Tone Detector
FX105A
1.6
Application Notes
1.6.1 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 (f0). The free running frequency of the VCO is set to 6 times
this frequency by observing the output across C1 or R1. (The frequency observed at pin 15 (R1) is
6 x f0 and the frequency observed at pins 13 or 14 (C1A or C1B) is 3 x f0).
(b) The FX105A Minimu mUsable Bandwidth (MUBW). This is obtained by taking into account the
worst case tolerances D(f0) of the input frequency and the variations in the FX105A VCO
frequency due to supply voltage DV(DD) and temperature D(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, VTH).
(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) f0 = 2800Hz
(b) DTEMP = 100°C, DVDD = 1V, Df0 = 0.5%
(c)
Maximum allowe dresponse time TON = 50msec
(d) Minimum input signal amplitude VIN MIN = 200mVrms
(e) Maximum input signal amplitude VIN MAX = 400mVrms
1.6.2.1 Calculate R1C1 (C1A = C1B
)
The components R1, C1A and C1B set the free running frequency of the VCO and therefore the f0 of
the FX105A. As shown below, the frequency of 2800Hz corresponds to a capacitor value of 220pF
and a resistor value of 385 Wk. This resistance can be achieved with a 330 Wkfixed resistor and a
100kW potentiometer. R1 should lie in the range 100 Wk to 680kW .
R1C1A = 1/ [2Kf0] = 1/ (2 x 2.1 x 2800 )= 85µsec
where K is a constan t= 2.1 ± 5%.
Note that the values of C1A and C1B 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 C1A = C1B = 233.2pF, then R1 » 364kohm
ã 1999 Consumer Microcircuits Limited
7
D/105A/5
CMLMICRO [ CML MICROCIRCUITS ]
