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Subel. E7
18 questions
Amateur Extra Class (2012-2016)
Subelement E7

PRACTICAL CIRCUITS

Section E7H

Oscillators and signal sources: types of oscillators; synthesizers and phase-locked loops; direct digital synthesizers

What are three oscillator circuits used in Amateur Radio equipment?

  • Taft, Pierce and negative feedback
  • Pierce, Fenner and Beane
  • Taft, Hartley and Pierce
  • Correct Answer
    Colpitts, Hartley and Pierce

Colpitts, Hartley, and Pierce oscillators are three common oscillator designs used to generate a stable signal in radio circuits.

  • Colpitts oscillator : It uses a capacitor divider to create feedback, with inductors and capacitors in the resonant circuit.
  • Hartley oscillator: It uses an inductor (with a tapped coil) for feedback, allowing it to generate a range of frequencies.
  • Pierce oscillator: It is a variant of the Colpitts, typically using a crystal to stabilize the frequency for accurate signal generation.

These designs are chosen based on their stability, ease of implementation, and frequency characteristics in RF applications.

How to rule out the distractors:

  • Taft: There is no widely recognized oscillator called "Taft."
  • Negative feedback: While it's a technique used in amplifiers, it is not an oscillator circuit.
  • Fenner and Beane: These are not recognized names for oscillators, they are probably just made up.

Memory tip: the correct answer starts with "C"

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What condition must exist for a circuit to oscillate?

  • It must have at least two stages
  • It must be neutralized
  • Correct Answer
    It must have positive feedback with a gain greater than 1
  • It must have negative feedback sufficient to cancel the input signal
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How is positive feedback supplied in a Hartley oscillator?

  • Correct Answer
    Through a tapped coil
  • Through a capacitive divider
  • Through link coupling
  • Through a neutralizing capacitor

The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by a tuned circuit consisting of capacitors and inductors, that is, an LC oscillator. The circuit was invented in 1915 by American engineer Ralph Hartley. The distinguishing feature of the Hartley oscillator is that the tuned circuit consists of a single capacitor in parallel with two inductors in series (or a single tapped inductor), and the feedback signal needed for oscillation is taken from the center connection of the two inductors.

http://en.wikipedia.org/wiki/Hartley_oscillator

Test Tip: Hartley is pretty close to Harley, which are notorious for leaking oil. Coil is pretty close to oil.

Test Tip: Hartley has "tl" in it. Tapped L

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How is positive feedback supplied in a Colpitts oscillator?

  • Through a tapped coil
  • Through link coupling
  • Correct Answer
    Through a capacitive divider
  • Through a neutralizing capacitor

http://en.wikipedia.org/wiki/Colpitts_oscillator

Think C for Colpitts and Capacitor, and voltages supplied by dividers.

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How is positive feedback supplied in a Pierce oscillator?

  • Through a tapped coil
  • Through link coupling
  • Through a neutralizing capacitor
  • Correct Answer
    Through a quartz crystal

The Pierce oscillator is a type of electronic oscillator particularly well-suited for use in piezoelectric crystal oscillator circuits. Named for its inventor, George W. Pierce (1872-1956), the Pierce oscillator is a derivative of the Colpitts oscillator.

Components: a single digital inverter, two resistors, two capacitors, and the quartz crystal, which acts as a highly selective filter element.

http://en.wikipedia.org/wiki/Pierce_oscillator

Memory aids:

  • Pierce = Piezo
  • Don't get pierced with a quartz crystal.
  • "Pierced" ears have crystal earrings.

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Which of the following oscillator circuits are commonly used in VFOs?

  • Pierce and Zener
  • Correct Answer
    Colpitts and Hartley
  • Armstrong and deForest
  • Negative feedback and balanced feedback

A Colpitts oscillator is a type of LC oscillator, so called because it consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C. It mimics a Hartley oscillator, in that the inductive voltage divider from a pair of coils, or a tapped coil is fed into the LC circuit.

by kd0swn

Hint: Colpitts and Hartley is the only answer with two types of oscillators. A Zener is a diode voltage regulator, Armstrong and De Forest were inventors and all oscillators employ some sort of negative feedback.

Memory tip: ...oscillator circuits are commonly... and Colpitts in the correct answer

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What is a magnetron oscillator?

  • An oscillator in which the output is fed back to the input by the magnetic field of a transformer
  • A crystal oscillator in which variable frequency is obtained by placing the crystal in a strong magnetic field
  • Correct Answer
    A UHF or microwave oscillator consisting of a diode vacuum tube with a specially shaped anode, surrounded by an external magnet
  • A reference standard oscillator in which the oscillations are synchronized by magnetic coupling to a rubidium gas tube

A UHF or microwave oscillator consisting of a diode vacuum tube with a specially shaped anode, surrounded by an external magnet.

