ANSI SCTE 257-2019 pdf free download – Techniques for Accurate Measurement of Cable TV Network Downstream RF Signal Levels.
For the example in Figure 5, five successive signal level measurements using the same test instrumentmight report +46.7 dBmV,+53.1 dBmV,+53.9 dBmV,+47.1 dBmV, and +54.0 dBmV.These fivemcasurements are neither precise nor accuratc.
6.3. Lab-Grade versus Field-Grade Measurement Devices
lt is important to choose the proper device for measuring RF signal levels.For instance, field-gradeinstruments are ideal for checking signal levels at the tap, ground block and input to CPE; measuringamplifier and node signal levels, and for routine signal level checks and adjustments in a headend or hub.
Manufacturers of headend and hub site signal sources such as CMTSs and QAM modulators use
specialized lab-grade test equipment to ensure that their products comply with published specifications.During manufacture, the signal sources are calibrated such that their configured RF output signal leveland their measured RF output signal level meet or exceed the specified power per channel absoluteaccuracy. In order to ensure the latter, the absolute measurement accuracy of the test equipment used bythe manufacturer must be better than the signal source’s specified power per channel absolute accuracy.
For example, if the signal source’s specified power per channel absolute accuracy is ±2 dB relative to theconfigured value, the test equipment used to calibrate the signal source must have an absolutely accuracybetter than +2 dB.Lab-grade instruments have accuracy specs on the order of士1 dB or less.
Field-grade measurement devices used by cable technical personnel have published absolute accuracyspecs that typically range from about土1.5 dB to 土2.5 dB, depending on make/model. A measurementdevice with1.5 dB absolute accuracy might be suitable for confirming whether a CMTS or QAMmodulator with+2 dB accuracy meets its own published spec, but instruments with+2 dB or 土2.5 dBabsolute accuracy would not be suitable in this example.
Some instruments support user-selectable detectors, and other instruments select the detector type
automatically without the need for user intervention.’ Incorrect detector type could impact the accuracy ofthe measurement results.Refer to the test equipment manufacturer’s instructions for recommendations ondetector selection (if applicable) for a given measurement and signal type.
6.4. A Closer Look at Factors Contributing to Measurement lnaccuracy
6.4.1. Signal source output signal level tolerance
The purpose of measuring a signal source ‘s output RF signal level or power is to confirm that it meets adesired value or perhaps some other specified value(e.g., meets the manufacturer’s published spec).
Modern QAM modulators, for instance, are required to comply with the technical parameters in the Data-Over-Cable Service Interface Specifications Downstream RF Interface Specification, also known as DRFI[2]. In particular, “Table 6 4 – DRFI Device Output Power,”requires power per channel absolute
accuracy to be土2 dB.That is, a DRFI-compliant modulator’s actual output power must be within+2 dBof the configured value.Equipment manufacturers often state compliance with DOCSIS R or DRFI specifications’ in published headend product data sheets.Amplifier and other active devices usually havepublished specs based upon typical intended use.Those specs can be adjusted by end users for specificdeployment scenarios (different channel loading, tilt, cascade depth, etc., compared to the assumptionsused in the manufacturer’s published specs).ANSI SCTE 257 pdf download.