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Subelement E7

PRACTICAL CIRCUITS

Section E7H

Oscillators and signal sources: types of oscillators; synthesizers and phase-locked loops; direct digital synthesizers; stabilizing thermal drift; microphonics; high-accuracy oscillators

What are three common oscillator circuits?

  • 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 is a microphonic?

  • An IC used for amplifying microphone signals
  • Distortion caused by RF pickup on the microphone cable
  • Correct Answer
    Changes in oscillator frequency caused by mechanical vibration
  • Excess loading of the microphone by an oscillator

Microphonics or microphony describes the phenomenon wherein certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise). The term comes from analogy with a microphone, which is intentionally designed to convert vibrations to electrical signals. In the case of oscillator frequency this mechanical vibration can sometimes cause interference.

More at Wikipedia: https://en.wikipedia.org/wiki/Microphonics

Test tip: the only answer without microphone.

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What is a phase-locked loop?

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

  • Wide-band AF and RF power amplification
  • Correct Answer
    Frequency synthesis and FM demodulation
  • Photovoltaic conversion and optical coupling
  • Comparison of two digital input signals and digital pulse counting

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 as an electronic circuit consisting of a variable frequency oscillator and a phase detector.

Phase-locked loops can be used to demodulate a signal (FM demodulation), recover a signal from a noisy communication channel, and generate a stable frequency at multiples of an input frequency (frequency synthesis).

Hint: Functions (from question), frequency (from correct answer)

Phase-locked Loop (Wikipedia page)

Block diagram of a phase-locked loop

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How can an oscillator’s microphonic responses be reduced?

  • Use NP0 capacitors
  • Reduce noise on the oscillator’s power supply
  • Increase the gain
  • Correct Answer
    Mechanically isolate the oscillator circuitry from its enclosure

Microphonic effects (think of a microphone) are electrical changes in component properties caused by mechanical effects such as vibration, sharp impulses, or the like.

In oscillator circuits, capacitors can slightly change their value due to mechanical stress, leading to changes in the resonant frequency of 'tank' circuits (remember \(\frac{1}{2\pi\sqrt{LC}}\)?) that form the heart of the oscillator. In vacuum tube oscillator designs, mechanical vibration of e.g. the grid or filaments can alter their electrical properties and cause the same thing.

Mechanical insulation / isolation of the oscillator circuit will minimize these microphonic effects.

For more, see these:

https://en.wikipedia.org/wiki/Microphonics

https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/12/19/stress-induced-outbursts-microphonics-in-ceramic-capacitors-part-1

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Which of the following components can be used to reduce thermal drift in crystal oscillators?

  • Correct Answer
    NP0 capacitors
  • Toroidal inductors
  • Wirewound resistors
  • Non-inductive resistors

Please refer to this link:

http://www.learningaboutelectronics.com/Articles/What-is-a-NPO-ceramic-capacitor

NP0 stands for negative-positive 0 ppm/°C, meaning that for negative or positive shifts in temperature, the capacitance changes 0 part per million (ppm), meaning that it has a flat response across a wide range of temperatures; the capacitance of the NP0 capacitor stays constant (at the same value) despite variations in temperature.

They are also very suitable for oscillator construction in order to compensate for frequency drift with temperature.

Hint: A Negative plus a Positive = 0, or NP0

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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 direct conversion synthesizer

The phrase "digital to analog converter" can only be present in "a direct digital synthesizer." None of the other answers has "Digital" in them.

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What information is contained in the lookup table of a direct digital synthesizer (DDS)?

  • The phase relationship between a reference oscillator and the output waveform
  • Correct Answer
    Amplitude values that represent the desired waveform
  • The phase relationship between a voltage-controlled oscillator and the output waveform
  • Frequently used receiver and transmitter frequencies

Direct Digital Synthesis (DDS) is a method of producing a sine wave by generating the wave in digital form and then converting it to analog using a digital-to-analog converter. Rather than computing the value of the sine function on every clock cycle, a lookup table is often used to store precomputed sine values. By stepping through this table at different strides, sine waves of various frequencies can be generated.

Hint: "Direct Digital" in the question, "Desired" in the correct answer. These are the only words that start with a 'D'.

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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
  • Harmonics of the local oscillator

Both the direct digital synthesizer (DDS) and the phase-locked loop (PLL) synthesizer have issues with spectral purity. The major spectral impurity components of direct digital synthesizers are spurious signals at discrete frequencies. (E7H11)

A spurious emission (signals) is any radio frequency not deliberately created or transmitted, especially in a device which normally does create other frequencies. A harmonic or other signal outside a transmitter's assigned channel would be considered a spurious emission.

More at: Wikipedia - Spurious emission

Trick: The question asks about "components", which is plural. The correct answer includes the word "signals" which is also plural. All other answer choices are singular.

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Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?

  • Provide the crystal with a specified parallel inductance
  • Correct Answer
    Provide the crystal with a specified parallel capacitance
  • Bias the crystal at a specified voltage
  • Bias the crystal at a specified current

Load capacitance is an important specification when using parallel-resonant oscillation mode.

http://www.maximintegrated.com/en/app-notes/index.mvp/id/726

Silly memory aid: provide "Crystal" with a "cap" (capacitance).

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Which of the following is a technique for providing highly accurate and stable oscillators needed for microwave transmission and reception?

  • Use a GPS signal reference
  • Use a rubidium stabilized reference oscillator
  • Use a temperature-controlled high Q dielectric resonator
  • Correct Answer
    All these choices are correct

A standard quartz crystal oscillator is usually of the AT cut type, which has a frequency deviation on the order of several parts per million over a commercial or industrial temperature range. This stability is unsuitable for many demanding applications where maximum deviations of hundreds of parts per billion are required over a large temperature range, or low drift vs. time is required (aging effect). The most basic is a temperature-compensated crystal oscillator, which as the name implies attempts to pull the crystal into tight tolerance by measuring a nearby temperature sensor. The second type mentioned is a rubidium reference, which is a type of atomic reference with good long-term stability characteristics derived from a rubidium-based physics package. Third, a GPS signal reference is one that can be used to create a GPS-disciplined oscillator, which is a type of oscillator that is kept synchronized by using signals derived from atomic references on GPS satellites, with corrections that are traceable to NIST. The result is an oscillator with zero effective long-term drift and good local stability provided by a high-quality ovenized oscillator.

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