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FX105A

Tone Detector

CML Microsystems Plc 

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 ∆(f0) 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, 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) ∆TEMP = 100°C, ∆VDD = 1V, ∆f0 = 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 Ωk. This resistance can be achieved with a 330 Ωkfixed resistor and a

100kΩ potentiometer. R1 should lie in the range 100 Ωk to 680 kΩ.

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

© 1999 Consumer Microcircuits Limited

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D/105A/5

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