Digital Power Measurements Demystified Presented by Sunrise Telecom Broadband … a step ahead

Introduction u Analog and digital carriers are very different in terms of the signal content and distribution of power over the channel and therefore need to be measured differently u The amount of distortion in a system is related to the total power of all of the carriers making accurate power measurements critical for optimum performance. u Instruments such as signal level meters that are designed to measure only analog carriers will not accurately measure digital carriers.

Absolute Vs Relative Power u Power is measured either as an absolute level or relative to another power level. u Carrier levels are absolute measurements and are measured in power units such as dBmV. u Examples of relative power measurements are C/N, delta audio to Video, CTB and CSO and these are always measured in dB. Relative Absolute

Average and Peak Power u Average power is the average of the power over a time period. u If the average power level varies such as in analog video, then the average needs to be over a long period of time to take into account the changes. X Voltage Current X = Power

Analog Video Power u The average power of an analog video carrier changes depending on the picture content. Dark scenes have a higher average power than bright scenes. u During the sync pulses the carrier is at its peak power and does not change from scene to scene. For this reason peak power has become the standard for analog video carrier level measurements. Peak Power Average Power Bright Scene Dark Scene Sync Video content

Analog Video Power u The vast majority of the power of a analog video signal is located right at the video carrier frequency, so analog carrier power measurements are only made at that one frequency. Peak Carrier Measured at Video Carrier Frequency

Digital Signal Power u On digital carriers average power is not affected by the programming content and is relatively fixed. u Unlike analog carriers, digital carrier power is spread fairly evenly across the entire channel, not just a one frequency. QAM Digital Carrier Power Not Effected by Programming Power Spread Across Channel

Average Power and Bandwidth u Digital signals require a specific bandwidth to transmit information. u The wider the bandwidth for a given peak power, the higher the average power. u Bandwidth must be taken into account in the measurement of digital signals. u An SLM designed for analog signals do not take into account bandwidth.

Digital Carrier Shape u The frequency shape of the carrier also affects the average power.

Digital Signal Average Power u Measuring digital signal power requires different techniques than we are used to with analog signals. u Measurements must measure all of the power at all frequencies within the channel, and reject any adjacent channel power. Broadband Signals Bandpass Filter Digital Average Power Reading Power Meter Basic Digital Average Power Measurement Setup Using Filter and Power Meter.

Digital Signal Average Power Measurements u SLM or Spectrum Analyzers do not have an IF bandwidth wide enough to measure a whole digital signal at once. u To correctly measure a digital carrier, SLMs or Spectrum Analyzers make multiple measurements across the frequency range of the carrier. The powers of each of these measurements is summed and the average power of the whole channel is calculated. u SLMs and Spectrum analyzers also correct for the shape factor or their IF filters to ensure a correct reading. SLM’s and Spectrum Analyzers Make Multiple Measurements Across the Channel and Sum the Power

Digital Carrier Levels Relative to Analog Carriers u Digital carriers are more robust than analog carriers making it more likely that the digital carriers will interfere with the analogs, rather than the other way around. u Typically digital signals are set between 6 to 12 dB below analog carriers when the analog carriers are measured using the standard peak method and the digital carriers are measured using the average method. u As long as the analog and digital signals are restricted to their own frequency ranges, differences as low as 6 dB should work fine and give good BER performance on the digital carrier. u Intermodulation products caused by digital signals typically look much like noise on analog carriers. If you find there is a poorer C/N on analog carriers than you would suspect, it’s possible the cause is digital carriers that are too high a level.

Conclusions u Digital power measurements are made using average power while analog signals are measured using peak power. u Average digital carrier power is not affected by content and the power is spread fairly evenly over the channel. u Digital carrier power must be measured over the entire bandwidth of the channel rather than just at one frequency as in analog channels. u Digital carriers are typically set between 6 and 12 dB below analog carriers with greater than 6 dB working in most situations. CM1000 Cable Modem System Analyzer AT2000 Spectrum Analyzer