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There are many different types of SAW filters, all consisting
of a metal film etched to a specificed geometry using a photolithography
process similar to that used for semiconductor processing. The
variety illustrates the versatility, which follows from the fact
that almost arbitrary shapes can be made on the surface. Another
factor is that a compact device, with length say 1 cm., can have
many SAW wavelengths inside it and hence many degrees of freedom.
As for semiconductors, the fabrication is done on a larger ‘wafer’
so that many devices are made simultaneously, giving economies
of scale.
The most common group are bandpass filters, which are in very
widespread use in radio systems (including mobile phone handsets
and base stations) and in domestic TV. There are many types with
differing advantages, such as low shape factor, low insertion
loss, small size, or high-frequency operation.
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The simplest type of SAW filter, illustrated
in Fig.1, consists of two interdigital transducers (IDTs)
on a piezoelectric substrate. The latter is a plate of crystalline
material such as quartz. The term ‘piezoelectric’
means that the material has a basic mechanism which couples
electrical and mechanical fields. |
Consequently, an acoustic wave such as a SAW will in general
have an associated electric field in such a material. The IDTs
have electrodes alternately connected to two bus-bars, so that
a voltage applied to the left IDT in Fig.1 causes electric fields
in the gaps between the electrodes. The piezoelectric effect couples
these fields to mechanical stresses which act as sources of SAWs,
and the SAWs travel out of the transducer. At the output transducer
on the right, the field associated with the incident wave induces
voltages on the electrodes, and hence a corresponding voltage
appears on the bus-bars connected to the output.
The device in Fig.1 is a basic delay line, because the wave takes
some time to travel between the transducers - typically 1 µs
for 3 mm of path length. This is very compact compared with EM
waves which, in free space, need 300 m of path for 1 µs
delay. This device can be regarded as a basic bandpass filter.
The reason is that the individual sources (electrode gaps) in
the input IDT generate waves with alternating signs, and they
add up in phase if the SAW wavelength 
equals the transducer pitch. This occurs at the ‘centre frequency’.
| If the frequency is changed the waves generated
by the sources are not quite in phase, and the total amplitude
decreases progressively as the frequency is changed. Hence
the device has a bandpass characteristic, with the strongest
response at the centre frequency. The bandwidth is approximately
1/T, where T is the transducer length in time units ( = physical
length ÷ SAW velocity). A typical SAW Bandpass Filter
characteristic is shown in Fig. 2. |
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The device will usually be hermetically packaged to protect the
sensitive surface
from contamination. Often, one or two reactive components must
be added at each end, outside the package. An inductor may be
needed because the IDTs have capacitance which may need to be
tuned out. Also, L-C circuits are often used to transform the
source or load impedance (usually 50  )
to an impedance more suitable for the device.
The maximum frequency possible is detemined by electrode width.
At the centre frequency the electrodes have spacing  /2,
and width typically /4.
In production, the smallest linewidths obtainable are about 0.3
micron, and for a typical SAW velocity of 3500 m/s this gives
a maximum centre frequency of about 3 GHz. A particular advantage
is the slow acoustic velocity, much slower than EM waves. This
enables long delays to be obtained in a small space.
The performance is constrained by the properties of the substrate
material. For SAW devices, the substrate is usually one of a number
of ‘standard’ materials already known to have suitable
SAW properties. One property of interest is the piezoelectric
coupling constant  ,
which determines the strength of coupling between electrical and
mechanical fields. Generally, larger
enables lower insertion loss to be obtained, or wider bandwidth
for the same loss. Another important property is the temperature
coefficient of delay (TCD), which specifies how the delay varies
with temperature (this involves velocity and dimensional changes).
This also gives the temperature coefficient for the centre frequency
of a filter. Some data for common materials is given in Table
1. Data for maximum bandwidth is only representative.
Quartz has low piezoelectric coupling, but particular substrate
orientations give good temperature stability. The TCD is zero
at a particular temperature, around 20°C. The fractional delay
change is  ,
where 
is the deviation from the 'turn-over temperature'. Lithium niobate
is the opposite, exhibiting strong coupling but rather bad temperature
stability. Lithium tantalate is intermediate in both respects.
The 42° rotated lithium tantalate is a special case, giving
a 'leaky surface wave', a special type of SAW which penetrates
deeper into the substrate. This tolerates higher power densities
and gives strong coupling with reasonable temperature stability.
It is often used for RF filters needing low insertion loss.
Another basic device is the SAW resonator. This uses arrays of
metal strips, with pitch  ,
as reflectors of the waves. These arrays can give strong SAW reflections,
and two arrays can be used to form a SAW cavity with high Q, up
to  .
Such resonators are often used for high-stability oscillators.
The above devices are just some basic types; a wide variety of
types is possible because almost arbitrary shapes can be defined
on the surface with very high precision, using lithography techniques
similar to those for semiconductor processing. Many unique variations
are possible for SAW devices, and their applications range from
piezoelectric strain gauges to pulse compression radar, to cellular
handsets.
Specific types of SAW filters include:
The performance of the various types is summarized in Table 2.
The data is only indicative of the performance obtainable, and
for a specific requirement it is best to consult COM DEV directly.
If appropriate, a better assessment can be obtained by doing a
preliminary design and simulation. Devices using tantalate or
niobate subtrates can often be used without any matching or tuning
components if the bandwidth is less than 4%.
Many of these devices can be supplied in balanced form, so as
to accept a balanced drive and load. Also, it is often possible
to have one port balanced and the other unbalanced, so that the
SAW device also serves the function of a balun transformer.
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