The power measurement of a digital channel differs from its vertical scale in the spectrum analyzer. Why?

May 10, 2019.

The power of a digital channel is calculated using algorithms that evaluate the energy received on its bandwidth. The result of this calculation will differ from the vertical scale depending on the signal bandwidth. The more bandwidth, the more different that value will be. How is this measurement calculated? How do we, at Promax, guarantee and improve its accuracy?

The vertical scale does not indicate the digital channel power

The following figure illustrates the spectrum of a digital channel. It seems that the digital channel has an approximate power of 40 dB µV. But only in a visual way! It would be a mistake to estimate that those 40 dB µV represent the measurement of the entire channel, because in reality it is only the power received at the point where the cursor is located. The cursor measures the power in a segment of 100 kHz (depends on the resolution filter) while the channel is occupying a much bigger frequency range, which in this picture is 8 MHz.

It would be a mistake to estimate that this digital channel has a power of 40 dBµV taking as reference the vertical scale at the graph.

This is how the channel power is calculated

In order to calculate the power of a digital channel, its spectral bandwidth is divided into a large number of segments. The power of each segment is measured (in linear units) and then all of them added to get a final value.

The RMS value of each measured segment is the actual power of the digital channel
The RMS value of each measured segment is the actual power of the digital channel.

To implement a measurement as described above, PROMAX has implemented sophisticated algorithms for digital signal processing in order to achieve a fast and accurate spectrum at the frequency domain. This spectrum is the basic tool to measure LEVEL, POWER and CN with the accuracy and speed that only our equipment can do. It also makes possible to provide our equipment with a wide variety of resolution filters that the user can apply to deal many specific situations.


This is how low-cost analyzers work

We have seen how PROMAX field strength meters calculate the power of a digital signal by integrating many segments along the channel bandwidth in order to obtain an accurate value in all circumstances.

This is done because the channel signal is never perfect: it can show distortions due to interferences, echoes, impairments in the SMATV installation, etc.

Low range field strength meters often assume that the signal spectrum is flat, with no impairments. They take a single segment and estimate that the rest of the channel has the same power level, discarding the other segments, which usually have different power levels:

Measurement method of a low-cost field strength meter (left) and a PROMAX field strength meter (right)
Measurement method of a low-cost field strength meter (left) and a PROMAX field strength meter (right). The low-cost analyzer assumes that the channel is constant and perfect. The PROMAX analyzer samples the channel to take the actual measurement.

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PROMAX is a leading manufacturer of test and measurement systems, broadcast and TV signal distribution equipment. Our product lines include measuring instruments for cable TV, satellite TV, broadcast, wireless and fibre optics networks, FTTH and GPON analyzers. DVB-T modulators, IP streamers or IP converters (ASI, DVB-T) are among the company’s latest developments.