Oscillators.
Subelement L07
Section L07
How is the positive feedback coupled to the input in a Hartley oscillator?
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Through link coupling
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Through a neutralizing capacitor
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Correct AnswerThrough a tapped coil
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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.
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How is positive feedback coupled to the input in a Colpitts oscillator?
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Through a link coupling
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Correct AnswerThrough a capacitive divider
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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?
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Correct AnswerThrough 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?
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It can be used with or without crystal lock-in
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The frequency is a linear function with load impedance
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It has high output power
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Correct AnswerIt 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?
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Any amplitude variations in the reference oscillator signal will prevent the loop from changing frequency
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Any amplitude variations in the reference oscillator signal will prevent the loop from locking to the desired signal
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Correct AnswerAny phase variations in the reference oscillator signal will produce phase noise in the synthesizer output
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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:
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Pierce
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Hartley
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Miller
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Correct AnswerColpitts
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:
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ceramic
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electrolytics
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Mylar
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Correct Answersilver 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:
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Colpitts
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Hartley
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Miller
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Correct AnswerPierce
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:
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detector
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audio amplifier
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Correct Answervariable-frequency oscillator
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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:
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Correct Answera two-stage CW transmitter
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a fixed-frequency single-sideband transmitter
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a two-stage frequency-modulated transmitter
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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:
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Correct AnswerPierce
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Colpitts
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Hartley
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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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