The first part of selecting an RF filter is selecting what technology to use. The main types stocked by Golledge are SAW and Crystal. We also have a range of ceramic resonator and LTCC LC filters. All our SAW and Crystal filters are bandpass types. There is an overlap between Crystal filters and SAW filters between 35MHz and 110MHz. this being the top of the standard crystal filter range and the bottom of the SAW filter range, 70MHz being popular for both SAW filters and crystal filters. For this centre frequency the choice is made by required bandwidth. SAW’s have bandwidths of 50kHz and higher, while crystal filters have bandwidths up to 30kHz.
Below is the minimum set of requirements we would like from a customer to specify a Crystal Filter.
Centre Frequency
Custom centre frequencies are available, but the filter will be cheaper sticking to one of the standard centre frequencies. These are 10.7MHz, 21.4MHz, 45MHz, and 70MHz. 10.7MHz is not available in surface mount types.
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. For a 1dB ripple 4 pole filter, the 3dB bandwidth is theoretically 5% larger than the ripple bandwidth. The smaller the ripple the larger the difference between the two. For a 0.1dB ripple filter this difference is 21%. As the filter designer will aim for a smaller ripple than the maximum allowed in the specification, in practice the difference will be larger. 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. For example, if a crystal filter was specified with a 45MHz centre frequency and ±15kHz passband then that passband would extend from 44.985MHz to 45.015MHz and would be said to have a 30kHz bandwidth.
Attenuation Band
These are the frequency offsets from the centre frequency at which the Rejection* passes the required figure. Often specified in ± format. For example, a 45MHz Centre frequency crystal filter with a -35dB @ ±50kHz specification would have its -35dB rejection points on the skirts of the filter response within the range 44.950MHz and 45.05MHz.
*Rejection - This term refers to the difference in loss through the filter between the point of minimum loss in the passband and at the point in question.
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.
Number of Poles
The choice is 2, 3, 4, 6, or 8. The maximum is 4 in surface mount packages. 3 poles is only available surface mount; this model is unusual as the number of poles is usually an even number.
The number of poles dictates the steepness of the sides (skirts) of the filter response shape. The bigger the number the steeper the sides.
6 and 8 pole filters are only available in 10.7MHz or 21.4MHz types.
Package Type
For 6 and 8 pole filters there is no choice. The package is a through hole mounted metal can.
For 4 pole filters at 21.4MHz or above, there are several choices. The recommended choice is our GSF-75 range which is a single 7.2 x 5.2 mm surface mount package. Other options have the filter split across a pair of packages. These pairs must be kept together; the halves cannot be mixed. This makes automated assembly very difficult. The package options for these pairs include 7.2 x 5.2 SM packages, UM-1 / UM-5 packages with bent legs suitable for surface mount, or UM-1 / UM-5 packages with straight legs for through hole mounting.
For 4 pole filters at 10.7MHz the packages are pairs of HC-49 cans with either straight legs for through hole mounting or bent legs for surface mount.
The options for 2 pole types are similar to the 4 pole types but without the complications of splitting across a pair of packages.
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
It is best to allow the manufacturer to set the termination impedance. If an alternative source for an existing filter is being sought, then the termination impedance will already be set, so needs to be specified.
Coupling Capacitance
For 4 pole filters an external capacitor to ground is required at the midpoint of the filter. It is best to allow the manufacturer to set the coupling capacitance. If an alternative source for an existing filter is being sought, then the coupling capacitance will already be set, so needs to be specified.
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.
Guaranteed Attenuation
This specifies rejection at higher offsets than the Attenuation Band. Usually specified at 910kHz below the passband it gives an indication of the stopband performance of the filter.
Storage Temperature Range
This is as the name suggests.