MX105AP datasheet(7/15 Pages) CMLMICRO

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Tone Detector

MX105A PRELIMINARY INFORMATION

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Doc.# 20480133.003

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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 (f0).

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 f0 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. f0 = 2800 Hz

∆TEMP = 100 °C, ∆ VDD = 1V, ∆fIN = 0.5%

C. Maximum allowed response time = 50ms

D. Minimum input signal amplitude = 200 mVRMS.

5.2 Define f0

The components R1, C1A and C1B set the free running frequency of the VCO and therefore the f0 of the MX105A. As

shown below, the frequency of 2800 Hz corresponds to a capacitor value of 220pF and a resistor value of 385 k

Ω. This

resistance can be achieved with a 300 k

Ω fixed resistor for R1F and for R1V a 100 kΩ potentiometer. The capacitance of

C1A and C1B should include 10-20pF parasitic capacitance due to the device and its package plus any board parasitic

capacitance.

K R1(C1

C1 )

R1 C1

2Kf

⋅+

⇒×

where : K

2.1 ± 5%

(R1F + R1V)

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 f0 temperature coefficient (TC = 100 ppm/°C)

C. MX105A f0 supply voltage coefficient (VC = 5000 ppm/V)

Note: Add A, B and C and express as TOTAL bandwidth, not as a ± percentage (%) value.

MUBW

TEMP

+ T

+ V

MUBW = 0.5 + 0.01

100 + 0.5

1 = 2%

∆∆

∆

××

5.4 Calculate The Recommended Operating Bandwidth

MUBW

+ =

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