While a "still wet behind the ears" student several hundred years ago, a 
professor told me that "The best filters for any receiver are at the 
antenna terminals, before any RF enters the receiver.  Once you let it 
in, it's hard to control.  Unfortunately, filters are a bit hard to 
implement that way, hence the superhetrodyne."

Fred K6DGW
Auburn CA CM98lw

Steven Pituch wrote:
> Hi John,
> The DSP is audio.  Its got a sharper cutoff but can be affected by the AGC
> which is also audio.  The crystals filters FL1 -FL4 are IF and closer to the
> front end and cut interference before the AGC.
> 
> Yes, in non critical situations, the DSP filters are sharper, but if a loud
> adjacent signal gets through the IF because a narrow IF filter is not being
> used it will activate the AGC and desense the signal you are listening to.
> 
> The best thing to do is narrow it down first with the IF filters, and then
> use the DSP narrow bandwidths if the IF filtering is not enough.
> 
> I have some samples of the bandwidths here.
> http://users.ev1.net/~spituch/Steve's%20Page/Radio/RTTY/RTTY.html
> By the way, using MixW to align the filters was very easy.
> 
> Regards,
> Steve, W2mY/5