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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
• Correct Answer
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
• Correct Answer
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
• Correct Answer
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"

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What is one way a single-sideband phone signal can be generated?
• Correct Answer
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.

Balanced Modulator (both side bands) + Filter (filter out 1 side band) = Single Side Band

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.

— mrdfox7

Hint: Think "Single Side Balanced"

— serif

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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
• Correct Answer
A 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?
• Correct Answer
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.

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.

Hint: Question has "commonly" and "communications".. correct answer is the one that also has a "c" word - "compatibility"

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
• Correct Answer
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.

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.

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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
• Correct Answer
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?
• Correct Answer
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?
• Correct Answer
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.

(mvs90)

Hint: Remember Rectification

kg5kou

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Which type of detector is used for demodulating SSB signals?
• Discriminator
• Phase detector
• Correct Answer
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
• Correct Answer
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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