Hint: Magnetrons are used in microwaves (kitchen appliance).

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What is a Gunn diode oscillator?

  • Correct Answer
    An oscillator based on the negative resistance properties of properly-doped semiconductors
  • An oscillator based on the argon gas diode
  • A highly stable reference oscillator based on the tee-notch principle
  • A highly stable reference oscillator based on the hot-carrier effect

Wikipedia.org - Gunn diode link:

http://en.wikipedia.org/wiki/Gunn_diode

Easy to remember Guns and dope go together.

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What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter?

  • Correct Answer
    A direct digital synthesizer
  • A hybrid synthesizer
  • A phase locked loop synthesizer
  • A diode-switching matrix synthesizer
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What information is contained in the lookup table of a direct digital frequency synthesizer?

  • The phase relationship between a reference oscillator and the output waveform
  • Correct Answer
    The amplitude values that represent a sine-wave output
  • The phase relationship between a voltage-controlled oscillator and the output waveform
  • The synthesizer frequency limits and frequency values stored in the radio memories
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What are the major spectral impurity components of direct digital synthesizers?

  • Broadband noise
  • Digital conversion noise
  • Correct Answer
    Spurious signals at discrete frequencies
  • Nyquist limit noise

Just Remember Triple S. Spectral Synthesizers Spurious

Al

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Which of the following is a principal component of a direct digital synthesizer (DDS)?

  • Phase splitter
  • Hex inverter
  • Chroma demodulator
  • Correct Answer
    Phase accumulator
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What is the capture range of a phase-locked loop circuit?

  • Correct Answer
    The frequency range over which the circuit can lock
  • The voltage range over which the circuit can lock
  • The input impedance range over which the circuit can lock
  • The range of time it takes the circuit to lock
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What is a phase-locked loop circuit?

  • An electronic servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator
  • An electronic circuit also known as a monostable multivibrator
  • Correct Answer
    An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator
  • An electronic circuit consisting of a precision push-pull amplifier with a differential input

A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. While there are several differing types, it is easy to initially visualize it as an electronic circuit consisting of a variable frequency oscillator and a phase detector.

More information....

Hint 1: The correct answer is the only one with "loop" and "phase" in the answer.

Hint 2: A "phase-locked loop" has to "detect" the phase to lock it up. The answer has "phase detector"

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Which of these functions can be performed by a phase-locked loop?

  • Wide-band AF and RF power amplification
  • Comparison of two digital input signals, digital pulse counter
  • Photovoltaic conversion, optical coupling
  • Correct Answer
    Frequency synthesis, FM demodulation
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Why is the short-term stability of the reference oscillator important in the design of a phase locked loop (PLL) frequency synthesizer?

  • Any amplitude variations in the reference oscillator signal will prevent the loop from locking to the desired signal
  • Correct Answer
    Any phase variations in the reference oscillator signal will produce phase noise in the synthesizer output
  • Any phase variations in the reference oscillator signal will produce harmonic distortion in the modulating signal
  • Any amplitude variations in the reference oscillator signal will prevent the loop from changing frequency

Hint: "Synthesizer" in the question and "Synthesizer" in the answer.

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Why is a phase-locked loop often used as part of a variable frequency synthesizer for receivers and transmitters?

  • It generates FM sidebands
  • It eliminates the need for a voltage controlled oscillator
  • Correct Answer
    It makes it possible for a VFO to have the same degree of frequency stability as a crystal oscillator
  • It can be used to generate or demodulate SSB signals by quadrature phase synchronization

REF: FUNDAMENTALS OF PLL

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What are the major spectral impurity components of phase-locked loop synthesizers?

  • Correct Answer
    Phase noise
  • Digital conversion noise
  • Spurious signals at discrete frequencies
  • Nyquist limit noise

A phase-locked loop uses an oscillator to provide the source-clock for state changes. The stability of that clock has a significant effect on the noise and when you consider the type of instability you'd expect in an oscillator used as a square wave source clock for a PLL the noise isn't going to be voltage shift in the clock, it's going to be slight early or late switches from high to low values in the clock and vice-versa. Viewed in the frequency domain that appears to be slight frequency drift in the clock giving the clock and PLL an output signal that isn't a single frequency but has a small bandwidth of frequencies (noisy).

Viewed in the time domain then because it's a square wave clock that noise appears to be phase-drift (early or late phase changes) in the clock and it is known as phase-noise.

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