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.
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