Monolithic crystal filter

- summary, tutorial or overview of the basics of the monolithic crystal filter describing its operation and giving its equivalent circuit.

Quartz crystal filters are widely used in many areas, and in particular in high performance radio receivers. They are able to offer unparalleled levels of performance at a cost that represents excellent value for the performance.

A development of the basic idea of a crystal filter is the monolithic crystal filter. These monolithic crystal filters are able to offer even higher levels of performance in some respects while costs are reduced slightly.

What is a monolithic crystal filter?

Traditionally a crystal filter is made from a number of discrete crystals with the circuit often based around the half lattice network. However these designs require the use of a number of individual crystals - often six or eight are used to give the required performance.

Rather than having a number of discrete crystals a monolithic filter uses a single crystal element and two sets of elements are placed on this. Like most crystals used for radio frequency applications an AT cut is used - the cut of the crystal is defined by the angle at which the blank is cut from the original quartz crystal and the angle of cut defines many of the properties of the crystal blank and hence the overall filter.

The two sets of electrodes are placed onto the crystal. The electrodes are coupled to each other by the mechanical resonances in the crystal to give a highly selective filter.

The filter crystal is grown and cut in the same way as that used for normal crystals. Although quartz occurs naturally, and was used at one time for crystal manufacture, most of the material used today is manufactured synthetically. This has the advantage that the crystals are formed under much tighter conditions quality is more uniform. Additionally some of the flaws existing in natural quartz are not present making the overall quality much higher.

Once the raw crystal has been formed it is cut into blanks which are lapped and polished to a very high degree. The final stages of manufacture usually involve chemical etching as this gives a much finer finish. As a result the effects of ageing are greatly reduced. The size is very important because this determines the final resonant frequency.,/p>

Once the blank has been completed the electrodes are deposited onto the quartz. These are normally aluminium, silver or gold, and they have to be deposited under very controlled conditions so that they cover the required area and have the correct thickness. The thickness of the electrodes can be used to trim the filter to its exact resonant frequency. Making the electrodes slightly thicker reduces the frequency allowing the final performance to meet very stringent requirements.

Monolithic crystal filter operation

Like the standard quartz crystal, the monolithic crystal filter relies upon the piezo-electric effect that is exhibited in quartz for its operation. When signals appear across one pair of electrodes they set up mechanical vibrations on the crystal. These are affected by the mechanical resonances of the crystal element, and only those signals within the pass-band of the filter are allowed across the crystal to be picked up by the second pair of electrodes.

The operation of the filter can be explained in terms of an equivalent circuit. From this it can be sent hat there are two series resonant circuits consisting of L1 / C1 and L2 / C2. The actual values of these equivalent components are determined by the mechanical dimensions and properties of the quartz element.

There are other constituents to the circuit. L3 represents the internal coupling between the two resonant circuits whereas Co and Cp are the parasitic capacitances in the circuit. Co is the capacitance between the top and bottom plates at either end of the quartz element. For the sake of this explanation it is assumed to be the same at either end. Cp is the parasitic leakage capacitance across the resonant element. This needs to be kept as small as possible to prevent signal leakage across the filter which would impair its stop band performance.,/p>

These filters are normally designed for operation below about 30 MHz as it is easier and cheaper to obtain a good level of performance at these frequencies. Additionally there is a lower reject rate because operation at higher frequencies results in the manufacture of crystals that are very thin and fragile. Where operation at high frequencies is required, they can be run in an overtone mode, and this results in them being larger, and hence more robust, than if they were designed for fundamental frequency operation.,/p>

Summary

Although often cheaper than a crystal filter made from discrete components, a monolithic crystal filter can still be expensive. Nevertheless monolithic crystal filters are able to provide a high level of performance Also with quartz crystal technology improving, their cost is likely to fall somewhat over the years and the performance improve steadily.