Tone Detector

10

MX105A PRELIMINARY INFORMATION

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5.10 Calculate Signal to Noise Performance

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

M

R3

C3

3R2

C2

A

A

=

×

××

substituting example values,

M

510

0.1

3200

=

×

××

=

001

85

.

.

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 then has to be increased depending on the input tone amplitude.

Number of Bandwidths Separation

24

68

10

1

M = 0.1

M = 0.33

M = 1

M =

∞

-4

-8

-12

-16

-20

-24

-28

0

Figure 5: S/N vs. BW Separation

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

required

S

N

= 20log

V

VV

+

S

N

IN

IN

TH

Figure 5

−













If this S/N is better than required for the application, R3

increased to obtain a faster tone detection time.

increased to improve S/N performance at the expense of slower response time.

5.11 Calculate C4 for 30° Phase Shift

Capacitor C4 is used to phase shift the input to the VCO commutating filter by 30°, thereby shifting the sampling clocks by

the same amount. This enables the Detect sampling filter to sample and integrate at the maximum and minimum of the

input tone.

C4

tan(30 )

2f

0.092

f

164pF

0V

0V

=

°

××

≈

×

≈

π

R

R