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y Frequency Measurement
y
Fixed Time methody Fixed Cycle method
y Power Measurement
Contents
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Frequency Measurement
Fixed Time methodCounting the number of pulses within a specific time
(t) window, such as 100 ms. The frequency is then
calculated by dividing the count by time (t).
This measurement approach inherently minimizes the
effect of error sources (such as jitter) by averaging many
oscillator pulses in the time window.
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Example:Measure the number of oscillation pulses in a 100 msec.
window and multiply by 10 to determine the frequency.
Algorithm
Count the number ofinput pulses in a time window.
F=
Counter ValueTime
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Microcontroller's Timer/ counter can be used to measurefrequencies by configuring one timer as a counter and
using the second timer to generate a timing interval over
which the first can count. The frequency of the countedpulse train is then,
Unknown Frequency = Count
Time
Hardware Implementation
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TimerClockSystem Clock
CounterClockFrequency Input
INT
Frequency Input
INT t
Count
Frequency = Count
t
Block diagram
System Clock
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Fixed Cycle method
In the fixed cycle method, the number of cycles
measured is fixed and the measurement time is
variable.
The counter is started at the rising edge of the
input signal and stopped at the next rising
edge. Now the counter reading is actually the
period of the input signal in multiples of the
master clock.
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Algorithm
Determine the time between a fixed number ofoscillation pulses.
Example: Measure time between four rising edges
of the oscillation signal.
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System Clock CounterClock
INT
Frequency Input
INT
Count
Block diagram
System Clock
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Sine wave input
Schmitt trigger can be used for converting a
sine wave to square wave signal.
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y The accuracy of the frequency measurement is
directly related to the quality of the
microcontrollers clock signal.
y Increasing the number of cycles measured will
increase the measurement time and reduce the error.
y Increasing Fosc will also decrease the minimum time
to resolve an edge, thereby reducing the error.
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y 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
yInstruments such as signal level meters that aredesigned to measure only analog carriers will not
accurately measure digital carriers.
Power Measurement
Introduction
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y Power is measured either as an absolute level orrelative to another power level.
y Carrier levels are absolute measurements and are
measured in power units such as dBmV.
Relative
Absolute
Absolute Vs Relative Power
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Average and Peak Power
y Average power is the average of the power over atime period.
y If the average power level varies such as inanalog video, then the average needs to be over
a long period of time to take into account thechanges.
X
Voltage CurrentX = Power
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Analog Video Power
y The average power of an analog video carrier changes
depending on the picture content. Dark scenes have ahigher average power than bright scenes.
y During the sync pulses the carrier is at its peak powerand does not change from scene to scene. For this
reason peak power has become the standard for analogvideo carrier level measurements.
Peak
Power
AveragePower
Bright Scene Dark Scene
Sync
Videocontent
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Digital Signal Power
y O
n digital carriers average power is not affected by theprogramming content and is relatively fixed.
y Unlike analog carriers, digital carrier power is spreadfairly evenly across the entire channel, not just a one
frequency.
QAM Digital Carrier
Power Not Effected byProgrammingPower Spread Across Channel
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Average Power and Bandwidth
y Digital signals require a specific bandwidth to transmitinformation.
y The wider the bandwidth for a given peak power, thehigher the average power.
yBandwidth must be taken into account in themeasurement of digital signals.
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Digital CarrierShape
y The frequency shape of the carrier also affects
the average power.
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Digital Signal Average Powery Measuring digital signal power requires different
techniques than we are used to with analog signals.
y Measurements must measure all of the power at all
frequencies within the channel, and reject any adjacent
channel power.
BroadbandSignals
BandpassFilter
DigitalAveragePowerReading
PowerMeter
Basic Digital Average Power Measurement SetupUsing Filter and Power Meter.
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Digital Signal Average Power
Measurementsy SLM or Spectrum Analyzers do not have an IF
bandwidth wide enough to measure a whole digitalsignal at once.
yTo correctly measure a digital carrier, SLMs or SpectrumAnalyzers make multiple measurements across thefrequency range of the carrier. The powers of each ofthese measurements is summed and the average powerof the whole channel is calculated.
SLMs and Spectrum Analyzers Make Multiple Measurements Across the
Channel and Sum the Power
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Conclusions
y Digital power measurements are madeusing average power while analogsignals are measured using peakpower.
y Average digital carrier power is notaffected by content and the power isspread fairly evenly over the channel.
y Digital carrier power must bemeasured over the entire bandwidth ofthe channel rather than just at one
frequency as in analog channels.
CM1000 Cable
Modem System
Analyzer
AT2000
Spectrum
Analyzer
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y The 8051 microcontroller, Kenneth J Ayala
y AN929, Temperature Measurement Circuits for
Embedded Applications , Jim Lepkowski
Microchip Technology Inc.
y http://www.techonline.com/showArticle.jhtml?art
icleID=192200826
References