E7H

Oscillators, PLLs and synthesizers

Explain how oscillators, phase-locked loops and DDS synthesizers generate frequencies, and how to keep an oscillator stable.

13 pool questions1 on the exam

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

An oscillator is an amplifier with positive feedback

Feed part of an amplifier's output back to its input in phase (positive feedback) with a loop gain of 1 or more, and it sustains its own signal. The three common types differ only in how the feedback is supplied.

Feed some output back to the input, in phase. If the loop gain is 1 or more, it keeps itself going.

Loop gain (amplifier gain × feedback fraction)1.10

Orange = where the feedback comes from: a capacitor divider, a tapped coil, or a crystal.

Oscillator	Positive feedback through
Colpitts	a capacitive divider
Hartley	a tapped coil
Pierce	a quartz crystal

RememberColpitts, Hartley, Pierce. Colpitts = capacitors, Hartley = tapped coil, Pierce = crystal.

A PLL locks an oscillator to a reference

A phase-locked loop is a servo: a phase detector compares a stable reference with the VCO, a low-pass filter smooths the error, and that voltage steers the VCO until they match.

The loop forces the divided VCO to match the reference. So the VCO runs at N times the reference.

Use the loop as

Divide-by value N÷ 144

That makes it a frequency synthesizer (a divider in the loop multiplies the reference) and an FM demodulator.

RememberPLL = phase detector + low-pass filter + VCO + stable reference. Does frequency synthesis and FM demodulation.

A DDS builds a sine from numbers

A direct digital synthesizer counts around a circle with a phase accumulator. A lookup table holds amplitude values of the waveform. A DAC and a low-pass anti-alias filter turn them into a smooth signal.

Bigger phase step, faster trip around the table, higher output frequency. Real accumulators wrap at a far larger count than 64.

Phase step per clock tick3

Its main impurities are spurious signals at discrete frequencies, not broadband noise.

RememberAccumulator, lookup table of amplitudes, DAC, low-pass filter. Impurity: discrete spurs.

Keep the frequency where you put it

Know the problem, then the fix.

Microphonic = oscillator frequency change from mechanical vibration. At microwave frequencies, a high-accuracy reference can be a GPS signal, a rubidium-stabilized oscillator, or a temperature-controlled high-Q dielectric resonator. All of these are correct.

Problem	Fix
Microphonics	mechanically isolate the oscillator from its enclosure
Thermal drift	NP0 capacitors
Crystal on the wrong frequency	its specified parallel capacitance
Highest accuracy	GPS, rubidium or temperature-controlled dielectric resonator

RememberVibration: isolate. Heat: NP0. Crystal frequency: specified parallel capacitance.

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