PRACTICAL CIRCUITS
Modulation and demodulation: reactance, phase and balanced modulators; detectors; mixer stages; DSP modulation and demodulation; software defined radio systems
Which of the following can be used to generate FM phone emissions?
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A balanced modulator on the audio amplifier
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Correct AnswerA reactance modulator on the oscillator
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A reactance modulator on the final amplifier
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A balanced modulator on the oscillator
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What is the function of a reactance modulator?
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To produce PM signals by using an electrically variable resistance
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To produce AM signals by using an electrically variable inductance or capacitance
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To produce AM signals by using an electrically variable resistance
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Correct AnswerTo produce PM signals by using an electrically variable inductance or capacitance
Remember that reactance has to do with inductors and capacitors, not resistors. This eliminates two of the distractors.
Inductance/reactance affect the phase (PM) of a wave, not the amplitude, which eliminates the final distractor.
Be aware that this question is a little misleading in that phase modulation (PM) is not the only use of a reactance modulator. Reactance modulation is also one way to modulate frequency, per the previous question.
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How does an analog phase modulator function?
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By varying the tuning of a microphone preamplifier to produce PM signals
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By varying the tuning of an amplifier tank circuit to produce AM signals
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Correct AnswerBy varying the tuning of an amplifier tank circuit to produce PM signals
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By varying the tuning of a microphone preamplifier to produce AM signals
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What is one way a single-sideband phone signal can be generated?
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Correct AnswerBy using a balanced modulator followed by a filter
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By using a reactance modulator followed by a mixer
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By using a loop modulator followed by a mixer
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By driving a product detector with a DSB signal
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What circuit is added to an FM transmitter to boost the higher audio frequencies?
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A de-emphasis network
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A heterodyne suppressor
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An audio prescaler
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Correct AnswerA pre-emphasis network
Emphasis is the boosting of lower-power frequencies to improve the signal-to-noise ratio.
Typically, higher-frequency components of a signal are "pre-emphasized" before transmission in order to produce a more equal modulation index, and therefore a better signal-to-noise ratio for the entire frequency range.
On reception, the signal is "de-emphasized" to recover the original power distribution.
Source: Wikipedia
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Why is de-emphasis commonly used in FM communications receivers?
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Correct AnswerFor compatibility with transmitters using phase modulation
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To reduce impulse noise reception
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For higher efficiency
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To remove third-order distortion products
This is a tricky subject to understand. Read thoroughly.
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What is meant by the term baseband in radio communications?
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The lowest frequency band that the transmitter or receiver covers
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Correct AnswerThe frequency components present in the modulating signal
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The unmodulated bandwidth of the transmitted signal
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The basic oscillator frequency in an FM transmitter that is multiplied to increase the deviation and carrier frequency
Baseband is commonly used to indicate the range (band) of source frequencies used to modulate the transmitted signal.
Generally, a transmission signal contains more than a single frequency. This is to say that there might be several different frequencies linked together or superimposed on each other.
Baseband refers to the original frequency range of a transmission signal before it is converted, or modulated, to a different frequency range. For example, an audio signal may have a Baseband range from 20 to 20,000 hertz. When it is transmitted on a radio frequency (RF), it is modulated to a much higher, inaudible, frequency range. Most telecommunication protocols require original Baseband signals to be modulated to a higher frequency before they are transmitted. These signals are then demodulated at the destination, so the recipient receives the original baseband signal.
Hint: The point prior to modulation
Another Hint: In the question it mentions radio communications so the answer would be the only one with message in it. Communications=message
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What are the principal frequencies that appear at the output of a mixer circuit?
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Two and four times the original frequency
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The sum, difference and square root of the input frequencies
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Correct AnswerThe two input frequencies along with their sum and difference frequencies
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1.414 and 0.707 times the input frequency
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What occurs when an excessive amount of signal energy reaches a mixer circuit?
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Correct AnswerSpurious mixer products are generated
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Mixer blanking occurs
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Automatic limiting occurs
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A beat frequency is generated
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How does a diode detector function?
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Correct AnswerBy rectification and filtering of RF signals
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By breakdown of the Zener voltage
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By mixing signals with noise in the transition region of the diode
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By sensing the change of reactance in the diode with respect to frequency
A Diode Detector is the simplest way of demodulating AM signals. It operates by detecting the envelope of the incoming signal which it does by rectifying the signal. Current is allowed to flow through the diode in only one direction, giving either the positive or negative half of the envelope at the output.
The AM detector or demodulator includes a capacitor at the output. Its purpose is to remove any radio frequency components of the signal at the output (Read Filtering). The value is chosen so that it does not affect the audio base-band signal.
Source: Radio-electronics.com - Diode detector basics
Hint: Remember Rectification
Memory Tip: Envelopes are RECTangles. You filter through mail envelopes.
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Which of the following types of detector is well suited for demodulating SSB signals?
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Discriminator
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Phase detector
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Correct AnswerProduct detector
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Phase comparator
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What is a frequency discriminator stage in a FM receiver?
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An FM generator circuit
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A circuit for filtering two closely adjacent signals
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An automatic band-switching circuit
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Correct AnswerA circuit for detecting FM signals
FM stands for Frequency Modulation and is a method of sending an audio signal by modulating the frequency of the signal. In order to convert that signal back to regular audio the receiver needs something that can discriminate (or detect) the changes in frequency.
Thus, the circuit that detects the FM signals is called a Frequency Discriminator (or sometimes Frequency Detector).
Hint: "Discriminator" and "Detecting" both start with the letter "D" -KC1OHT
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Which of the following describes a common means of generating an SSB signal when using digital signal processing?
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Mixing products are converted to voltages and subtracted by adder circuits
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A frequency synthesizer removes the unwanted sidebands
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Emulation of quartz crystal filter characteristics
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Correct AnswerThe quadrature method
SSB has the mathematical form of quadrature amplitude modulation (QAM) in the special case where one of the baseband waveforms is derived from the other.
Hint: The only answer with "Signal(s)" in it as "Signal" is in the question.
https://en.wikipedia.org/wiki/Single-sideband_modulation#Mathematical_formulation
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What is meant by direct conversion when referring to a software defined receiver?
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Software is converted from source code to object code during operation of the receiver
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Incoming RF is converted to the IF frequency by rectification to generate the control voltage for a voltage controlled oscillator
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Correct AnswerIncoming RF is mixed to "baseband" for analog-to-digital conversion and subsequent processing
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Software is generated in machine language, avoiding the need for compilers
In state of the art SDR Radios, RF is received and sent directly to an analog-to-digital (A/D) converter. In other words the RF is digitized and processed by digital circuits from that point.
There are no mixers. local oscillators or intermediate frequencies, avoiding distortion and other undesired effects of mixing. This is a major benefit of SDR receivers.
Direct conversion, RF to digital is the modern trend in amateur radio receivers and transmitters.
HINT: The correct answer is the only one with language about digital. i.e analog-to-digital and digitized
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