a computer program is available that eliminates the need for empirical approaches or cut-and-try methods and allows the designer to shape the filter response with great accuracy.the simple topology is conducive to a high number of poles, which allows very steep skirts and A) Lattice Crystal Filter.the absence of coils allows a compact assembly and reduces the possibility of dynamic range degradation.no adjustable components are required after alignment is completed.the inherently simpler filter topology results in simple construction methods.there is no need to pick crystals for proper frequency separation and no need for matched crystal pairs.Ladder filters offer several advantages to the amateur experimenter: But as we'll see, with a sufficient number of poles this asymmetry is significantly reduced. It typically has an asymmetrical response and is sometimes called the "lower-sideband ladder" configuration. In addition, the coils used for lattice filter alignment often use small cores, which can result in the degradation of dynamic range because of core saturation at high signal levels.Īnother form of filter-which is the subject of this article-is the ladder filter shown in Fig 2. While it is reasonably easy to obtain matched crystal pairs for CW filters, it becomes considerably more problematic to obtain pairs of crystals separated by a couple of thousand hertz for use in SSB filters. If a steeper response is desired, designing a half-lattice filter with more than four crystals becomes more complex, requiring matched pairs of crystals and several adjustments. But the bandwidth of such filters is a function of the frequency separation of the crystals. A two- or four-crystal filter of this type can provide a symmetrical response with reasonably steep skirts. Most of the crystal filters described in amateur projects and those being sold commercially are lattice, half-lattice or cascaded half- lattice filters like those shown in Fig 1. The resulting filters achieve top-quality performance at a fraction of the cost of commercially available crystal filters. I will describe a simple, practical step-by-step procedure to design, construct and align crystal filters using equipment available to most construction-minded amateurs. So, the goal of this article is to provide design and building methods that can be used to construct crystal filters that rival or exceed the quality of commercially available filters. In addition, studies conducted in recent years conclude that in a high-performance receiver, a crystal filter may become the "bottleneck" restricting the receiver's dynamic range. On the other hand, a good crystal filter can significantly enhance receiver performance, whether in a simple "weekend" project or in a competition-grade station.ġ100 N Sunset Canyon Dr Burbank, CA 91504Ĭommercially available crystal filters are usually expensive and often discourage construction-minded amateurs from pursuing projects that include crystal filters. A low-quality crystal filter in even a high-priced commercial transceiver can degrade its selectivity and dynamic range. Despite the several excellent articles about crystal filters that have been published in amateur magazines over the years, building high-quality crystal filters is still seen by many amateurs as either black magic or as a complicated procedure beyond the reach of the average builder.Ī crystal filter, being the heart of a superheterodyne receiver, has a profound effect on its selectivity.
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