Amateur Extra E7H: Oscillators and synthesizers
These are the 13 official questions in group E7H of the 2024–2028 Amateur Extra question pool, with the correct answer marked and a plain-English explanation of why it's right. The real exam asks one question from this group.
- E7H01Question 1
What are three common oscillator circuits?
- ATaft, Pierce, and negative feedback
- BPierce, Fenner, and Beane
- CTaft, Hartley, and Pierce
- DColpitts, Hartley, and Pierce Correct
Why: Colpitts, Hartley and Pierce are common oscillators.
- E7H02Question 2
What is a microphonic?
- AAn IC used for amplifying microphone signals
- BDistortion caused by RF pickup on the microphone cable
- CChanges in oscillator frequency caused by mechanical vibration Correct
- DExcess loading of the microphone by an oscillator
Why: Microphonics are frequency changes caused by vibration.
- E7H03Question 3
What is a phase-locked loop?
- AAn electronic servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator
- BAn electronic circuit also known as a monostable multivibrator
- CAn electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator Correct
- DAn electronic circuit consisting of a precision push-pull amplifier with a differential phase input
Why: A PLL is a servo loop: phase detector, low-pass filter, VCO and reference oscillator.
- E7H04Question 4
How is positive feedback supplied in a Colpitts oscillator?
- AThrough a tapped coil
- BThrough link coupling
- CThrough a capacitive divider Correct
- DThrough a neutralizing capacitor
Why: A Colpitts gets positive feedback through a capacitive divider.
- E7H05Question 5
How is positive feedback supplied in a Pierce oscillator?
- AThrough a tapped coil
- BThrough link coupling
- CThrough a neutralizing capacitor
- DThrough a quartz crystal Correct
Why: A Pierce gets positive feedback through a quartz crystal.
- E7H06Question 6
Which of these functions can be performed by a phase-locked loop?
- AWide-band AF and RF power amplification
- BFrequency synthesis and FM demodulation Correct
- CPhotovoltaic conversion and optical coupling
- DComparison of two digital input signals and digital pulse counting
Why: A PLL can synthesize frequencies and demodulate FM.
- E7H07Question 7
How can an oscillator’s microphonic responses be reduced?
- AUse NP0 capacitors
- BReduce noise on the oscillator’s power supply
- CIncrease the gain
- DMechanically isolate the oscillator circuitry from its enclosure Correct
Why: Mechanically isolating the oscillator reduces microphonics.
- E7H08Question 8
Which of the following components can be used to reduce thermal drift in crystal oscillators?
- ANP0 capacitors Correct
- BToroidal inductors
- CWirewound resistors
- DNon-inductive resistors
Why: NP0 capacitors reduce thermal drift.
- E7H09Question 9
What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter?
- AA direct digital synthesizer Correct
- BA hybrid synthesizer
- CA phase-locked loop synthesizer
- DA direct conversion synthesizer
Why: Phase accumulator, lookup table, DAC and anti-alias filter make up a DDS.
- E7H10Question 10
What information is contained in the lookup table of a direct digital synthesizer (DDS)?
- AThe phase relationship between a reference oscillator and the output waveform
- BAmplitude values that represent the desired waveform Correct
- CThe phase relationship between a voltage-controlled oscillator and the output waveform
- DFrequently used receiver and transmitter frequencies
Why: The lookup table holds amplitude values for the waveform.
- E7H11Question 11
What are the major spectral impurity components of direct digital synthesizers?
- ABroadband noise
- BDigital conversion noise
- CSpurious signals at discrete frequencies Correct
- DHarmonics of the local oscillator
Why: A DDS's main impurities are spurs at discrete frequencies.
- E7H12Question 12
Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?
- AProvide the crystal with a specified parallel inductance
- BProvide the crystal with a specified parallel capacitance Correct
- CBias the crystal at a specified voltage
- DBias the crystal at a specified current
Why: The crystal needs its specified parallel capacitance to run on frequency.
- E7H13Question 13
Which of the following is a technique for providing highly accurate and stable oscillators needed for microwave transmission and reception?
- AUse a GPS signal reference
- BUse a rubidium stabilized reference oscillator
- CUse a temperature-controlled high Q dielectric resonator
- DAll these choices are correct Correct
Why: Several techniques provide stable microwave oscillators. All the choices are correct.