The "DUAL PASSBAND" makes a pass at providing an APF type of function where you make the DSP passband as narrow as you choose, but there's a wider passband that's 20 dB down. Undoubtedly it isn't the same shape as APF amplitude v.s. frequency, but conceptually "a signal peak floating on a sea of weaker undesired signals" is similar.
G3XJP implementated a DUAL PASSBAND function (that terminology hadn't been invented then) in his STAR transceiver, but there the amplitude of the wider passband can be dialed to be -20, -30, -40, .... -60 dB. It probably isn't too big a deal to make the "skirt attenuation" number a user-adjustable variable but I enjoyed being able to dial it up and down as QRM changed....which would ite up another programmable functio button. For those who suggested kicking the gain compensation up when narrower roofing filters are switched in, that's exactly what I did. I'm well aware that IF the roofing filter is setting the IMD capability of the receiver adding pre-filter gain reduces the intercept/increases IMD WHEN that IMD happens to occur, and IF it happens to fall into the passband with the desired signal. I intentionally emphasized the string of IF..WHEN..IF.. because depending upon you penchant for living in pileups, or turning the rig off on big contest weeks, a 4 dB IMD increase IF all of those things happened to occur may, or may not, be that big a deal. A post-roofing filter gain increment would not affect intercept or IMD and could be a smoothly varying increase that could conceivably be matched to a users memory of APF or Heath QF-1 function. Bill ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
