Leaks are unacceptable. What plumbers and secret agents have known for many years has now become a critical issue for TV technicians. Unwanted emissions from cable TV networks represent a huge potential source of interference for new communications networks.
Since the digitization of terrestrial TV, many countries have discontinued TV broadcasting in parts of the UHF range. The freed-up frequency band is known as the digital dividend. It is now available for other services and has been distributed to private operators.
These operators plan to set up LTE networks in the UHF band to provide fast, mobile Internet and to improve Internet access in rural areas. To ensure the quality of services and prevent any negative impact on their business, the new license owners take extreme care to ensure that no interference occurs in their part of the spectrum.
Unlike terrestrial television, cable TV continues to use the UHF band and overlaps with the new LTE networks. If leaks occur in cable TV networks, interference in LTE networks is inevitable.
FCC leadership position
The U.S. FCC regulatory authority long ago defined strict limits for radiated emissions in the aeronautical radio band from 108-139 MHz. Limits for the range between 700-800 MHz in which the new LTE networks operate have recently been introduced. The objective of the FCC limits is to protect LTE networks from unwanted radiated emissions from cable TV networks.
This places an enormous amount of pressure on cable TV network operators because, as owners of the spectrum, LTE network operators can force cable TV network operators to comply with these limits.
Cable TV network operators accountable
Many cable TV network operators still do not have measuring equipment capable of detecting radiated emissions in the UHF band. Measuring equipment for the VHF aeronautical radio band has been around for some time now, but it is narrowband and cannot detect UHF signals.
However, this is necessary since leaks generate far more emissions at higher frequencies than at lower frequencies. Sometimes the field strength of radiated emissions from a poorly tightened screw connector can barely be measured in the VHF band, whereas in the UHF band it is significantly above the permissible limit.
Measuring instruments for detecting leaks in cable TV networks must fulfill several requirements: They must be sufficiently sensitive and designed for broadband operation in order to reliably detect limit violations in both the VHF and UHF band. They must be fast enough to detect even sporadic interference, and they must also be able to make interference visible long enough for users to see it.
Even though a radiated emission from a cable TV network is constant, directional antennas make them appear to be sporadic, occurring only when the antenna is pointing at the leak.
Interference detection using FFT analysis and a directional antenna
A combination of fast Fourier transform (FFT) analysis and spectrogram display is the right method for this task. FFT analysis covers the entire frequency band of interest or individual sub-bands and calculates the signal spectrum.
The spectrogram display is used to make sporadic interference visible for a while. The current FFT spectrum is color-coded and displayed in the top line of the spectrogram. Previous measurement values slowly move downwards. This type of display is also referred to as a waterfall diagram.
But simply detecting radiated emissions is not enough; the source of interference also has to be located. This requires a directional antenna. When the directional antenna points at an interference source, the test receiver immediately registers an increase in level.
The measured value is visible for a while in the spectrogram display, allowing users to see radiated emissions on the display that were only briefly pointed at by the antenna. The spectrogram helps users to move the antenna back until it is pointed at the interference source, making the interference reappear on the display.
Because the user tracks the movement on the antenna with the eyes during this process, it is particularly helpful to have measuring instruments that generate an acoustic signal where the pitch of the tone is proportional to the measured field strength.
