Transcript QAM Digital Measurements Seminar

Digital Power Measurements Demystified
Presented by
Sunrise Telecom Broadband …a
step ahead
Introduction



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
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.
Instruments such as signal level meters that are designed to
measure only analog carriers will not accurately measure
digital carriers.
Absolute Vs Relative Power



Power is measured either as an absolute level or relative to
another power level.
Carrier levels are absolute measurements and are measured
in power units such as dBmV.
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


Average power is the average of the power over a time
period.
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 X Current =
Power
Analog Video Power


The average power of an analog video carrier changes
depending on the picture content. Dark scenes have a
higher average power than bright scenes.
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
Sync
Average
Power
Video
content
Bright Scene
Dark Scene
Analog Video Power

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


On digital carriers average power is not affected by the
programming content and is relatively fixed.
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




Digital signals require a specific bandwidth to transmit
information.
The wider the bandwidth for a given peak power, the higher the
average power.
Bandwidth must be taken into account in the measurement of
digital signals.
An SLM designed for analog signals do not take into account
bandwidth.
Digital Carrier Shape

The frequency shape of the carrier also affects the average
power.
Digital Signal Average Power


Measuring digital signal power requires different
techniques than we are used to with analog signals.
Measurements must measure all of the power at all
frequencies within the channel, and reject any adjacent
channel power.
Power
Meter
Broadband
Signals
Bandpass Filter
Basic Digital Average Power
Measurement Setup Using Filter and
Power Meter.
Digital
Average
Power
Reading
Digital Signal Average Power Measurements



SLM or Spectrum Analyzers do not have an IF bandwidth
wide enough to measure a whole digital signal at once.
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.
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




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.
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.
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.
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




Digital power measurements are made
using average power while analog
signals are measured using peak
power.
Average digital carrier power is not
affected by content and the power is
spread fairly evenly over the channel.
Digital carrier power must be
measured over the entire bandwidth of
the channel rather than just at one
frequency as in analog channels.
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