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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
