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 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of AnnLatz Sent: Wednesday, February 20, 2008 11:38 AM To: [email protected] Subject: [Elecraft] Roofing Filters 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. Thanks, Alan KA9UCP _______________________________________________ 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 _______________________________________________ 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
