AN84 데이터 시트보기 (PDF) - Silicon Laboratories

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AN84

DIGITAL HYBRID WITH THE Si305X DAAS

Silicon Laboratories 

AN84

1.2. Digital Hybrid Calculation Tool

Silicon Labs has developed a useful graphical user

interface tool (shown in Figure 5) that will assist in

calculating the coefficients to use with the digital hybrid

in the Si305x DAA product family. The tool allows the

user to enter the reference termination of the central

office (in an R + R||C format) and the model for the

phone line between the DAA and the central office. The

line can be represented by one of the EIA models,

shown in " Appendix C—EIA Line Models" on page 24,

or as a specified length of wire. The software then

executes the Matlab code found in " Appendix A—

Sample MATLAB Code" on page 7, which graphically

shows the expected trans-hybrid response of the digital

hybrid and lists the best hybrid coefficients to use given

the line characteristics.

Three graphs are shown in Figure 6. An echo graph is

created by intentionally mismatching the 600 Ω ac

termination with the TBR21 mode CO termination. The

digital hybrid response is the 8-tap FIR filter response

calculated using the sample code found in Appendix A.

The cancelled graph is obtained by adding the echo and

the digital hybrid response. The digital hybrid response

looks very similar to an echo. Figure 7 shows the phase

of the echo and the digital hybrid response. The phase

of an echo and the digital hybrid have the opposite

polarity. Figure 8 compares the rejection in dB with and

without the digital hybrid. By properly using the digital

hybrid, near-end echo cancellation has increased by

approximately 20 dB.

To use the Digital Hybrid Calculation Tool, simply enter

the ACIM value recommended in Table 13 of the Si3050

datasheet into the ACIM control. This value determines

the impedance presented by the DAA to the line. It is

governed by the region in which the application will be

deployed. Next, enter values for R1 and R2 in ohms and

C in farads into the appropriate controls. These values

will represent the impedance presented by the central

office to the line. This value is also governed by the

region in which the application will be deployed. Also,

select the line-side device used in the application in the

pull-down box. Finally, select the line model to be used

that will most closely model the line connecting the DAA

to the central office. This is done by either picking a

specific EIA line model or by specifying a wire gauge

and length. Once this is complete, hitting the

"CALCULATE" button will generate the coefficients that

provide the best performance.

For example, if we assume that an application will be

deployed within the U.S., we enter a 0 for the ACIM

value. Also, the central office impedance in the US is

900 Ω in series with 2.16 µF. To enter this information in

the GUI, we enter 900 for R1 and a fairly large value for

R2, since it is not present. For this example, the value of

100,000 Ω was used. For the C value, we enter 2.16e-6

since the expected units are farads. Also, for this

example, we use an EIA model of 0. This means

essentially no loop length, and the central office

impedance is connected directly to the application. Now,

the "CALCULATE" button is pressed, and the resulting

coefficients, 0xF8, 0xF9, 0x03, 0xFE, 0xFE, 0x00,

0xFE, and 0x00, are generated.

Rev. 0.6

3

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