COMMUNICATION ICs Application Note Filter Coefficient Generator for CMX860, CMX867, and CMX868 1. Introduction The Filter Coefficient Generator is a Microsoft Excel spreadsheet that assists the design engineer with the use of the programmable tone pair detectors (TPD) included in the CMX860, CMX867, and CMX868. This spreadsheet calculates the required IIR filter coefficients for the programmable tone pair detectors (TPD) included in these MX-COM devices. The programmable TPD in these devices use "coefficients" to define the IIR filter shape, and this filter shape determines the tones that can be detected. The coefficients, once determined, can be programmed into the "Receive Tone Pair Programming Register" of the respective device. The CMX860, CMX867, and CMX868 provide programmable detection of single and dual tones. This feature is particularly important when a customer wishes to use one of these devices in a situation where non-standard tones must be detected. Specific details of TPD functionality are covered in detail in the relevant data bulletin, so a thorough description will not be given in this document. For clarity, however, the basic functional blocks will be discussed. The TPD consists of a pair of programmable 4th order Infinite Impulse Response (IIR) filters to select the response bandwidth, a pair of frequency measurement blocks that allow the designer to determine the measurement resolution and response time, and a level detector. These three functional blocks work together to provide a very reliable method of detecting non-standard tones. 2. Spreadsheet When the Filter Coefficient Generator spreadsheet is opened, a message box may be launched asking whether macros should be enabled; respond "yes" to continue the loading sequence. Once the spreadsheet has opened, you will be presented with the following: Figure 1: Filter Coefficient Generator Spreadsheet 2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830151.001D 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All Trademarks and service marks are held by their respective companies. Filter Coefficient Generator for CMX860, CMX867, & CMX868 2 Application Note The spreadsheet includes three auto-ranging charts, which illustrate the gain, phase and group delay of the 4th order IIR filter used in the TPD. These charts, particularly the gain chart, are used when assessing the performance of a particular filter. To proceed further, the "Run" button should be clicked. 3. Coefficient Generator Window Figure 2: Coefficient Generator Window The Coefficient Generator Window (Figure 2) is the main input and output window and is where the calculated filter coefficients appear. It is divided into the following four panels: 3.1. BiQuad A and BiQuad B These two panels are where the required filter shape can be defined. The edit boxes allow for the insertion of an appropriate filter gain, pole frequency, zero frequency, and Q values. Clicking on the "Enable Zero Frequency" button allows the pole frequency to be used in the calculation and will therefore define a high-pass or low-pass characteristic. Clicking on "Enable Zero Frequency" again will disable the zero frequency value and return the filter response back to band-pass only. 3.2. Remaining Coefficients The Remaining Coefficients Panel adds three final coefficients that are used to limit the response time of the detector against the accuracy of the result. The three-edit boxes are where the user enters "ncyc", "mintime" and "maxtime", where; i) ncyc is an integer relating to the period of the incoming tone and required response time ii) mintime & maxtime define the selectivity of the detector. More details can be found in the relevant data bulletin. 2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830151.001D 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All Trademarks and service marks are held by their respective companies. Filter Coefficient Generator for CMX860, CMX867, & CMX868 3.3. 3 Application Note CMX868 2's Complement Coefficients This is the output panel where the desired filter coefficients are located. The values are displayed in 2's complement decimal and can be written, in hex format, to the Programming Register of the respective MX-COM device. The coefficients are "words", numbered 2 to 14 (for Filter 1) and 15 to 27 (for Filter 2), where the numbers relate to the coefficients' location in the Programming Register. Word 1 should be set to 32769 (8001 hex) and must not be changed. Once the basic characteristics are entered, the user clicks the "Ok" button. After a short delay, the resultant 2's complement coefficients will be displayed in the "CMX868 2's Complement Coefficients" window. The user will now be able to examine the filter frequency response, as illustrated by the graphs, and decide whether the required performance has been reached. If the user is not satisfied with the results, fine adjustments can be made to the coefficients by clicking the "IIR Adjust" button; doing so will cause the "Z-Coefficient User Fine Tweak" window to appear. 3.4. Z-Coefficient User Fine Tweak Figure 3: Z-Coefficient User Fine Tweak The "Z-Coefficient User Fine Tweak" window allows for the direct manipulation of the coefficients, one iteration at a time. In use, there will probably be no need to alter the values by more than a few units, but more are possible if so desired. Please note that the 2's complement values displayed in the main "Z-Coefficient User Fine Tweak" window are not saved upon exiting the application. 3.5. Notes When designing the IIR filter performance for these devices, allowance must be made for the fixed receive filtering, which has the low-pass characteristic shown in Figure 4. 2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830151.001D 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All Trademarks and service marks are held by their respective companies. Filter Coefficient Generator for CMX860, CMX867, & CMX868 4 Application Note Attenuation dB Fixed Receive Signal Filtering 5 4 3 2 1 0 4.1 2.6 1.2 0 1 0 2 0.4 0 3 4 5 6 7 Frequency (kHz) Figure 4: Fixed Receive Signal Filtering 4. Conditions MX-COM does not assume any responsibility for the use of this software. MX-COM reserves the right to alter or update the software at any time without notice. While all reasonable efforts have been made to ensure that software contained in this product is virus free, MX-COM accepts no responsibility whatsoever for any contamination which results from using this software, and the responsibility for checking that the software is virus free is placed on the user of the software. 2001 MX-COM, Inc. www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054 Doc. # 20830151.001D 4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All Trademarks and service marks are held by their respective companies.