Communication Systems

ETE 335

Course Description:

        Introduction to periodically gated, amplitude, single sideband, frequency and phase modulation methods involved in communications systems. Introduction to digital modulation communication techniques.

Lab RL LP, RC HP, and Series and Parallel LC circuits        1

Electronic Communication Oscillators        1

RMS Signal Values        3

Rigol Spectrum Analyzer Fourier series lab        4

Specialized Waveforms Fourier Spectrum and the Scope FFT analyzer        5

Using PSpice with Fourier Analysis And Excel with FFT        6

Amplitude Modulation Lab        7


FM – Frequency Modulation        9

Lab RL LP, RC HP, and Series and Parallel LC circuits

In this lab we worked with basic filters, low-pass, high-pass, and band-pass. Testing them at different frequencies to see how they operate. We first began with first order low pass LR(inductor/resistor) and RC(resistor/capacitor) circuits. The purpose was to find their critical frequency fc; which is where the phase is at 45 degrees, as well as to record common data about the circuit. We used this data to graph the transfer functions vs frequency, phase vs frequency. We than tested high pass and bind band circuits measured and recorded then analysed the data like before. We noticed how the low pass filters only allow frequencies below about 1 MHz, the high pass roughly above 1 MHz, and the band-pass filter allowed 400kHz above and below the cutoff frequency of 1MHz.

Electronic Communication Oscillators

        We modified the bubba oscillator circuit, using four RC filters and four gain amplifiers to create a 360 degree phase shift from dc. We use bipolar op amps in a quad package, two potentiometers for the gain adjustment and DC bias, as well as four low pass RC circuits. We set the gain and bias to give us a positively amplification on the first op amp. This voltage is than passed through the first LP RC filter and to the op amp which if the voltage is positive enough it than switches to negative and shifts 45 degrees to counter the positiveness. We than pass that negative voltage to the next LP RC filter and op amp at which it goes back to positive and shifts another 45 degrees. We do this again giving us a negative and another 45 degree shift. This than feeds back into the negative of the initial op amp giving us a positive voltage and repeating the cycle thus we get our oscillation.

RMS Signal Values

This lab is to show us how the RMS values are calculated and measured for different types of waves. Including the Sine,Triangle, Square, and Pulse waves, we did this using the waveform generator and the oscilloscope to view the waves.





DC offset


DC offset


w/offset =

w/o offset =



Symmetrical AC sine wave with an amplitude of 2Volts

Symmetrical AC square wave with an amplitude of 2Volts and a DC offset of +2Volts

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Symmetrical AC triangle wave with an amplitude of 2Volts

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Rigol Spectrum Analyzer Fourier series lab

In this lab we learned about the Rigol Spectrum Analyzer, by analyzing a sine, square, triangle, and pulse at varying duty cycles. We say how the spectrum analyzer graphs voltage vs frequency. We learned were looking for the harmonics of the waves in which we saw that sinusoidal waves have 1 fundamental harmonic, square waves are at every odd harmonic, triangle are at every harmonic except every third, pulse has harmonics at every one but the multiple of 1 over the duty cycle.









Specialized Waveforms Fourier Spectrum and the Scope FFT analyzer

        In this lab we took a look at FFT, Fast Fourier transform, an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse. Fourier analysis converts a signal from its original domain (often time or space) to a representation in the frequency domain and vice versa. Using the Scope’s FFT function we took a look at sinusoidal, triangle, square, and arb waves. We notice it is similar to that of the spectrum analyzer however not as good of quality, though still reliable enough if a spectrum analyzer is not able to be used. We also listened to the sound that each wave produces.


Using PSpice with Fourier Analysis And Excel with FFT

In this lab we took another look at FFT, Fast Fourier Transform. Fourier analysis converts a signal from its original domain (often time or space) to a representation in the frequency domain and vice versa. We simulated basic circuit with a resistor and source, measuring the voltage of the resistor in the time domain. We used a sinusoidal nada triangle source. In PSpice we did a time sweep to get one period of the wave. We than used the built in FFT function to view the voltage in the frequency domain. Next we created a excel document in which we generated voltage values for a set of time values where the voltage was that of a sine wave. We than ran FFT with in excel to see the results in which we only saw one harmonics as to be expected.

Amplitude Modulation Lab

In this lab it was our goal to understand and be able to explain/show the relationship of all AM signal variables Ec, Fc, Em, and Fm in time and frequency domains. When changing Ec, the voltage of the carrier, it changes the amplitude of our carrier. When changing the Fc, The frequency of the carrier,  it changes the time of the carrier represented with Tc=1/fc. Depending on the frequency it causes the gap( within the time domain) to grow or shrink horizontally. When EM, the voltage of the modulation changes it causes the ma = EM/Ec to change thus giving us a flatter modulation. As ma gets lower the modulated signal seems to get weaker. When Fm, Frequency of the modulation changes it causes the time of the modulation to changing the period of the modulated signal.DS2_QuickPrint1.png







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A mixer is a type of device that does what it is called “mixes”. It combines two signals together by multiplying them, getting a sum and difference.  Which results in two signals our LO (local oscillator) combined with our RF (our selected radio frequency). In a sum of the two and a difference LO - RF. Though because of this there is an issue that develops which is the image, this happens when The RF - LO equals what what we are intending the difference LO - RF equals, this causes an issued that in order to fix we have a filter apply to the RF antenna that blocks out RF signals above the LO frequency.

FM – Frequency Modulation

Frequency modulation is the the act of storing data within the frequency domain, in order to do this we use a VCO, a voltage controlled oscillator similar to a mixer but instead of multiplying a the information with another signal. The VCO converts the amplitude of the information signal into a frequency change of the Carrier thus giving us a signal like the modulated result below. Side bands are a result of the modulation factor, we use Bessel chart which determines the 1 % bandwidths of the FM signal. 1 % bandwidth means we only look at all the bandwidths that have a amplitude one percent or more than the carrier amplitude.