Tone Detector
7
MX105A PRELIMINARY INFORMATION
5. Application
The external components shown in Figure 2 are used to adjust the various performance parameters of the MX105A. The
signal-to-noise performance, response time and signal bandwidth are all interrelated factors which should be optimized to
meet the requirements of the application.
By selecting component values in accordance with the following formulas, optimum circuit performance is obtained for any
given application.
First define the following application parameters:
A. The center frequency to be detected (f
0
).
B. The MX105A Minimum Usable Bandwidth (MUBW). This is obtained by taking into account the worst case
tolerances on the input tone frequency and variations in the MX105A f
0
due to supply voltage and any temperature
effect of the MX105A and its supporting components.
C. The maximum permissible MX105A response time.
D. The minimum input signal amplitude.
Note:
Using this information the appropriate component values can be calculated, and the signal-to-noise performance
can be read from a chart. Do not add large safety margins for response time and minimum signal amplitude;
reasonable margins are already included in the formulas. Excessive margins may result in reduced noise immunity.
5.1 Method for Calculating External Component Values
The examples on the following pages demonstrate the calculation of component values for any given application. For the
purpose of the examples, the values below are used:
A. f
0
= 2800 Hz
B.
∆TEMP
= 100 °C,
∆
V
DD
= 1V,
∆f
IN
= 0.5%
C. Maximum allowed response time = 50ms
D. Minimum input signal amplitude = 200 mV
RMS
.
5.2 Define f
0
The components R1, C1
A
and C1
B
set the free running frequency of the VCO and therefore the f
0
of the MX105A. As
shown below, the frequency of 2800 Hz corresponds to a capacitor value of 220pF and a resistor value of 385 kΩ. This
resistance can be achieved with a 300 kΩ fixed resistor for R1
F
and for R1
V
a 100 kΩ potentiometer. The capacitance of
C1
A
and C1
B
should include 10-20pF parasitic capacitance due to the device and its package plus any board parasitic
capacitance.
f
0
=
1
1
⇒
R1
×
C1
A
=
K
⋅
R1(C1
A
+
C1
B
)
2Kf
0
where : K
R1
=
=
2.1 ± 5%
(R1
F
+ R1
V
)
5.3 Calculate Minimum Usable Bandwidth
Minimum Usable Bandwidth (MUBW) is the TOTAL bandwidth required for the following:
A. Input signal frequency tolerance
B. MX105A f
0
temperature coefficient (T
C
= 100 ppm/°C)
C. MX105A f
0
supply voltage coefficient (V
C
= 5000 ppm/V)
Note:
Add A, B and C and express as TOTAL bandwidth, not as a ± percentage (%) value.
MUBW =
∆
f
0
+ T
C
∆
TEMP + V
C
∆
V
MUBW = 0.5 + 0.01
×
100 + 0.5
×
1 = 2%
5.4 Calculate The Recommended Operating Bandwidth
BW
=
10
+
MUBW 10
+
2
=
=
6%
2
2
© 1997 MX•COM, INC.
www.mxcom.com Tele: 800 638 5577 910 744 5050 Fax: 910 744 5054
Doc.# 20480133.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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CMLMICRO [ CML MICROCIRCUITS ]
