The system allows lumped element circuits to be generated for low pass, high pass and band pass filters of Chebyshev and Butterworth types. Additionally, the software generates coupling matrices for these approximations and allows the matrix to be modified. This matrix provides the information necessary to build a coupled resonator filter circuit. Graphs of frequency response can be generated from the coupling matrix.
Question marks (?) next to an interface element provide links to the relevant section of the interface help.
S21 is the transmission loss. This shows the attenuation of various frequencies of the output signal with respect to the input signal. S11 is the return loss, which corresponds to the signal appearing at the input terminals.
It is possible that the order is very high, or selected combination of attenuation and stop band frequency is high. Calculations take longer for higher order filters.
The system imposes no limit on the order of filter that can be specified, but bear in mind that calculation speed decreases with order. There should be no problems with filters of a reasonable order (i.e. less than 20)
The focus of the system is on coupled resonator circuits, so only the most important filter types are included (Chebyshev, Butterworth and Elliptic). The are many other packages around that deal with a wider selection of approximations, like Nuhertz' Filter Solutions
The frequency response plot is calculated from the coupling matrix. A coupling matrix cannot be determined for a low pass or high pass filter. Frequency response plots can be obtained by other means, such as from the Chebyshev or Butterworth equations, but this has not been incorporated as the focus of the system is on coupled resonator circuits.
There are two methods for achieving this. Firstly, it is possible to click anywhere on the graph using the mouse. The area below the graph then displays the frequency clicked at, and the values of S11 and S21. Alternatively, if the attenuation values at an exact frequency are desired, this frequency can be entered into the field labelled "find attenuation at frequency", and the results obtained by clicking "Find Att".
The important part of the response will be that surrounding the centre frequency. Provided the "Scale frequency axis to __ bandwidths" is checked, then any time new data is sent to the graphing applet, the axes will be automatically centred around this region.
Simply enter the new values into the "max" and "min" fields on each axis and press the "set limits" button.
Click the "Spec" or "Coupling" buttons, under "View results". This opens a new window containing a list of all of the parameters and the matrix. Copy and paste this text into another file, such as a simple text file.
Copy and paste the text from a results file into the text area marked "load results file" and press "Spec" or "Coupling", depending on whether the data is for a specification or a coupling circuit.
Yes, provided that the structure is maintained. No lines must be deleted or added, but any of the values may be modified.
Saving files directly onto internet user's computers is a major security issue, as it allows the potential distribution of viruses. Applets that do so require additional security privileges on the user's computer. Many users are unwilling to reduce security settings in this way and so introducing a system like this would discourage people from using the software. Additionally, on some computer networks such as those in academic institutions, users are forbidden from changing the access privileges required to view such applets.
These are the external coupling coefficients, for the input and output respectively.
The coupling matrix represents the degree of electromagnetic coupling between resonators in the coupled resonator circuit. Each position in the matrix corresponds to a particular pair of resonators. For example, element (i,j) gives the coupling coefficient between resonator i and j.
Yes. If the filter is to be physically realisable, the matrix must be symmetrical about the diagonal.
Yes. Once the matrix has been generated from a particular approximation, it can be entirely user customised.
The coupling matrix must be symmetrical about the diagonal if the filter is to be physically realisable. The software ensures this by automatically updating the opposite field each time a value is entered.
The filter type governs the shape of its frequency response. There are three kinds catered for. The Butterworth is the simplest, and is used in situations where distortion must be minimised as it has a flat pass band. The Chebyshev design introduces an amount of "ripple", or fluctuation, into the pass band, but produces an improvement in attenuation at frequencies outside the pass band. The elliptic design is the most complicated and it has ripple in both stop band and pass band, but displays a very sharp cut off.
This refers to the frequency range being filtered. Due to the calculation methods used, low pass and high pass options are not available for Elliptic filters.
For low and high pass filters, the specification is simply the frequency where the transition is made from pass band to stop band, otherwise known as the cut off frequency. For band pass filters, the range of frequencies being passed, or bandwidth, is specified, along with the central point of this range, the centre frequency
For elliptic filters, the sharper cut off is created by introducing an extra point of infinite attenuation. This is specified as a 'pole frequency', outside the defined pass band.
