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E7H

PRACTICAL CIRCUITS

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

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E7H011 of 13

What are three common oscillator circuits?

Why The three classic feedback oscillator topologies are Colpitts, Hartley and Pierce. A Colpitts takes its feedback from a capacitive voltage divider across the tank, a Hartley takes it from a tapped inductor, and a Pierce is essentially a Colpitts that uses a quartz crystal as the resonant element, which is why it dominates crystal oscillator design. All three meet the Barkhausen requirement: loop gain of at least 1 with 360 degrees (in phase) feedback at the desired frequency.
Watch out The other choices sprinkle in invented names like Taft, Fenner and Beane; negative feedback is also a giveaway, since oscillators require positive (regenerative) feedback, not negative.
CHP: Colpitts = Capacitors, Hartley = Henrys (coil tap), Pierce = Piezoelectric crystal. Any other surname is fake.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H022 of 13

What is a microphonic?

Why Microphonics is unwanted modulation of a circuit by mechanical vibration or shock. In an oscillator, physical movement of a capacitor's plates, a crystal, a coil, or even a circuit board flexing slightly changes the tank's L or C, which shifts the resonant frequency. The result is a warble or 'ping' on the signal when the rig is tapped or when nearby sound or vibration reaches it. Cures include shock-mounting the oscillator, using rigid components, and potting or bracing the tuned circuit.
Watch out The choices mentioning a microphone are traps based on the word root only; microphonics has nothing to do with microphones or microphone cables, though the effect is named after it because the circuit behaves like one, turning vibration into an electrical signal.
Microphonic = the circuit acts like a microphone: tap the rig and you hear the frequency wobble.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H033 of 13

What is a phase-locked loop?

Why A phase-locked loop is a feedback (servo) control system for frequency. A phase detector compares the output of a voltage-controlled oscillator (usually after a divider) against a stable reference oscillator, the resulting error voltage is smoothed by a low-pass loop filter, and that filtered voltage steers the VCO until its phase and frequency track the reference. Those four blocks, phase detector, low-pass filter, VCO, and reference oscillator, are the defining parts, and the arrangement is the basis of frequency synthesizers and FM demodulators.
Watch out The choice listing a ratio detector and reactance modulator names FM discriminator and modulator parts, not the PLL blocks; a monostable multivibrator is a one-shot timer with no frequency-locking feedback at all.
PLL = Phase detector, Low-pass filter, VCO, plus reference. Look for the answer with all four.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H044 of 13

How is positive feedback supplied in a Colpitts oscillator?

Why An LC oscillator needs a fraction of the tank's output fed back to the input in phase to sustain oscillation, and the different classic topologies are named for how that tap is made. In a Colpitts oscillator the tank uses two series capacitors across the coil, and feedback is taken from the junction between them, so the capacitive divider sets the feedback ratio.
Watch out The tapped coil describes the Hartley oscillator, which splits the inductor instead of the capacitance; link coupling through a separate winding is the Armstrong arrangement. A neutralizing capacitor does the opposite job, cancelling unwanted feedback in an amplifier.
Colpitts has a C, so Capacitors; Hartley has no C, so the coil is tapped; Armstrong uses a transformer link.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H055 of 13

How is positive feedback supplied in a Pierce oscillator?

Why The Pierce oscillator is essentially a Colpitts topology in which the inductive element is replaced by a quartz crystal operating slightly above its series resonance, where it looks inductive. The crystal sits between the output and input of the amplifying device (drain to gate, or collector to base) and returns the in-phase energy needed to sustain oscillation. Because the crystal's very high Q sets the loop phase, the Pierce is both simple and very frequency stable.
Watch out The tapped coil answer describes a Hartley oscillator, where an inductive divider provides the feedback; a capacitive divider would make it a Colpitts, and link coupling suggests an Armstrong or tickler-coil design.
Learn the feedback family: Hartley = tapped coil, Colpitts = capacitor divider, Pierce = crystal, Armstrong = transformer.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H066 of 13

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

Why A phase-locked loop compares the phase of a reference signal with the output of a voltage-controlled oscillator, filters that error into a DC control voltage, and steers the VCO until the two stay locked. Insert a programmable divider in the feedback path and the VCO must run at N times the reference, which is exactly how a synthesizer generates many frequencies from one crystal. Feed an FM signal into the phase detector instead, and the control voltage the loop generates to keep up with the deviation is the recovered audio, so the same circuit demodulates FM.
Watch out The choice about comparing two digital signals and counting pulses describes digital logic like comparators and counters; a PLL does compare phase, but it is an analog feedback loop, not a pulse counter, and power amplification is not a PLL function at all.
PLL = phase detector + loop filter + VCO: divide the feedback for synthesis, tap the control voltage for FM audio.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H077 of 13

How can an oscillator's microphonic responses be reduced?

