Error Vector Spectrum (Evs) Function - Deviser S7200 User Manual

Part II: Settings and Measurements
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Error Vector Spectrum (EVS) Function

The EVS function is a powerful tool for locating signal interference under the QAM (DVB-C or DOCSIS) in-
service. Traditional methods require briefly interrupting the QAM signal, in order to see any distortion
products mixed with the noise floor underneath. This is non-ideal in today's HFC preventive network
maintenance practices.
The EVS function helps find interference signals under the QAM signal without interrupting service. It works
two different ways: in the time domain, and in the frequency domain. The frequency domain function is
typically used to find interference signals of much narrower bandwidth than the QAM signal.
Error Vector Spectrum – Frequency Domain (left) and Time Domain (right)
However, when the interference is broadband in nature – meaning that its bandwidth is close or wider than
the QAM bandwidth (5MHz, or 10MHz in the case of LTE signals) – the time domain function is the right tool.
In recent years, the most common and disruptive broadband signals are cellular LTE signals that may creep
into an HFC network and interfere with QAM signals being transmitted over Cable at the same frequencies.
Frequently used LTE bandwidths include 5, 10, and 20MHz. These types of bandwidth are all close to or
wider than the DVB-C signal bandwidth. Both LTE and DVB-C are digital signals, and are similar to noise. The
S7200 uses the EVS time-domain function to display LTE sub-frames coming in every 1ms (above).
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