Tone Detector
10
MX105A PRELIMINARY INFORMATION
5.10 Calculate Signal to Noise Performance
Worst-case S/N calculations depend on calculation of a value “M” using the formula shown below:
R3 × C3
A
M =
3 × R2 × C2
A
substituting example values,
510 × 0.1
M =
= 8.5
3 × 200 × 0.01
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.
-28
M =
∞
-24
-20
-16
-12
-8
M = 1
M = 0.33
M = 0.1
-4
0
1
2
4
6
8
10
Number of Bandwidths Separation
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
V
TH
.
V
S
N
S
N
IN
required
= 20log
+
V
− V
TH
Figure 5
IN
If this S/N is better than required for the application, R3×C3 can be reduced, or the operating bandwidth can be
A
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 R3C3 can be
A
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.
tan(30°)
0.092
C4 =
≈
≈ 164pF
2π ×f0× RV f0× RV
© 1997 MX•COM, INC.
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