Tone Detector
8
MX105A PRELIMINARY INFORMATION
5.5 Select R4 for Operating BW
R4
=
4.8
×
BW
4.8
×
6
=
≈
6.8k
Ω
10.35
−
BW 10.35
−
6
The exact bandwidth given by any value of R4 will vary slightly. In applications where an exact bandwidth is required, R4
should be a variable resistor to permit adjustment.
5.6 Calculate R2×C2
A
R2
×
C2
A
≈
100
3
×
f
0
×
BW
For a frequency of 2800 Hz, a bandwidth of 6%, and a choice of
C2
A
=
0.01
µ
F
⇒
R
V
=
200k
Ω
.
Note:
Use nearest preferred values.
5.7 Define Maximum Allowed Response Time
The maximum response time (T
ON
) is the sum of the VCO lock time (T
LOCK
) and the DETECT integration time (T
DETECT
).
The MX105A’s T
ON
must not exceed the maximum time allowed for the application, but a value lying near the maximum
gives the best S/N performance.
A. Calculate T
LOCK
T
LOCK
=
150
f
0
×
BW
Using the formula above, for a frequency of 2800 Hz and a bandwidth of 6% the approximate Lock time (T
LOCK
) will be 9
ms. Since the maximum response time is 50 ms, a DETECT time of 41 ms is allowed.
Note:
T
LOCK
may vary from near zero to the value given, causing corresponding variations in actual T
ON
.
B. Calculate Maximum Allowable T
DETECT
T
DETECT
=
T
ON
MAX
−
T
LOCK
C. Define Minimum Expected Signal Amplitude V
IN
MIN
This is used in calculating T
DETECT
components.
(
)
5.8 Calculate R3×C3
A
R3
×
C3
A
≈
T
DETECT
V
−
3
×
ln
1
−
TH
V
IN
MIN
where: V
TH
is the detect filter sensitivity.
Note:
1. For a signal amplitude of 200 mV
RMS
, a resistor value R3 of 510 kΩ with a 0.1µF capacitor for C3
A
and C3
B
will yield
a T
DETECT
time of 20ms. This in turn yields a response time of 9ms + 20 ms = 29ms.
2. Use nearest preferred values.
© 1997 MX•COM, INC.
www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054
Doc.# 20480133.003
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