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Subelement L07
Oscillators.
Section L07
How is the positive feedback coupled to the input in a Hartley oscillator?
  • Through link coupling
  • Through a neutralizing capacitor
  • Correct Answer
    Through a tapped coil
  • Through a capacitive divider

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How is positive feedback coupled to the input in a Colpitts oscillator?
  • Through a link coupling
  • Correct Answer
    Through a capacitive divider
  • Through a tapped coil
  • Through a neutralizing capacitor

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How is positive feedback coupled to the input in a Pierce oscillator?
  • Correct Answer
    Through capacitive coupling
  • Through a neutralizing capacitor
  • Through link coupling
  • Through a tapped coil

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why is the Colpitts oscillator circuit commonly used in a VFO?
  • It can be used with or without crystal lock-in
  • The frequency is a linear function with load impedance
  • It has high output power
  • Correct Answer
    It is stable

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why must a very stable reference oscillator be used as part of a phase-locked loop (PLL) frequency synthesizer?
  • Any amplitude variations in the reference oscillator signal will prevent the loop from changing frequency
  • 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

Key words: PHASE-LOCKED. As the name implies, a PLL synthesizer includes a voltage-controlled oscillator (VCO) whose output is constantly compared to a stable crystal reference. If the phase of the output signal begins to lead or lag the reference (a phase error), a correction is applied to the oscillator tuning. If the reference is noisy or subject to phase jitter, the output is similarly corrupted.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Positive feedback from a capacitive divider indicates the oscillator type is:
  • Pierce
  • Hartley
  • Miller
  • Correct Answer
    Colpitts

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In an RF oscillator circuit designed for high stability, the positive feedback is drawn from two capacitors connected in series. These two capacitors would most likely be:
  • ceramic
  • electrolytics
  • Mylar
  • Correct Answer
    silver mica

In an oscillator, stability is paramount. Silver Mica, NP0 (N-P-Zero, negative-positive-zero) ceramic and polystyrene capacitors are temperature-stable. The electrolytic is suitable as a filter or audio bypass. The plain ceramic capacitor is good for coupling or RF bypass.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In an oscillator circuit where positive feedback is obtained through a single capacitor in series with the crystal, the type of oscillator is:
  • Colpitts
  • Hartley
  • Miller
  • Correct Answer
    Pierce

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A circuit depending on positive feedback for its operation would be a:
  • detector
  • audio amplifier
  • Correct Answer
    variable-frequency oscillator
  • mixer

An oscillator, fixed or variable, is an amplifier with a positive feedback path from output to input used to start and maintain oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

An apparatus with an oscillator and a class C amplifier would be:
  • Correct Answer
    a two-stage CW transmitter
  • a fixed-frequency single-sideband transmitter
  • a two-stage frequency-modulated transmitter
  • a two-stage regenerative receiver

A simple two-stage CW transmitter comprises an oscillator and a Class-C power amplifier. A transformer at the output of the oscillator serves the dual purpose of tuned circuit and coupling to the next stage. The DC supply to the final amplifier is bypassed to ground with a capacitor and decoupled through an RF choke so RF is kept out of the supply.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In an oscillator where positive feedback is provided through a capacitor in series with a crystal, that type of oscillator is a:
  • Correct Answer
    Pierce
  • Colpitts
  • Hartley
  • Franklin

Hartley = a tap on the inductor of the tuned circuit permits inserting positive feedback from output to input. Colpitts = the inductor tap of the Hartley is replaced by two series capacitors in a capacitive divider configuration. Relatively large capacitor values, when compared to the Hartley, mean less influence from internal capacitance changes in the device, hence stability. Pierce = derived from the Colpitts, a piezoelectric crystal replaces the tuned circuit. Capacitive coupling maintains oscillation.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

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