Telephone Line Monitor for Critical Communication Systems

Combilent develops and delivers state of the art Radio Frequency conditioning products for critical communication systems. The listening equipment is used by Combilent’s customers to monitor the quality of analog telephone lines in these systems.

For monitoring the signal quality on analog telephone lines Combilent needed the development of the new listening equipment. Analog telephone phone lines can be defective making it difficult to get the desired analog signals from a to b. Combilent has contacted Develco to develop the monitoring equipment for analog telephone lines that measures and analyzes the analog signals in a digital domain to determine the quality of the signals. In the development process Develco has participated in the concept and the development phase in developing the electronic printed circuit board and the embedded software for monitoring analog telephone lines. Combilent has itself tested the finished developed printed circuit board and produced prototypes.

The printed circuit board measures signals on 4 telephone lines at a time via 4 galvanically separated Analog Digital Converters (ADC). To measure the quality of the analog telephone signals Develco has developed an algorithm that starts at sudden changes in the signal strength. The algorithm uses Fast Fourier Transformation (FFT) on the collected analog signal to convert it to the frequency domain to determine the presence as well as the frequency and the strength of the expected signals.

The result of the frequency analysis is sent serially from the printed circuit board developed by Develco to Combilent’s device. With “Telephone Line Monitor” Combilent has the possibility to analyze and present the quality of the signals from the analogue telephone lines. In addition, Combilent can change the setup of the monitor and update the firmware from the serial interface.

Combilent Telephone Line Monitor


  • STM32 Microprocessor
  • External Flash
  • 4 Light Emitting Diodes
  • 4 Analog Digital Converters
Combilent Telephone Line Monitor


Normally we develop our own new product solutions, but owing to lack of time we needed external resources to develop the electronic printed circuit board and the embedded software for monitoring analog telephone lines. After reading a professional advertisement I contacted Develco as they had the right competences for the development of our new listening equipment. My decision was right. In the development process we have had a good dialogue at the development meetings with continuous status reports. We are very satisfied with the finished technical solution and the test report as the termination of the development project“, says Jesper Trier, Sales Director at Combilent.