ANSI SCTE 257:2019 pdf free download – Techniques for Accurate Measurement of Cable TV Network Downstream RF Signal Levels
6. Downstream Signal Level Measurements
What is signal level? As stated in 5.2 Definitions , signal level is “The amplitude of an RF signal, specifically the power of that signal.” In the case of an analog TV signal’s visual carrier, signal level is the carrier’s peak envelope power (PEP). The signal level of a quadrature amplitude modulation (QAM) signal or other digital signal is its average power (integrated over the signal’s occupied bandwidth), also called digital channel power or digital signal power. Orthogonal frequency division multiplexing (OFDM) power is expressed in average power per CTA channel (that is, power per 6 MHz) as well as total power. 2 Signal level in cable networks is expressed using decibel millivolt (dBmV), where dBmV is a unit of power expressed in terms of voltage. 3 It is incorrect to state signal level using just decibel (dB), because by itself dB is a “logarithmic-based expression of the ratio between two values of a physical quantity such as power.” For example, one can correctly say that a two-way splitter’s nominal insertion loss is 3.5 dB, or the gain of an amplifier is 20 dB, but not something like “the per-channel input to the modem is 2 dB.” For the latter, the correct expression is “the per-channel input to the modem is 2 dBmV.”
The first component, the signal source, provides an RF signal (or signals) whose level is an assumed or perhaps unknown value. The second component, the interconnection, could be something as simple as a short length of coaxial cable, or may be more complex (e.g., headend combining/splitting network) and include a combination of gain and loss. The third component is the measurement device, the test instrument used to measure the RF signal level. All three components shown in the figure have an impact in one or more ways on the outcome of signal level measurement. Among the major factors that affect the measurement results are the signal source’s accuracy and stability, and depending on the nature of the source, its calibration; the net attenuation in the interconnection; and the measurement device’s accuracy and calibration. Other factors such as impedance mismatches, frequency response flatness, and temperature also have an effect. Indeed, the latter factors can have a significant impact on the measurement results. The signal level present at the measurement device is the net signal level, which is the difference between the signal source output signal level and the net attenuation through the interconnection. In the following sections, RF power is intended to mean the same thing as RF signal level. The following table summarizes typical ranges of downstream per-channel RF signal levels that can be expected in various parts of a cable network. The actual values in any given cable network can be different from what is shown here.