CMX860, CMX867, & CMX868 - CML Microcircuits

COMMUNICATION ICs

Application Note

Filter Coefficient Generato r

for CMX860, CMX867, and

CMX868

¤¤

¤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 respec t ive companies.

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 “coeffici ents” 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

Filter Coefficient Generator for CMX860, CMX867, & CMX868 2 Application Note

¤¤

¤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 respec t ive companies.

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 Genera tor Window

Figure 2: Coef ficient 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.

Filter Coefficient Generator for CMX860, CMX867, & CMX868 3 Application Note

¤¤

¤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 respec t ive companies.

3.3. 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.

Filter Coefficient Generator for CMX860, CMX867, & CMX868 4 Application Note

¤¤

¤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 respec t ive companies.

Fixed Receive Signal Filtering

0000.4 1.2

2.6

4.1

1234567

Frequency (kHz)

Attenuation

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.