Q Microwave´s design tools are provided to assist our customers in specifying a filter requirement. It is not intended to make you a filter designer. The below list identifies the key decisions made by our filter designers when developing a solution for your specific requirement. Obviously, the more detail you provide as to critical features or specifications, the more relevant the proposal provided by our staff. Additional pertinent technical information is offered in our
Reference Data page.
DEFINITION OF REQUIREMENT - Before proceeding, the following information must be collected with as much detail as possible. Noting specifications of little concern or areas with flexibility is equally important.
Critical Specifications:
Center frequency and passband
Rejection in terms of signal attenuation level or sections in the filter circuit.
Optional Specifications:
Insertion loss limit
Phase or time domain requirements
Size constraints
Other critical specifications
Price targets
FILTER TRANSFER FUNCTION - The filters shape factor is primarily determined by the transfer function, or mathematical function, used to realize the circuit. Chebychev transfer functions are typically used in applications requiring higher performance in the frequency domain (i.e., a sharp cutoff between passband and rejected frequency bands).
Transfer Function
Time Domain
Frequency Domain
Comments
Elliptic
Poor transient response.
Sharpest possible rejection response
Typically not feasible at higher frequencies (>1 GHz)
Chebychev
Degraded time delay performance & phase flatness.
Relatively steep rejection skirts with significant passband ripple
Optimal for most filters with significant rejection requirements.
Butterworth
Good time delay or phase response.
Degraded amplitude response
Bessel
Same as Butterworth
Poor amplitude response
Gaussian
Maximum phase delay flatness across the passband
Poor rejection - Very gradual or shallow rejection skirts.
Optimal for filters with critical phase delay flatness and minimal rejection requirements.
TOPOLOGY SELECTION - The filter's topology is typically selected based upon the frequency of operation, relative bandwidth, shape factor, and insertion loss, and rejection requirements:
Operating Frequency - The following plot provides the approximate frequency capability of various topologies available at Q Microwave. Customers should note that this relationship is an approximation using typical performance requirements and does vary based upon other specification issues. These capabilities also do vary from manufacturer to manufacturer.
Relative Bandwidth - The passband´s relative size as defined by the 3 dBc bandwidth divided by the center frequency. As noted in the below plot, this can restrict the available filter topologies.
Passband Shape Factor - The relative rolloff or shape of the filter's passband, as defined by the loss characteristics between the 1 dBc and 3 dBc bandwidths, is provided below for comparison of available topologies. Customers should note that this relationship is an approximation using typical performance requirements and does vary based upon other specification issues.
A more detailed estimate of rejection performance is available by following this link.
Insertion Loss Requirements - Passband loss will limit the available topologies as illustrated in the figure below. Typically, larger topologies provide lower loss solutions. Customers should note that this relationship is an approximation using typical performance requirements and does vary based upon other specification issues.
A more detailed estimate of insertion loss is available by following this link. These capabilities also do vary from manufacturer to manufacturer.
Rejection Performance - With all other factors being equal, each topology provides the same rejection performance outside of the 3 dBc bandwidth. Some exceptions include the following:
Ultimate Rejection - Combline/cavity topology can provide up to 100 dBc of rejection while other topologies will consistently provide no more than 50 dBc.
Resonances - Circuit re-resonances will limit rejection performance at higher frequencies. This will limit the upper rejection band frequency range per the following table:
Topology
Re-resonance Frequency
Ceramic Resonator
2x Fc
Combline/Cavity
3x Fc
Lumped Element
4x Fc
Physical Topology Issues - Physical concerns may dictate the selection of one topology over another. Most of these concerns relate to size limitations. A rough tradeoff analysis with size verses electrical performance can be done using the above plots and discussions.
PACKAGING SELECTION- Considerations for packaging options are provided for review. Variables considered include topology, size constraints, installation method, and desired input/output interface.
OTHER CONCERNS - Probably the single most important issue not addressed here is cost. Cost tradeoffs are not easily quantified without a specific requirement for evaluation and tradeoff comparisons. Q Microwave´s sales and engineering staff will gladly provide the necessary to ensure that the optimal solution is proposed.