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CMX641A

Dual SPM/Security Detector/Generator

MX-COM Inc  

Dual SPM/Security Detector/Generator

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CMX641A Advance Information

5.5 Setting Level Sensitivity via External Components

In Fixed Bandwidth Operating state, Fixed Sensitivity mode, the sensitivities of the two channels are set by the

correct selection of the components around the Channel Input Amplifiers.

Input Gain Calculation: The input amplifiers, with their external circuitry, are available to set the sensitivity

of the CMX641A to conform to the user’s national level specification with regard to ‘Must’ and ‘Must-Not’

decode signal levels. With reference to the graph in Figure 6, the following steps will assist in the

determination of the required gain/attenuation.

Step 1

Draw two horizontal lines from the Y-axis {Signal Level dB(ref)}

The upper line will represent the required ‘Must’ decode level

The lower line will represent the required ‘Must-Not’ decode level.

Step 2

Mark the intersection of the upper horizontal line and the upper sloping line; drop a vertical line from this point

to the X-axis {Amplifier Gain (dB)}.

The point where the vertical line meets the X-axis will indicate the MINIMUM Input gain required for reliable

decoding of valid signals.

Step 3

Mark the intersection of the lower horizontal line and the lower sloping line; drop a vertical line from this point

to the X-axis.

The point where the vertical line meets the X-axis will indicate the MAXIMUM allowable Input amp gain. Input

signals at or below the ‘Must-Not’ decode level will not be detected as long as the amplifier gain is no higher

than this level.

Step 4

Refer to the gain components shown in Figure 2. The user should calculate and select external components

(R1/R3/C3, R2/R4/C4 and R5/R7/C5, R6/R8/C6) to provide amplifier gains within the limits obtained in Steps 2

and 3.

Component tolerances should not move the gain figure outside these limits. Resistors R3, R4, R7 and R8

should always be greater than or equal to 100kΩ. It is recommended that the designed gain is near the center

of the calculated range.

Note that the device sensitivity is directly proportional to the applied power supply (VDD). The graph in Figure 6

is for the calculation of input gain components for the CMX641A using a VDD of 5.0 (±0.1) volts.

Subtract 4.44dB from the amplifier gain for operation at 3.0V volts.

5.6 Aliasing

Due to the switched capacitor filters employed in the CMX641A, care should be taken to avoid any aliasing

effects by removing all frequencies above 579.390kHz (16kHz mode) or 434.543kHz (12kHz mode). This can

be achieved by adding bypass capacitors across R3, R4, R7, and R8, setting the –3dB breakpoint of each

resistor-capacitor combination such that there is sufficient attenuation at the alias frequency and negligible

effect at the desired SPM frequency.

ã2001 MX-COM, Inc.

www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054

Doc. # 20480227.002

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

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