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Exam 2 - RF Communications
For proper credit show all formulas and all work; if you make an assumption note it in your work. All appropriate
answers should be denoted in a box, circled, or marked in a manner it is clear that is your final answer. In this
exam you will be tested on your understanding of communication system concepts such as, transfer functions, waveforms,
frequency spectra, bandwidth principles, Bessel functions, wide band and narrow band properties, filter components,
nonlinear devices, image frequency, spurs, and noise figure calculations.
1. Given the following voltage controlled oscillator (VCO) transfer function data graphics depicting the time and
frequency domains, plot the output voltage frequency spectrum. Also, answer the following:
b) Based on a
Bessel Table
, what is the required bandwidth for this signal?
c) Is this a Wideband Frequency Modulation (WBFM) or a Narrowband Frequency Modulation (NBFM) system?
2. The radio frequency (RF) block diagram system in the figure below shows an antenna, band-pass filter, a mixer,
and an amplifier. Based on the variable specifications provided in the figure calculate the following:
a) Find the input radio-frequency at the antenna, fRF assuming high side injection.
b) Determine what the image frequency is, fimage.
c) Find the center frequency and shape factor of the bandpass filter to attenuate the image frequency by 60dB.
d) In the 3dB pass band of the bandpass filter, is there any 3rd order or 4th order
Spurs
present at the image frequency? If so, list the frequencies and show your
work how you came to this conclusion.
3. For a frequency modulated (FM) modulator with a 40KHz frequency deviation and a
modulating signal frequency Fmod = 10KHz, determine the bandwidth using a
Bessel Table
and Carson’s Rule.
4. Given the radio frequency system block diagram in the figure below comprised of an RF signal generator,
two amplifier stages, a mixer stage, and a local oscillator, and tied to an output load resistance of 50 ohm;
determine the following:
a) Find the receiver systems Noise Figure (NF)?
b) With the input of the receiver connected to the RF generator, which is set to a value of -50dBm, find
the RMS voltage across the 50 ohm load resistor?
Solutions: Provided in the Exam Notes Calculations Section below.
Engineering key words: Frequency modulation, spurs, narrowband, wideband, antenna, isotropic, attenuation,
harmonics, image frequency, mirrored, shape factor, period, pulse width, waveform, RF, duty cycle, noise figure,
gain, power loss, power gain, computational techniques, spectrum, time domain, frequency domain, voltage, log,
resistance, pulse shape, VCO, voltage controlled oscillator, bandwidth, transfer function, Bessel Function, tables,
band-pass filter, linear, nonlinear, devices, specifications, specs, FLO, FIF, FRF, high side injection, low side injection,
gigahertz, megahertz, center frequency, 3dB, third order, forth order, IF, roll off, transmitter, receiver, transceiver,
link budget, spurious, predicted, products, down-converter, up-converter, circuit configuration, FM, AM, modulator,
deviation, deflection, Carson Rule, RF generator, conversion loss, KTB, RMS.
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