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Reference Data ] Rejection ] Insertion Loss ]

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.

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:

Top-v-freq

plot bw v topology

plot il vs q
 

Topology

Re-resonance Frequency

Ceramic Resonator

2x Fc

Combline/Cavity

3x Fc

Lumped Element

4x Fc

 

 

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.

 

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