This is the minimum set of requirements we would like from a customer to specify a SAW Filter:
Centre Frequency
If the application is for a receiver front end filter, then this is set by the application. Where the SAW filter is for an IF, then design freedom in choosing an IF should be used to find a part with appropriate bandwidth and rejection.
Passband
There are two types of bandwidth specification used, 3dB bandwidth and ripple bandwidth, often referred to as the passband. Except in the case of a 3dB ripple specification, these are not the same. It is therefore important to be clear which definition is being used. Bandwidth should be just wide enough to encompass signals of interest, bearing in mind the frequency tolerance of those signals, which adds to the required bandwidth. If the passband is referred in a ± format, then that refers to that deviation from the nominal centre frequency. The most common format for specifying a SAW passband is using absolute values for the lower and upper limits of the passband. When specified this way it is assumed this applies over the whole temperature range. The practical filter will have a larger bandwidth than this range to accommodate the change in centre frequency with temperature of the filter.
Attenuation
For SAWs this refers to the rejection at various frequencies away from the passband. These are expressed relative to the passband insertion loss. These requirements are derived from a RF system analysis.
Insertion Loss
This is the loss through the filter within the passband measured at the point of minimum loss.
Ripple
The difference in loss through the filter in the passband between the points of minimum and maximum loss. It is an important parameter for systems employing complex digital modulation types that will be degraded by high group delay variation causing high EVM.
Input Power Level.
Often quoted as 10dBm (10mW). Power levels above this require special design features to enable this that may compromise other performance parameters.
Package Type
There are many different packages for SAW filters. Lower frequency types require bigger packages because the size of the active area of the filter is proportional to acoustic wavelength in the passband. One of the most popular packages is the 3x3mm 6 pad one. Much smaller packages for the higher frequency parts are available where space is at a premium.
Operating Temperature Range
This is taken from the temperature range of the equipment the part is designed into, with a little extra at the hot end to allow for local heating on the PCB.
Optional Specifications
The above will give a supplier the basics to select a suitable Crystal Filter for an application where there is design flexibility.
Termination Impedance
For SAWs this is often 50Ω. Where this is not the case external matching components will be required.
Group Delay Variation
This is calculated from the phase shift v frequency response of the filter. It is an important parameter for systems employing complex digital modulation types that will be degraded by high group delay variation causing high EVM.
Storage Temperature Range
This is as the name suggests.
*Rejection
This term refers the difference in loss through the filter between the point of minimum loss in the passband ant the point in question.