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.
ã 1999 Consumer Microcircuits Limited
10
D/105A/5
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