or
Amateur Extra Class (2016-2020)
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 oscillator circuits used in Amateur Radio equipment?
• Taft, Pierce and negative feedback
• Pierce, Fenner and Beane
• Taft, Hartley and Pierce
Colpitts, Hartley and Pierce

just remember CHiPs!

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Which describes a microphonic?
• An IC used for amplifying microphone signals
• Distortion caused by RF pickup on the microphone cable
Changes in oscillator frequency due to mechanical vibration

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

-KE0IPR

Test tip: the only answer without microphone.

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How is positive feedback supplied in a Hartley oscillator?
Through a tapped coil
• Through a capacitive divider
• 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

-KE0IPR

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

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How is positive feedback supplied in a Colpitts oscillator?
• Through a tapped coil
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 a neutralizing capacitor
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

Hint: Pierce = Piezo

Dumb tip: Don't get pierced with a quartz crystal.

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Which of the following oscillator circuits are commonly used in VFOs?
• Pierce and Zener
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.

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How can an oscillator's microphonic responses be reduced?
• Use of NP0 capacitors
• Eliminating noise on the oscillator's power supply
• Using the oscillator only for CW and digital signals
Mechanically isolating 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?
NP0 capacitors
• Toroidal inductors
• Wirewound resistors
• Non-inductive resistors

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.

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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?
A direct digital synthesizer
• A hybrid synthesizer
• A phase locked loop synthesizer
• A diode-switching matrix 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 frequency synthesizer?
• The phase relationship between a reference oscillator and the output waveform
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

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.

--edited, removed incorrect hint KE7KTO

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?
• Digital conversion noise
Spurious signals at discrete frequencies
• Nyquist limit noise

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 must be done to insure that a crystal oscillator provides the frequency specified by the crystal manufacturer?
• Provide the crystal with a specified parallel inductance
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

KM4IFJ

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
All of 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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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
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.

Click for a full explanation

Hint: surprisingly only one answer has the word "phase" in it

kg5kou

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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
Frequency synthesis, FM demodulation

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)