One of my pet peeves is people who refuse to admit they are wrong even when presented with irrefutable evidence. Well, I've recently seen some irrefutable evidence so it's time to admit I was wrong. :=)
I had claimed that US AM broadcast stations must not have an audio bandwidth greater than 5-6 kHz or so because FCC regulations <http://www.fcc.gov/mb/audio/bickel/amfmrule.html#AM> require that the emissions be attenuated a minimum of 25 dB at +/-10.2 kHz from the carrier. It would be difficult to design a reasonable analog filter that is -25 dB at 10.2 kHz and has a flat passband much more than 5 or 6 kHz. I've recently spent some time with a spectrum analyzer looking at on-the-air signals on the AM band. For example, my local station (KSRO 1350 kHz) clearly has modulation sidebands out to almost 10 kHz from the carrier. It looks to me like the passband starts to roll off about 9.7 kHz. They must be using a "brick-wall" digital low-pass filter to obtain the 25 dB attenuation at 10.2 kHz while maintaining the passband as wide as possible. Such filters were not available until relatively recently. But apparently many AM stations are using them now, although I'm not sure why since the additional bandwidth is only important for music and most AM stations are talk-only format these days. (The one exception being the Spanish-language stations, for some reason.) So if your goal is to listen to music, an IF filter with a bandwidth of up to 19-20 kHz might be advantageous. The K3's "FM" filter (13 kHz bandwidth) would work well if you detune plus or minus 5 kHz to tune in only one sideband. For DXing or for non-music formats, a narrower filter would be better to improve the signal-to-noise level. Al N1AL _______________________________________________ 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
