Design Considerations
Transfer Function
A filter's transfer function determines both its frequency and time domain characteristics. Transfer function selection is based on trade-offs between frequency domain and time domain performance characteristics required for the particular application. In general, filters with good time domain response will have poorer roll off characteristics in the frequency domain and vice versa.
The REMEC D&S staff will be pleased to discuss specific requirements.
Refer to the table below to select a filter transfer function that best suits your requirements:
Filter Temperature Drift
REMEC D&S filters are compensated for known temperature-related variations in performance. Exposure to cold temperature results in an increase in frequency response and decrease in insertion loss response. Exposure to hot temperature results in a decrease in frequency response and increase in insertion loss response. Typical insertion loss, or Q drift, is 1677 ppm degrees C.
To reduce frequency drift, filter designs can use temperature-compensating materials that maintain frequency drift as low as 2 ppm degrees C.
Filter Q vs. Component Q
Filter Q (Qf) is a measure of relative bandwidth and equals the center frequency divided by the 3 dB bandwidth of the filter. Thus narrow filters may be referred to as "high Q" filters and wideband filters may be referred to as "low Q." "High Q" or narrowband filters have higher insertion loss and sharper rejection roll-off than "low Q" or wideband filters implemented using the same structure. This is a parameter of the filter itself and not the structure that is used to implement it.
Component Q or resonator Q is the measure of resistance loading or lossiness of the resonator. Component Q will vary with the type of filter structure and resonator characteristics of that structure. Component Q is proportional to the size of a resonator. For a lumped element filter this will be the outside diameter of an inductor. For a cavity filter this will be the volume of the resonator cavity. The references to Q here refer to component Q. The accompanying graph shows the relationship between component Q and volume for the various filter technologies.
