Doug, KR2Q, presented a superb explanation of why one needn't sweat too much over the "roofing" (first I.F.) filter in the K3.
I'm enough of an O.T. to remember the famous 1950's articles in QST "What's Wrong With Our Present Receivers?" by Byron Goodman, W1DX, who pointed out the astonishing idea that double and triple conversion receivers were not necessarily better than a simple, single-conversion designs. Virtually all "high end" receivers of the day used double conversion formats (like the K3). Some even used triple conversion designs. In spite of what 'everyone knew', those behemoths of the airwaves weren't as good as a simple single-conversion design, thanks to the limitations of the circuits and active devices available in those days. Those very expensive, top-end receivers offered astonishing features and flexibility, but at a huge cost in raw ability to hear signals. Today engineers have vastly better devices to use in designing receivers, and they allow high-performance multiple conversion designs with many features and flexibility, if the receivers are designed carefully with attention to having as few "birdies" and other spurs as possible and with very careful management of the gain distribution throughout the design. However, multiple conversion receivers are far more complicated than a single-conversion design. The more complicated something becomes, the fewer people really understand the trade-offs involved. It gets especially difficult to give up some basic "truths" we've lived with for many years that have stood the test of time. One of those is that all the selectivity in a receiver is best taken as close to the antenna as possible, so the first i.f. (so-called 'roofing') filter should set the overall selectivity. In principle, that is true: we should strive to take all the selectivity possible as close to the antenna as possible. But, in practice, it's not really needed in a well-designed receiver using modern devices under almost any real-world operating conditions. Taking the selectivity in stages, such as with a roofing filter followed by a second i.f. variable filter (such as the DSP filter in the K3), offers the chance to have features and flexibility we can't get otherwise. Like all designs, it's compromise. There is *no* perfect design, no perfect receiver. Engineering is *always* about compromises. 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
