What Is A Crystal Lattice Filter?

Designing crystal filters far SSB is made easier using readily bachelor software

By Jacob Makhinson, N6NWP

Despite the several fantabulous articles almost crystal filters that have been published in amateur magazines over the years, building loftier-quality crystal filters is still seen by many amateurs every bit either blackness magic or as a complicated procedure beyond the achieve of the average builder.

A crystal filter, being the eye of a superheterodyne receiver, has a profound issue on its selectivity. A low-quality crystal filter in even a high-priced commercial transceiver tin can degrade its selectivity and dynamic range. On the other mitt, a good crystal filter can significantly enhance receiver operation, whether in a unproblematic "weekend" project or in a contest-grade station.

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Commercially available crystal filters are usually expensive and ofttimes discourage construction-minded amateurs from pursuing projects that include crystal filters. In addition, studies conducted in recent years conclude that in a high-functioning receiver, a crystal filter may get the "bottleneck" restricting the receiver's dynamic range. And 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 draw a simple, practical step-by-pace procedure to blueprint, construct and align crystal filters using equipment available to about construction-minded amateurs. The resulting filters achieve pinnacle-quality performance at a fraction of the cost of commercially available crystal filters.

Well-nigh 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 one. A ii- or 4-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 than 4 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, information technology becomes considerably more problematic to obtain pairs of crystals separated by a couple of thousand hertz for utilise in SSB filters. In improver, the coils used for lattice filter alignment frequently use small cores, which tin result in the degradation of dynamic range because of core saturation at high signal levels.

Another form of filter—which is the subject field of this article—is the ladder filter shown in Fig 2. It typically has an asymmetrical response and is sometimes chosen the "lower-sideband ladder" configuration. Merely as we'll see, with a sufficient number of poles this disproportion is significantly reduced. Ladder filters offer several advantages to the amateur experimenter:

• in that location is no need to pick crystals for proper frequency separation and no need for matched crystal pairs;

• the inherently simpler filter topology results in uncomplicated construction methods;

• no adjustable components are required after alignment is completed;

• the absenteeism of coils allows a compact associates and reduces the possibility of dynamic range deposition;

• the unproblematic topology is conducive to a high number of poles, which allows very steep skirts; and

: A) Lattice Crystal Filter

B) Half-Lattice Crystal Filter

Crystal Filter Ssb

C) Cascaded Half-Lattice Crystal Filter

• a computer program is available that eliminates the need for empirical approaches or cut-and-endeavour methods and allows the designer to shape the filter response with dandy accuracy.

This work was inspired by an article past Pecker Carver, K60LG/7.ane Carver's work is quite remarkable; first, it proves that it is possible to build loftier-quality CW and SSB crystal filters with a predetermined frequency response "without black magic," and 2d (but of no less importance), it proves that the performance of filters built in a dwelling lab using home-built equipment successfully rivals that of filters built using sophisticated professional equipment.

This article builds on Carver's piece of work, refines the crystal filter design criteria and methodology, walks the reader through a consummate design example, provides the results of measurements on several crystal ladder filters and analyzes the results.

The scope of this report 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 past Wes Hayward, W7ZOI. The ARRL has but republished Wes Hayward's textbook Introduction to Radio Frequency Design, now including the software as part of the packet.ii The computer programs (which I will refer to as IRFD) run on an IBM PC or compatible estimator. The figurer requirements are minimal, since IRFD fits on a single floppy disk and the computer'due south speed is of no concern. A VGA carte du jour is required for graphic display, however.

The Design Procedure

Pattern 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.

Frequency Selection

If the required filter frequency is non already defined, you can select an IF to arrange your needs. In doing so, consider that certain frequencies may outcome in in-band intermodulation products. Tables and charts have been developed to helj) designers avoid these frequencies. Practical considerations besides impose some limitations on IF pick.

The crystals used in color-burst generators at three.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 chiliad£2, which necessitates an impedance transformation with a very high ratio (for a fifty-Q system). Equally a result, very high voltage levels may be adult at the filter input, which may crusade an overload condition. In addition, the required values of the coupling capacitors may be nether 5 pF, making construction difficult due to stray capacitances. For these reasons, crystal filters with center frequencies under vi 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 cheap, but, being manufactured in big 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 inside a batch. Therefore, the just fashion to find the Q of a specific type of crystal is to obtain several samples and to measure out the parameters. This should be done earlier 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)

Continue reading here: Kvg Crystal Filters Xf-9b

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