Radio receiver filter options

- summary, tutorial or overview of the basics of radio receiver filter options including LC filter, crystal bandpass filter, mechanical filter, ceramic filter and roofing filter for use in radio communications receivers.

There is a wide variety of different types of RF filter used within superhet radio receivers to provide the main selectivity within the IF stages of the receiver. Some radio receivers will simply use RF filters in their IF stages made up from the tuned transformers (LC filters based on capacitors and inductors) linking the different intermediate frequency stages within the radios or used with an IC in the radio. Other radio receivers may incorporate highly selective crystal filters, whereas others may use mechanical filters (like those used by the Collins Radio Company some years ago) or ceramic filters. Each radio receiver will have its own requirements for its RF filter according to the form of radio communications application for which it will be used. The choice of RF filter will depend upon a variety of parameters including cost, performance frequency of operation and many other elements. Often the choice of RF filter will be a compromise, but with the technology available today, very high levels of performance can be achieved.

There is a variety of different types of RF filter that can be used. The main types that are used include the following:

• LC tuned circuit



• Crystal filter



• Monolithic crystal filter



• Ceramic filter



• Mechanical filter



• Roofing filter

Descriptions of each type of RF filter is given below in more detail

LC tuned circuits

The simplest type of RF filter is an ordinary L-C tuned circuit. In many older radio receivers using discrete semiconductors, or older radio receivers using vacuum tubes they take the form of transformers to couple the individual stages in an IF amplifier chain. Often there are two or three stages with tuned circuits. Using them it is usually possible to achieve sufficient selectivity for a medium wave AM or VHF FM broadcast radio. However for a good quality communications receiver used for professional radio communications systems, it is rarely possible to be able to achieve the required degree of selectivity using just L-C filters.

In more modern radios using integrated circuits a single tuned circuit could be used in conjunction with an integrated, as the concept of inter-stage coupling is not employed in the same manner. Typically a ceramic filter, rather than an LC circuit is more likely to be used.

If L-C filters were used in a radio using inter-stage transformers then it would be possible to increase the degree of selectivity by increasing the number of tuned circuits between each stage. This is not ideal for a number of reasons. In the first case it increases the difficulty of aligning the set. In addition to this each tuned circuit will introduce a certain amount of loss. Increasing the number of tuned circuits will increase the amount of gain required, sometimes necessitating a further stage of gain. A further disadvantage is that it is not easy to alter the degree of selectivity by switching in additional L-C filters. If this is to be achieved then it is often preferable to switch in a further type of RF filter such as a crystal filter.

Crystal Filters

Crystal filters provide the main selectivity in of most of today's high performance radio receivers used for professional radio communications applications. These crystal filters provided exceedingly high degrees of selectivity which are hard to equal in terms of performance and cost.

The crystals in the RF filters are made from a substance called quartz. This is basically a form of crystalline silicon. Originally natural deposits were used to manufacture the crystals required for the electronics industry. Now quartz crystals are grown synthetically under controlled conditions to produce very high quality material.

The crystals use the piezo-electric effect for their operation. This effect occurs in a number of substances and it converts a mechanical stress into a voltage and vice versa. Many electrical transducers use the effect converting electrical impulses or signals into mechanical vibrations and vice versa.

In quartz crystal resonators the piezo-electric effect is used in conjunction with the mechanical resonances which occur in the substance. The electrical signals passing into the crystal are converted into mechanical vibrations which interact with the resonances of the crystal. In this way the crystal uses the piezo-electric effect to enable the mechanical resonances to tune the electrical signals. These mechanical resonances have exceedingly high Q factors. Many crystals exhibit values of several thousand. This is many orders of magnitude higher than ordinary LC tuned RF filters where values of a hundred or so are considered high. Typically the Q of an LC tuned circuit may be reach values of a few hundred. For quartz crystals values of Q may exceed 100 000.

Further details about quartz, its properties and the ways in which crystals are manufactured and used can be found on the Electronic components section of this site - see side menu for the link.

The response of a single crystal is too narrow for many applications. Normally an RF filter is required to have a passband, possibly of a few hundred Hertz, or a few kilohertz, and outside this bandwidth, other signals should be totally rejected. While it is not possible to achieve the perfect filter very high degrees of selectivity can be achieved. By adding several crystals together it is possible to obtain the performance that is required. Often crystal filters are referred to as having a certain number of poles. This terminology comes from the filter analysis design process, but effectively there is one crystal in the filter for every pole.

A two pole filter (i.e. one with two crystals) is not normally adequate to meet many requirements. The shape factor which is the ratio between the bandwidth where the stopband attenuation starts and the bandwidth of the passband) can be greatly improved by adding further sections. Typically ultimate rejections of 70 dB and more are required in a receiver. As a rough guide a two pole filter will generally give a rejection of around 20 dB; a four pole filter, 50 dB; a six pole filter, 70 dB; and an eight pole one 90 dB.

Monolithic filters

With more items being integrated onto single chips these days it is hardly surprising to find that a similar approach is being adopted for crystal filters. Instead of having several separate or discrete crystals in an RF filter, even if they are all contained in the same can, it is possible to put a complete filter onto a single quartz crystal, hence the name monolithic crystal filter.

