Posted by
Dave Van Wallaghen on
Feb 20, 2008; 10:20pm
URL: http://elecraft.85.s1.nabble.com/Roofing-Filters-tp461501p461505.html
First, thank you Ron for putting that in a very comprehensive manner that
even I could understand ;-) As usual, you've turned on the "light bulb".
Now, when I purchased my K3 and debated on my purchase of the roofing
filters, I spoke with a few hams who said that the roofing filters were
primarily for blocking out close in and powerful signals and that if I
didn't have that problem where I live, that the standard DSP filter with the
standard 2.7khz filter would probably suffice. And that is what I did.
But I wonder (always dangerous), if operating on some of the noisier bands,
would there be a benefit to using a narrower filter to cut down on the
amount of energy hitting the DSP in order to help with digging out weak
signals? Or is that not a consideration with this particular implementation
of DSP in the K3?
As is, it has been fun playing around with digging out weak signals and
enhancing them. I need to get a better antenna to rig switching setup so I
can make a/b comparisons with my K2.
Thanks in advance,
Dave W8FGU
>
> Can someone explain the meaning of "roofing filters". I have searched
> everywhere and can't find a good explaination. Lisa recommended the FAQ
> secftion of Elecraft and there they just mention them. Maybe I should
> order
> an updated Ham bible from ARRL.
>
> --------------------------------------------
>
> The K3 receiver, like many current superhetrodyne receivers, has more than
> one high-performance filter in the system.
>
> I don't know how long you've been tinkering with superhets, but a few
> decades we ago we just called the "roofing filter" the first I.F. filter.
>
> You probably know that a superheterodyne receiver takes the selected
> incoming signal, no matter where it is in the tuning range of the
> receiver,
> and converts it to one fixed frequency before it is demodulated. That
> fixed
> frequency is called the intermediate frequency (I.F.).
>
> >From a design standpoint, it's a good idea to put all the selectivity as
> early in the receiver chain as possible so the first I.F. filter has been
> the most important in the system. Indeed, we normally tried to do all the
> significant filtering there. That's who the K2 is designed.
>
> Nowadays higher performance amplifiers and mixers allow more signal
> processing before it's necessary to use a filter to strip off all but the
> exact signal we want to hear.
>
> We use the first I.F. filter to reject signals that are completely outside
> the range of interest, then use a second filter further along in the
> signal
> path to finish the filtering to set the final bandwidth to just what we
> want.
>
> The first I.F. filter, therefore, defines how far off each side of the
> center frequency we can "hear". That is, it sets the limit or "roof" on
> the
> bandpass. (I think of it as the upper and lower "sides" to the bandpass,
> but
> no one checked with me before choosing the name "roofing filter".)
>
> The K3's second filter is provided by digital signal processing (DSP) and
> allows controlling the bandpass and center frequency to set it anywhere
> within the range of the first I.F. (roofing) filter bandpass.
>
> Even though mixers and amplifiers are much better today than a few years
> ago, it's still a good idea to reject all unwanted signals as early in the
> signal path as possible. That's why the K3 offers up to five "roofing
> filters" of various bandwidths to fit the sort of signal you're receiving
> from very wide FM signals down to a very narrow CW/PSK signal. Within the
> bandpass of the roofing filter, the DSP filter provides additional
> flexibility to set the exact upper and lower frequency limits, provide a
> notch filter to remove a specific heterodyne, etc.
>
> Ron AC7AC
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