David Gilbert <xda...@cis-broadband.com> wrote: > Actually, a roofing filter does exactly what it says. It protects against > ... i.e., does not allow to pass ... out-of-passband signals from affecting > the ADC or hardware AGC. It's a "roof" against unwanted energy and a window > for desired signals. So would you prefer to call it a "window?" Have at it, > but that doesn't alter the fact that its fundamental purpose is to protect > ... protect the ADC and protect the AGC so that they can perform their > function properly. And it doesn't mean that anybody will recognize what > you're talking about, which is probably more relevant than the semantics > involved.
In receive I.F. applications, a crystal filter is used as a very narrow (very high-Q) band-pass filter. It cuts off unwanted signals above and below the desired passband. In a modern superhet like the K3, the crystal filter augments the DSP in a subsequent I.F., providing improved ultimate rejection and steeper skirts. In some implementations you can slide the I.F. associated with the crystal filter to the left or right of the nominal receive passband, so that the filter acts, in effect, like a low-pass or high-pass filter, working against the center frequency of later IFs. This can be useful in protecting the downstream I.F. on one side, but not the other. Narrow filtering takes care of both ends. Radios that don't use hardware roofing filters of some kind typically have worse blocking dynamic range (BDR) than those that do. For example, the K3 has typical BDR of over 140 dB, 15 to 20 dB better than radios with no hardware filtering (i.e., pure SDRs). This is why a K3 outfitted with narrow filters works well for contesting, Field Day, and DXpeditions; it won't get desensed by either the wide- or narrow-spaced onslaught often experienced in these situations. 73, Wayne N6KR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
