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FX105A

Tone Detector

CML Microsystems Plc 

Tone Detector

FX105A

1.6.2.9 Calculate Signal to Noise Performance

Worst-case S/N calculations depend on calculation of a value “M” using the formula shown below:

M = RWC3A/ [3 RVC2A]

substituting our example values,

M = 465 x 0.22 / (3 x 200 x 0.01) = 17.05

By substituting this value for M in Figure 5 the minimum required S/N of an in band tone with respect

to an adjacent interfering tone can be found. This has to be increased if the required tone amplitude is

close to the word filter sensitivity VTH.

Figure 5 S/N vs. BW Separation

The following formula expresses the reduction in noise immunity as the input signal approaches the

word filter sensitivity VTH.

required S/N = 20 log (VIN/ [VIN - VTH] ) + S/NFigure 5

If this S/N is better than required for the application, RWC3A can be reduced, or the operating

bandwidth can be increased to obtain a faster tone detection time.

If the S/N performance is not adequate, the operating bandwidth can be reduced toward the MUBW,

or RWC3A can be increased to improve S/N performance at the expense of a slower response time.

© 1999 Consumer Microcircuits Limited

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D/105A/5

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