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Subelement E7
PRACTICAL CIRCUITS
Section E7E
Modulation and demodulation: reactance, phase and balanced modulators; detectors; mixer stages
Which of the following can be used to generate FM phone emissions?
  • A balanced modulator on the audio amplifier
  • A reactance modulator on the oscillator
  • A reactance modulator on the final amplifier
  • A balanced modulator on the oscillator

It's the oscillator that is being modulated by changing the reactance to vary the frequency.

-KE0IPR

You can immediately eliminate two options: an audio amp does not by itself generate FM emissions, and the job of the final amp is to amplify a pre-modulated signal. The answer must be one of the options pertaining to an oscillator, but it’s less obvious which of these two remaining options is correct.

You know that the resonant frequency of a capacitive-inductive (i.g. RLC) oscillator is determined by its inductance and capacitance, so modulating reactance (by modulating inductance or capacitance) is one way to modulate frequency.

A balanced oscillator, on the other hand, is used to generate dual sideband carrier-suppressed signals, not FM. Read more about how this works: https://electronicspost.com/explain-the-generation-of-dsb-sc-signal-with-balanced-modulator-using-diodes/

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What is the function of a reactance modulator?
  • To produce PM signals by using an electrically variable resistance
  • To produce AM signals by using an electrically variable inductance or capacitance
  • To produce AM signals by using an electrically variable resistance
  • To 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?
  • By varying the tuning of a microphone preamplifier to produce PM signals
  • By varying the tuning of an amplifier tank circuit to produce AM signals
  • By varying the tuning of an amplifier tank circuit to produce PM signals
  • By varying the tuning of a microphone preamplifier to produce AM signals

The question is about an analog phase modulator. The answer is "By varying the tuning of an amplifier TANK circuit to produce PM signals"
Hint - Think PM=Phase Modulator.

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What is one way a single-sideband phone signal can be generated?
  • By using a balanced modulator followed by a filter
  • By using a reactance modulator followed by a mixer
  • By using a loop modulator followed by a mixer
  • By driving a product detector with a DSB signal

One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter.


Two methods of SSB Transmissions

A balanced oscillator produces a dual sideband carrier-suppressed (DSB-CS) signal. Filtering out one of the sidebands results in an SSB signal.

Only one answer has the word "filter" in it.

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What circuit is added to an FM transmitter to boost the higher audio frequencies?
  • A de-emphasis network
  • A heterodyne suppressor
  • An audio prescaler
  • A pre-emphasis network

In telecommunications emphasis is the intentional alteration of the amplitude-vs.-frequency characteristics of the signal to reduce adverse effects of noise in a communication system.

The whole system of pre-emphasis and de-emphasis is called emphasis.

The high-frequency signal components are emphasized to produce a more equal modulation index for the transmitted frequency spectrum, and therefore a better signal-to-noise ratio for the entire frequency range.

Pre-emphasis typically happens during transmission and de-emphasis typically happens during reception.

Source: Wikipedia

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Why is de-emphasis commonly used in FM communications receivers?
  • For compatibility with transmitters using phase modulation
  • To reduce impulse noise reception
  • For higher efficiency
  • To remove third-order distortion products

De-emphasis means attenuating higher received frequencies by the amount by which they were boosted prior to transmission. The purpose is to improve signal-to-noise ratio.

From: http://www.daenotes.com/electronics/communication-system/pre-emphasis-and-de-emphasis


Phase modulation (PM) inherently emphasizes higher frequencies at 6 dB/octave, requiring PM receivers to de-emphasize the signal upon reception. In the early days of frequency modulation (FM), common PM receivers with in-built de-emphasis were sometimes re-purposed for FM. Artificially adding emphasis to FM signals prior to transmission made PM receivers more easily converted for FM, with the side benefit of substantially improving signal-to-noise ratio for FM.

http://www.repeater-builder.com/tech-info/pdemph-post.html

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What is meant by the term baseband in radio communications?
  • The lowest frequency band that the transmitter or receiver covers
  • The frequency components present in the modulating signal
  • The unmodulated bandwidth of the transmitted signal
  • 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. So, while a 2 meter FM ham transceiver might have deviation of +/-5kHz and while that appears to be 10kHz bandwidth, the rate of change of frequency comprising the modulation means the actual bandwidth is probably up to 16kHz, all of this despite the fact that it probably has a baseband component of 5kHz or less, which is the actual modulating audio from zero to no more than five kHz.

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What are the principal frequencies that appear at the output of a mixer circuit?
  • Two and four times the original frequency
  • The sum, difference and square root of the input frequencies
  • The two input frequencies along with their sum and difference frequencies
  • 1.414 and 0.707 times the input frequency

The sum and difference frequencies are the frequencies added together and subtracted apart, i.e 10kHz + 15 kHz will mix to 25kHz and 5kHz

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What occurs when an excessive amount of signal energy reaches a mixer circuit?
  • Spurious mixer products are generated
  • Mixer blanking occurs
  • Automatic limiting occurs
  • A beat frequency is generated

Radio Frequency (RF) mixer circuits operate at radio wave frequencies and are designed for some maximum amount of input signal.

Think of what happens when you yell into a microphone on SSB - your voice won't sound louder, it will sound distorted. In this case, the spurious products are in the audio range. This is an example of an excessive amount of signal from the microphone going into an audio amplifier.

In an RF mixer, the effect is to generate mixer products which are not linear representations of the input signal. These are called "spurious mixer products" and can be at audio and RF frequencies.

So, we don't want the maximum amount of RF signal applied to a mixer, because it might exceed the design parameters and could generate spurious mixer products. We want the input to the mixer to be something that won't generate spurious mixer products.

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How does a diode detector function?
  • By rectification and filtering of RF signals
  • By breakdown of the Zener voltage
  • By mixing signals with noise in the transition region of the diode
  • 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

(mvs90)

Hint: Remember Rectification

kg5kou

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Which type of detector is used for demodulating SSB signals?
  • Discriminator
  • Phase detector
  • Product detector
  • Phase comparator

How to remember it, is both the question and answer have the word "detector", and you are detecting a product.

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What is a frequency discriminator stage in a FM receiver?
  • An FM generator circuit
  • A circuit for filtering two closely adjacent signals
  • An automatic band-switching circuit
  • A 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).

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