In essence the RF filter is made up by placing two sets of electrodes at opposite sides of a single AT cut crystal. The coupling between the two electrodes acts in such a way that a highly selective RF filter is produced.

Monolithic filters have only been available since the 1970s. Even now a large number of RF filter manufacturers do not produce them, preferring to use the more traditional filters made from individual crystals.

While it had been known for a long while that a two pole filter could be made up on a single crystal, the idea was not developed because the way in which it worked was not understood. After much work, scientists at Bell Laboratories in the USA discovered its mode of operation. Very simply it consists of two acoustically coupled resonators.

A monolithic crystal filter consists of a crystal blank onto which two sets of electrodes or plates are placed at opposite ends of the blank. Each set consists of an electrode on either side of the blank. When the electrical signal is placed across one pair of electrodes, the piezo-electric effect converts this into mechanical vibrations. These travel across the crystal to the other electrodes where they are converted back into an electrical signal again. However if the acoustic signal is to travel across the crystal then its frequency must match the resonance of the crystal.

Often these RF filters are manufactured for operation below about 30 MHz, because above these frequencies the manufacturing costs tend to rise. However manufacturing techniques are improving all the time it is possible to use them above this. If this is required then the normal way of accomplishing this is to use an overtone mode. This considerably increases the maximum possible frequencies, although the performance is not usually quite as good.

Monolithic filters are used in many areas now. They offer better performance than their discrete counterparts and they can be made smaller - a feature which is becoming increasingly important in today's miniaturised electronics industry. The main drawback of these filters is that they require very specialised equipment for their manufacture.

Ceramic filters

Quartz is not the only substance to exhibit the piezo-electric effect combined with a sharp resonance. A number of ceramics are also used successfully to perform this function. Although filters made from these ceramics are not nearly as selective as their higher quality quartz relatives, they are cheaper and offer great improvements over their L-C counterparts.

Ceramic filters are made from a specialised family of ceramics, and the elements for filters are normally in the form of a small disc. They operate in exactly the same way as crystal filters, the signal being linked to the mechanical resonances by the piezo-electric effect. Generally ceramic filters have a much wider bandwidth and a poorer shape factor than their crystal counterparts. As a result they are rarely used in high performance communications receivers as the primary form of filtering, although their performance has improved dramatically in recent years and some examples of ceramic filters offering exceedingly good levels of performance are available. As a result they find widespread use in broadcast receivers for AM and VHF FM reception and some wireless applications.

Mechanical filters

When high performance filters are needed there is another type which can be considered. Although not nearly as popular as crystal filters these days, mechanical filters found widespread use a number of years ago. The Collins Radio Company (now Rockwell Collins) was a famous manufacturer of these devices, introducing their first designs in 1952, these filters are still manufactured.

In essence their operation is very similar to that of a crystal, although the various functions are performed by individual components within the RF filter. At either end of the filter assembly there are transducers which convert the signals from their electrical form to mechanical vibrations, and back again at the other end. These vibrations are applied to a series of discs which are mechanically resonant at the required frequency. Each of these discs has a Q of which can be about 5000 or more, and they are arranged close to one another but not touching to form a long cylinder. A number of coupling rods are attached to run along the side of the assembly to transfer the vibrations from one section to the next. By altering the amount of coupling between the sections and the resonance of each disc, the response of the overall unit can be tailored to meet the exact requirements.

Operation of these mechanical filters is normally confined to frequencies between about 50 and 500 kHz. Below these frequencies the discs become too large, whilst at the top end of the range they are too small to manufacture and mount in the filters with any degree of reliability. Apart from the limited frequency range the other disadvantage is that the resonant frequency of these filters drifts with temperature. However one of their main advantages is that exceedingly narrow bandwidths can be achieved relatively easily, and the low levels of intermodulation distortion they introduce. Additionally the costs of these devices have been reduced over the years and the number of resonators that can be used can be between 2 and 12 dependent upon the requirements.

Roofing filters

In many radio receivers the main RF filter occurs only after there have been many stages of amplification. This means that a strong signal which is outside the pass-band of the main receiver filter can cause overloading especially in the early IF stages before the filter. This occurs because the AGC does not see the signal and reduce the gain of the earlier stages to take account of it, or the operator may not be aware of the signal and reduce the RF gain if a control is available.

To overcome this problem a wider bandwidth filter is placed early on in the IF stages to reduce the level of any strong off channel signals. The main filtering, however, is still provided late on in the receiver by the main full specification filter.

Roofing filters are often found in multi-conversion superheterodyne receivers where the main filter is found after two or possibly three conversion stages. The roofing filter can be placed soon after the first mixer to reduce the effects of any strong off-channel signals.

Summary

There is a good selection of RF filters that can be used in radio receivers. The actual type that is eventually decided upon a balance of performance, cost and other factors dependent upon the radio communications application for which the receiver will be used. For many radio communications applications where the highest levels of performance are not needed, ceramic filters provide the ideal solution being very cheap and easy to use while providing levels of performance that are quite adequate for many applications. For applications where only the highest levels of performance are required, crystal filters are the most common solution either as units made from discrete crystals or as monolithic filters. However mechanical filters could be considered for some applications. These days LC filters are not widely used because the cost of winding coils is high, and often ceramic filters are more convenient, cheaper, and offer a better level of performance.