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Mikrokontrollerid ja robootika kodutöö 4 (0)

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Read Chapter_4_Time_Based_Measurements.pdf


Question 1
When converting an analogue value to a frequency , consider the following diagram describing the system.
The frequency changes from 20 MHz to 18 MHz and the system samples at an interval of 2ms. How many counts does the microprocessor detect at,
  • 20 MHz?
  • 18 MHz?
    What is the difference in terms of number of counts detected by the microprocessor?
    Between 20MHz and 18MHz are 4000 counts.

    Question 2

    Consider the following diagram
    The frequency changes from 20 MHz to 18 MHz and the system samples at an interval of 100ns.
  • What is the difference in terms of number of counts detected by the microprocessor?
    Between 20MHz and 18MHz are 555,555 counts.
  • Explain why this method results in higher resolution.
    Higher resolution gained by increased sampling rate .
    Consider the following simple mixer circuit .
  • Why do we require a low- pass filter when using this mixer circuit?
    Mixer needs more parts
  • Name another disadvantage of such a mixer circuit.
    The mixer approach multiplies the frequency shift you want to measure , but also any other frequency shift. This includes drift caused by component heating, noise, etc.

    Question 4


    Consider the following circuit and input waveform.
    Draw an equivalent output waveform.
    Consider the following LM231 voltage to frequency converter (see LM331.pdf). Assume Rs = 17 kΩ. (Hint: Find the formula to calculate the output frequency fOUT and the values of Rs, RL, Rt, and Ct in LM331.pdf).
    Determine the output frequency (fOUT). a) Vin = 0.0836 V
    = 100kHz
  • Vin = 0.418 V
    = 500kHz
  • Vin = 0.836 V
    = =1000kHz

    Question 6


    One of the reasons for using time/frequency based measurements in microcontroller systems is to be able eliminate ground loops. Explain the unwanted ground loop that may occur in an electrical scheme with the help of a figure.
    Ground loop refers to an unwanted current in an electrical system. This is caused by a
    conductor that is connecting two points that are supposed to have same potential, but actually
    have different potentials, often ground. Noise and interference in electronics are mostly
    caused by falsely installed or created ground loops. Ground loop occurs when there are more
    than one gorund connection paths between many parts of equipment .
    Assume we have a period based measurement system as shown in the below figure. Design the sample clock frequency that we can measure the frequency input with the change from 5 MHz to 5.0001 MHz in 50 counts.
    5MHz / 10000 = 500Hz
    5.0001 MHz / 10000 = 500,01 Hz
    Fs = 1250 MHz
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