By Jacob Makhinson, N6NWP
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.
A crystal filter, being the heart of a superheterodyne receiver, has a profound effect on its selectivity. A low-quality crystal filter in even a high-priced commercial transceiver can degrade its selectivity and 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.
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Commercially available crystal filters are usually expensive and often discourage construction-minded amateurs from pursuing projects that include crystal 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. 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. 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. The resulting filters achieve top-quality performance at a fraction of the cost of commercially available crystal filters.
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. A two- or four-crystal filter of this type can provide a symmetrical response with reasonably steep skirts. But the bandwidth of such filters is a function of the frequency separation of the crystals. 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. 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. 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.
Another form of filter—which is the subject of this article—is the ladder filter shown in Fig 2. It typically has an asymmetrical response and is sometimes called the "lower-sideband ladder" configuration. But as we'll see, with a sufficient number of poles this asymmetry is significantly reduced. Ladder filters offer several advantages to the amateur experimenter:
• there is no need to pick crystals for proper frequency separation and no need for matched crystal pairs;
• the inherently simpler filter topology results in simple construction methods;
• no adjustable components are required after alignment is completed;
• the absence of coils allows a compact assembly and reduces the possibility of dynamic range degradation;
• the simple topology is conducive to a high number of poles, which allows very steep skirts; and
B) Half-Lattice Crystal Filter
B) Half-Lattice Crystal Filter
C) Cascaded Half-Lattice Crystal Filter
• 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.
This work was inspired by an article by Bill Carver, K60LG/7.1 Carver's work is quite remarkable; first, it proves that it is possible to build high-quality CW and SSB crystal filters with a predetermined frequency response "without black magic," and second (but of no less importance), it proves that the performance of filters built in a home lab using home-built equipment successfully rivals that of filters built using sophisticated professional equipment.
This article builds on Carver's work, refines the crystal filter design criteria and methodology, walks the reader through a complete design example, provides the results of measurements on several crystal ladder filters and analyzes the results.
The scope of this study has been limited to SSB filters, although most of the methods and conclusions are also applicable to CW filters.
The computer-design stage is based on a collection of computer programs designed by Wes Hayward, W7ZOI. The ARRL has just republished Wes Hayward's textbook Introduction to Radio Frequency Design, now including the software as part of the package.2 The computer programs (which I will refer to as IRFD) run on an IBM PC or compatible computer. The computer requirements are minimal, since IRFD fits on a single floppy disk and the computer's speed is of no concern. A VGA card is required for graphic display, however.
The Design Procedure
Design and construction of these ladder crystal filters are performed using these steps:
• selection of the filter center frequency;
• measurement of crystal parameters;
1 Notes appear on page 17.
C) Cascaded Half-Lattice Crystal Filter
Fig 1—Lattice crystal filter circuits.
Fig 2—Circuit of a ladder crystal filter.
• selection of the shape of the response;
• computer design of the filter; and
• construction and alignment.
If the required filter frequency is not already defined, you can select an IF to suit your needs. In doing so, consider that certain frequencies may result in in-band intermodulation products. Tables and charts have been developed to helj) designers avoid these frequencies. Practical considerations also impose some limitations on IF selection.
The crystals used in color-burst generators at 3.579 and 4.433 MHz are the most inexpensive crystals around and are widely available as surplus components. Unfortunately, the required termination resistances of filters built with such crystals may exceed 10 k£2, which necessitates an impedance transformation with a very high ratio (for a 50-Q system). As a result, very high voltage levels may be developed at the filter input, which may cause an overload condition. In addition, the required values of the coupling capacitors may be under 5 pF, making construction difficult due to stray capacitances. For these reasons, crystal filters with center frequencies under 6 MHz are not recommended.
The useful upper frequency limit is determined by the influence of stray capacitances at frequencies above 10 MHz and by the limitations imposed on the VFO circuit for multiband HF operation. Consequently, the recommended frequency range for an HF SSB crystal filter is between 6 and 12 MHz. The remaining criteria for the crystal frequency selection are the crystal Q and the price. Microprocessor crystals in HC18/U or HC49/U cases are reasonably inexpensive, but, being manufactured in large quantities, they are optimized for parameters other than Q.
Q is typically not specified by the manufacturer, and it varies significantly from batch to batch and from device to device within a batch. Therefore, the only way to find the Q of a specific type of crystal is to obtain several samples and to measure the parameters. This should be done before buying a large batch of crystals.
I originally intended to build crystal filters at 9 MHz, which is a popular IF within the amateur community, but it turned out that all the 9-MHz crystals I obtained (from different vendors)
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