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12kHz and 16kHz SPM Detector



  • Low Power Operation
  • Detects 12kHz and 16kHz SPM Frequencies
  • High Speechband Rejection Properties
  • Tone Follower and Packet-mode Outputs


  • Complex and/or Simple Telephone Systems
  • Hotel and Office Call-Charge/Logging Systems
  • Billing Systems
  • Public Call Monitoring
  • Payphone Systems
  • PCMCIA Fax and Data Modems

Supply Requirement:

  • 3.0 to 5.5 V power supply

The CMX631A is a low-power, system-selectable Subscriber Pulse Metering (SPM) detector to indicate the presence, on a telephone line, of both 12kHz and 16kHz telephone call-charge tone frequencies.

Deriving its input directly from the telephone line, input amplitude/sensitivities are component adjustable to the user’s national ‘Must/Must-Not Decode’ specifications via an on-chip input amplifier, whilst the 12kHz and 16kHz frequency limits are accurately defined by the use of an external 3.579545MHz telephone-system Xtal or clock-pulse input.

The CMX631A, which demonstrates high 12kHz and 16kHz performance in the presence of both voice and noise, can operate from either a single or differential analogue signal input from which it will produce two individual logic outputs.

This system (12kHz/16kHz) selectable microcircuit, which may be line-powered, is available in: 24-pin SSOP, 16-pin SOIC and 16-pin plastic DIP packages.

Block Diagram
CMX631A: a low-power, system-selectable Subscriber Pulse Metering (SPM) detector

Design Support Information



Q. 631 - What is the difference between the FX631 and the CMX631A?

A. The CMX631A is a development of the FX631 to give lower voltage operation. The FX631 is not recommended for new designs. The CMX631A is intended as a pin and function equivalent part. New designs should use the CMX631A.

The FX631 is a 3.3V-to-5V part whereas the CMX631A will operate between 3.0V and 5.0V. It is also possible to operate the CMX631A down to 2.7V over a reduced temperature range although there may be some small parametric differences.

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