Cable and Telephony - Diplex Filters

August 11th, 2014


Diplex filters are used in the amplifiers at about every mile of cable for high speed internet, phone, and interactive television programming.  The diplex filter allows for a separation of the low frequency and high frequency radio frequency (rf) signals to be transmitted over the single pipe of the cable. The filter facilitates the ability to two communications to take place over cable, sending and receiving.  Precise tuning of the filters assures that the demilitarized zone between Low and High frequencies is consistant within a specified frequency range.  The issue comprised the following:

  • Tuning at the filter level was taking well over 1 hour per filter and was not consistant.
  • No clear specification for the required waveform existed to for the tuning of the filter
  • Filters would not plug and play within an amplifier requiring a day of swapping filters and testing until the amplifier was tuned properly.


A team was assembled that included design engineering, production engineering, line supervisors, and production, quality, and testing personnel. The initial activity was process mapping to understand the suppliers, the inputs they provided, the individual process steps, the outputs, and the customers both internal and external. Quality Function Deployment (QFD) was used to identify and prioritize both visual and functional requirements for both the diplex filters and the amplifiers. FMEA, Failure Mode and Effects Analysis, was used to identify the process steps that had the highest chance of creating issues during production.  To develop an official "gold standard" for the diplex filter waveform from 0-1.5 GHz known acceptable amplifiers had each of there 4 diplex filters removed and tested to capture the waveform. This was done for 25 amplifiers. The average wave form value was determined at increments of 5 Hz from 0-1.5 GHz and this became the gold standard. QFD was then used to narrow the field of potential variation contributors to achieving the desired quality characteristics for the optimal application of Design of Experiments (DOE).  The DOE focused on the wrapping and wire locations of the 10 torroids on the diplex filter.  Adjustment of the wrapping and position of the windings is how the tuning was accomplished for each filter.

Solutions and Results:

  • The "gold standard" for the frequency waveform was implemented on all oscilliscopes used for testing.
  • The DOE identified which torroids and where to wind them would best achieve the target value of the "gold standard" at each increment of the frequency range. Taguchi's Target Signal to Noise performance metric was applied to accomplish the analysis.  The result was how to wind and adjust each torroid if adjustment was required.
  • Diplex filters could now be tuned in less than 1.5 minutes each and they would plug and play in any combination in any amplifier.
  • Amplifier tuning was reduced to the 20 minute test cycle.
  • Total labor savings was in excess of $500,000 annually in a factory in Mexico.

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