Why Microphonics are frequency or amplitude variations caused by mechanical vibration or shock reaching the oscillator, where movement of components, leads, or the tuned circuit slightly changes capacitance or inductance. The cure is mechanical, not electrical: shock-mount or isolate the oscillator board from the chassis and enclosure, keep leads short and rigid, and secure components so nothing can vibrate. Potting or clamping critical parts works on the same principle.
Watch out NP0 (C0G) capacitors are the standard fix for thermal drift, since they have near-zero temperature coefficient, but they do nothing about vibration; cleaning up power supply noise reduces hum and phase noise, not microphonics.
Microphonic = mechanical. A mechanical problem needs a mechanical fix: isolate and mount rigidly.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H088 of 13

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

Why A crystal oscillator's frequency depends on the crystal plus the loading capacitance around it, so any capacitor whose value drifts with temperature drags the frequency with it. NP0 (also called C0G) ceramic capacitors are specified for a near-zero temperature coefficient, roughly 0 +/- 30 ppm per degree C, so the load capacitance stays put as the circuit warms up. Using them in the frequency-determining part of the oscillator is the standard cure for thermal drift.
Watch out Toroidal inductors are chosen for self-shielding and high Q, not temperature stability, and resistor types (wirewound or non-inductive) affect inductance and noise, not the oscillator's frequency-setting reactance.
NP0 = Negative-Positive-Zero drift: zero temperature coefficient, zero frequency wander.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H099 of 13

What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter?

Why A direct digital synthesizer builds the waveform numerically: a phase accumulator adds a tuning word to a running phase value on each clock tick, the accumulated phase indexes a sine lookup table, a DAC turns those samples into an analog staircase, and a low-pass anti-alias filter smooths it into a clean sine wave. Output frequency is set by the tuning word and the reference clock, so frequency changes are instant and extremely fine in resolution. The blocks named in the question are exactly the DDS signal chain.
Watch out A phase-locked loop synthesizer is the other common approach, but it works with a VCO, phase detector, loop filter and programmable divider, not a lookup table and DAC. Direct conversion is a receiver architecture, not a synthesizer type.
Phase accumulator plus lookup table plus DAC equals DDS: digital numbers in, sine wave out.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H1010 of 13

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

Why A DDS works by stepping a phase accumulator at the clock rate; the accumulator's output is a phase angle, and the lookup table (usually a ROM holding one cycle of a sine wave) converts each phase value into the corresponding amplitude sample. Those amplitude numbers go to a DAC, which reconstructs the waveform, and a low-pass filter smooths the steps. So the table is simply a stored map of phase angle to waveform amplitude.
Watch out The choices about phase relationships describe a phase-locked loop, where a phase detector compares a reference with a VCO; a DDS has no such feedback loop, and its phase information lives in the accumulator, not the table.
DDS chain: phase accumulator makes the angle, lookup table supplies the amplitude, DAC makes the wave.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H1111 of 13

What are the major spectral impurity components of direct digital synthesizers?

Why A DDS builds its output by stepping a phase accumulator and looking up sample values that a DAC converts to analog. Because the phase word is truncated and the DAC output is sampled and quantized, the errors are periodic rather than random, so they fold back into the passband as discrete, repeatable spurs (aliases and phase-truncation lines). That is why DDS specs are dominated by a spurious-free dynamic range (SFDR) number rather than a noise floor figure.
Watch out Broadband noise is the usual limitation of analog PLL synthesizers and amplifier stages; DDS phase noise is actually very low, and its problem is the sharp discrete spurs instead.
DDS = Discrete Digital Spurs. Sampling and truncation errors repeat, so they land on specific frequencies, not spread as noise.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H1212 of 13

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

Why Quartz crystals cut for parallel resonance are ground to hit their marked frequency only when they see a specified load capacitance across their terminals, typically something like 18, 20 or 32 pF. The circuit capacitance combines with the crystal's motional and holder capacitance to set the exact resonant point, so the oscillator must present that load value. Too little or too much load capacitance pulls the crystal slightly high or low in frequency, which is why many oscillators include a small trimmer for netting.
Watch out Adding a parallel inductance would pull the crystal far off frequency rather than set it; bias voltage or current affects amplitude and drive level, not the specified operating frequency.
Crystals are spec'd with a load capacitance: match the C, get the marked frequency.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7H1313 of 13

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

Why Microwave work needs frequency references good to parts per billion, and there are several proven ways to get there. GPS satellites carry atomic clocks, so a GPS-disciplined oscillator locks a local 10 MHz source to that timing signal for long-term accuracy. A rubidium reference is itself an atomic standard, excellent for both short and long term stability. A high-Q dielectric resonator held at constant temperature gives a very stable mechanical/electrical resonance directly at microwave frequencies, avoiding the drift of ordinary LC or crystal circuits.
Watch out Each single choice looks like the whole answer, but the question asks which technique is used, and all three are in common use in microwave stations and lab references.
GPS, rubidium, temperature-controlled dielectric resonator: three roads to microwave stability, all valid.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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