Quartz Crystal Filter: bandpass filter

Quartz crystal filters are often used as high performance bandpass filters in radio receivers where their performance is necessary for communications receivers and the like.

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Quartz crystals can be used to form the basis of very high performance crystal bandpass filter.

These quartz crystal filters are used in applications such as high performance radio receivers and transmitters.

Quartz crystal filters are able to provide superior performance to most other types of filter and as such they find uses in a variety of areas.

Today, quartz crystal filters can be designed with pass bands ranging from frequencies in the kilohertz region up to many Megahertz - with the latest technology this can rise to 100 MHz and more. However for the best performance and lowest costs the passband of the filter is generally kept to below about 30 MHz.

Quartz crystal filter parameters

There are two main areas of interest for a filter, the pass band where it accepts signals and allows them through, and the stop band where it rejects them. In an ideal world a filter would have a response something like that shown below. Here it can be seen that there is an immediate transition between the pass band and the stop band. Also in the pass band the filter does not introduce any loss and in the stop band no signal is allowed through.

Ideal response for a bandpass filter
Ideal response for a bandpass filter

In reality it is not possible to realise a filter with these characteristics and a typical response more like that shown in Figure 3. It is fairly obvious from the diagram that there are a number of differences. The first is that there is some loss in the pass band. Secondly the response does not fall away infinitely fast. Thirdly the stop band attenuation is not infinite, even though it is very large. Finally it will be noticed that there is some in band ripple.

Response of a typical quartz crystal-bandpass filter showing passband, stopband, insertion loss, etc
Response of a typical quartz crystal-bandpass filter

In most filters the attenuation in the pass band is normally relatively small. For a typical crystal filter figures of 2 - 3 dB are fairly typical. However it is found that very narrow band filters like those used for Morse reception may be higher than this. Fortunately it is quite easy to counteract this loss simply by adding a little extra amplification in the intermediate frequency stages and this factor is not quoted as part of the receiver specification.

It can be seen that the filter response does not fall away infinitely fast, and it is necessary to define the points between which the pass band lies. For receivers the pass band is taken to be the bandwidth between the points where the response has fallen by 6 dB, i.e. where it is 6 dB down or -6 dB.

A stop band is also defined. For most receiver filters this is taken to start at the point where the response has fallen by 60 dB, although the specification for the filter should be checked this as some filters may not be as good. Sometimes a filter may have the stop band defined for a 50 dB attenuation rather than 60 dB.

Shape factor

It can be seen that it is very important for the filter to achieve its final level of rejection as quickly as possible once outside the pass band. In other words the response should fall as quickly as possible. To put a measure on this, a figure known as the shape factor is used. This is simply a ratio of the bandwidths of the pass band and the stop band. Thus a filter with a pass band of 3 kHz at -6dB and a figure of 6 kHz at -60 dB for the stop band would have a shape factor of 2:1. For this figure to have real meaning the two attenuation figures should also be quoted. As a result the full shape factor specification should be 2:1 at 6/60 dB.

Quartz crystal filter design parameters

When a quartz crystal filter is designed factors such as the input and output impedance as well as bandwidth, crystal Q and many other factors need to be taken into account.

Some of the chief factors are obviously the bandwidth, shape fact, and ultimate cutoff. Although it is very much a simplification, these factors are dependent upon the number of poles (equivalent to the number of crystals), their Q value, and their individual frequencies.

Further factors such as the maximum bandwidth that can be achieved is controlled by the filter impedance and also the spurious responses that are present in the individual quartz crystal elements. The location of the important responses for quartz crystal filters can be controlled by the size of the plates deposited onto the crystals. By making them smaller the responses also become less critical. The down side of this is that the impedance of the overall quartz crystal filter rises. This means that the quartz crystal filter will need impedance transformers at the input and the output. This obviously needs to be avoided if at all possible, but for wide band filters it is often the only option.

Quartz crystal filters are able to provide levels of performance that most other forms of bandpass filter are unable to achieve. However their use tends to be limited to professional and semi-professional applications. High performance radio receivers and transmitters in which the selectivity is of paramount importance are key uses for these filters.

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