Tone Detector
FX105A
1.6
1.6.1
Application Notes
General
The external components shown in Figure 2 are used to adjust the various performance parameters of
the FX105A. The signal-to-noise performance, response time and signal bandwidth are all interrelated
factors which should be optimised to meet the requirements of the application.
By selecting component values in accordance with the following formulae, optimum circuit
performance is obtained for any given application.
First define the following application parameters:
(a) The input frequency to be detected (f
0
). The free running frequency of the VCO is set to 6 times
this frequency by observing the output across C
1
or R
1
. (The frequency observed at pin 15 (R
1
) is
6 x f
0
and the frequency observed at pins 13 or 14 (C
1A
or C
1B
) is 3 x f
0
).
(b) The FX105A Minimu mUsable Bandwidth (MUBW). This is obtained by taking into account the
worst case tolerances
∆
(f
0
) of the input frequency and the variations in the FX105A VCO
(
frequency due to supply voltage
∆
V
DD
) and temperature
∆
TEMP) variation of the FX105A and its
(
supporting components.
(c) The maximum permissible FX105A response time.
(d) The minimum input signal amplitude. (This must be larger than the threshold voltage, V
TH
).
(e) The maximum input signal amplitude.
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 formulae. Excessive
margins may result in reduced noise immunity.
1.6.2
Method for Calculating External Component Values
The example on the following pages demonstrates the calculation of component values for any given
application. For the purpose of this example, the values below are used:
(a)
(b)
(c)
(d)
(e)
f
0
= 2800Hz
∆
TEMP = 100°C,
∆
V
DD
= 1V,
∆
f
0
= 0.5%
Maximum allowe d
response time T
ON
= 50msec
Minimum input signal amplitude V
IN
MIN
= 200mVrms
Maximum input signal amplitude V
IN
MAX
= 400mVrms
1.6.2.1 Calculate R
1
C
1
(C
1A
= C
1B
)
The components R
1
, C
1A
and C
1B
set the free running frequency of the VCO and therefore the f
0
of
the FX105A. As shown below, the frequency of 2800Hz corresponds to a capacitor value of 220pF
and a resistor value of 385
Ω
. This resistance can be achieved with a 330
Ω
kfixed resistor and a
k
k
100k
Ω
potentiometer. R
1
should lie in the range 100
Ω
to 680 k
Ω
.
R
1
C
1A
= 1/ [2Kf
0
] = 1/ (2 x 2.1 x 2800 )
=
85µsec
where K is a constan t
=
2.1 ± 5%.
Note that the values of C
1A
and C
1B
need to include the stray capacitance
attributable to the package style and printed circuit board layout. A typical
value of 6.6pF per pin should be assumed.
If C
1A
=
C
1B
=
233.2pF, then R
1
»
364kohm
©
1999 Consumer Microcircuits Limited
7
D/105A/5
CMLMICRO [ CML MICROCIRCUITS ]
