Digital Measurement 1

8/8/2019 Digital Measurement 1

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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

Digital Measurement 1

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