For elliptic and Chebyshev filters, there is a trade off between the amount of variation permitted in the pass band and the sharpness of the decay into the stop band. This can either be specified directly as the maximum deviation from zero of the transmission loss, as the minimum value of the return loss (S11 min).
The order of a filter is its most fundamental characteristic. Filters with higher orders have better performance, but require more components to make them. The software allows the order of the filter to be specified directly, or to be calculated as the minimum order needed to meet a particular attenuation.
These are displayed as a 'netlist', or a sequential list of component values. RS and RL relate to the source and load impedances, with the number components being inductors and capacitors, counting from the left of the diagram. This software creates circuits which follow the "Pi" Topology, rather than the "Tee" Topology, which requires the circuits shown below.
For band pass filters, the capacitors in the low pass design are replaced by a parallel LC combination and the inductors are replaced by a series LC combination.
Pressing the "get" button calculates the circuit components based on the filter specification.
The source and load impedance of the filter must be matched to the devices preceding and succeeding in order to achieve optimum performance. The software assumes that these impedances are also matched to each other.
Due to the security limitations of Java Applets, no direct file saving utility is provided for data produced. However, a facility exists where a new window can be opened, containing the data for a filter specification or a coupling circuit. This data can be cut and pasted into a blank text file for saving purposes. The coupling matrix can also be loaded into any spreadsheet package, by importing it as CSV (Comma Separated Values) data. Press the "spec" button, in the "view" section to open a new window containing the filter parameters. Press the "Coupling" button, in the "view" section to open a new window containing the matrix and other coupling parameters.
Data can be loaded from a previous session by copy and pasting the results file into the text area under "Load results file". The submit button for "Spec" or "Coupling" must then be pressed, depending on the type of data being entered. The software will then update the relevant fields on the interface.
Saving a filter specification
Saving a coupling circuit
Loading Data from a previous session
Graphing panel
Frequency response
Both scales are logarithmic, with an appropriate number of ticks being made on each axis to enable easy viewing. The red line corresponds to the return loss (S11) of the filter, with the blue line corresponding to the transmission loss (S21).
Pressing the "Plot from coupling" button generates the frequency response plot based on the coupling matrix and associated parameters.
By default, the graph axis limits are automatically set to fit the data generated. This means that the frequency axis is scaled to a multiple of the bandwidth and the attenuation axis is then scaled to ensure that all of the data is displayed. The respective checkboxes allow this feature to be overridden, useful in situations where a change in the response needs to be observed. In this case any new data is simply displayed on the existing scale regardless.
The fields at the extremities of the axes show the existing graph limits and allow new ones to be entered. The scales are updated by pressing the "Set Limits" button.
Use the "Show grid" option to turn gridlines on or off on the frequency response plot.
This can be achieved by clicking the mouse at any point in the graph area. The fields below the plot will then show the frequency and attenuation values at the nearest point on the response line. Alternatively, an exact frequency, on or off the graph, can be entered into the frequency field, with the "Get" button updating the attenuation readings.
Clicking "Generate from spec" Calculates the coupled resonator circuit required to realise the specification given.
This corresponds to the resonant centre frequencies of the individual resonators, and also determines the centre frequency of the filter itself.
This parameter takes account of the fact that the resonators used in a coupled resonator circuit with not have ideal characteristics. Ideally, this quality factor would be infinite, so a lower number results in reduced performance. The default Q value is 30 000.
Qea and Qeb represent the external coupling coefficients of the input and output respectively.
This shows the electromagnetic coupling between each resonator and each other resonator in a coupled resonator circuit. For simple filters, like Chebyshev and Butterworth, this matrix will be zero except for those elements on either side of the diagonal. Further pairs of non zero elements are added to improve the filter response, as in the case of Elliptic filters.
The order field specifies the number of poles in the filter. Enter a value here and click "Set" to define the size of a new matrix. The new matrix will be blank.
The "Clear" button provides a quick way of setting the elements to zero before entering values.
Nuhertz Technologies freeware download of a package performing basic filter calculations.
Includes basic theories on transmission lines, network analysis, and filter design, in addition to detailed information on coupled resonator circuits and cross coupling.
Transmission line theory and information on the design of lumped element low, high and band pass filters.