The three classic oscillators
What are three common oscillator circuits?
Colpitts, Hartley, and Pierce.
They differ only in where the positive feedback comes from, and that is exactly what the pool asks.
| Oscillator | Feedback via |
|---|---|
| Colpitts | a capacitive divider |
| Hartley | a tapped inductor |
| Pierce | a quartz crystal |
Colpitts: capacitors. Hartley: inductor. Pierce: crystal. Everything else about them is the same amplifier-plus-filter-in-a-loop from the General track.
Pierce is the one inside every crystal oscillator module ever made, which is why it is the most common of the three by an enormous margin.
Phase-locked loops
What is a phase-locked loop?
An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator.
Four blocks. The phase detector compares the VCO against the reference and produces an error; the filter smooths it; the error steers the VCO until the two agree. A servo, in electronics.
Which of these functions can be performed by a phase-locked loop?
Frequency synthesis and FM demodulation.
Synthesis: put a divider in the loop and the VCO must run at N times the reference to satisfy it — so you get any multiple of the reference with the reference’s stability.
FM demodulation: lock a PLL to an FM signal and the control voltage that keeps it locked is the demodulated audio, because it is tracking the frequency deviation.
One circuit, two apparently unrelated jobs, both following from what a loop does.
Direct digital synthesis
What type of frequency synthesizer circuit uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter?
A direct digital synthesizer.
Four blocks again, and the chain is worth following:
- The phase accumulator adds a fixed increment on every clock — a counter that represents the phase of the output, advancing at a rate you choose.
- The lookup table converts that phase to an amplitude.
- The DAC turns the amplitude into a voltage.
- The anti-alias filter removes the sampling artefacts.
What information is contained in the lookup table of a direct digital synthesizer (DDS)?
Amplitude values that represent the desired waveform
Because the increment can be any integer, the output frequency has extremely fine resolution — the source of the General track’s “variable output frequency with the stability of a crystal oscillator.”
What are the major spectral impurity components of direct digital synthesizers?
Spurious signals at discrete frequencies.
Not broadband noise — discrete spurs, from the finite word length of the accumulator and the DAC. That is the characteristic DDS defect, and it is why a DDS signal generator’s spectrum has small isolated lines rather than a raised noise floor. A PLL, by contrast, has phase noise skirts and few spurs.
Stability
Three enemies of a stable oscillator, with their cures.
What is a microphonic?
Changes in oscillator frequency caused by mechanical vibration.
How can an oscillator’s microphonic responses be reduced?
Mechanically isolate the oscillator circuitry from its enclosure.
Vibration moves components minutely, which changes stray capacitance, which changes frequency. You hear it as your voice modulating your own signal — literally the microphone effect, and the origin of the word.
Which of the following components can be used to reduce thermal drift in crystal oscillators?
NP0 capacitors.
NP0 (also called C0G) ceramic has a near-zero temperature coefficient. Ordinary ceramics change capacitance substantially with temperature, so an oscillator using them drifts as it warms up.
Which of the following ensures that a crystal oscillator operates on the frequency specified by the crystal manufacturer?
Provide the crystal with a specified parallel capacitance.
A crystal’s specified frequency assumes a particular load capacitance. Present a different one and it oscillates slightly off — which is a nuisance and also the mechanism behind crystal trimming, where a small variable capacitor pulls it onto frequency deliberately.
Which of the following is a technique for providing highly accurate and stable oscillators needed for microwave transmission and reception?
All these choices are correct — an oven-controlled crystal oscillator (OCXO), a temperature-compensated crystal oscillator (TCXO), and a GPS-controlled oscillator.
Three tiers. A TCXO compensates for temperature electronically — cheap and good. An OCXO holds the crystal in a small oven at constant temperature — better and power-hungry. A GPS-disciplined oscillator steers a local oscillator to match GPS timing, giving atomic-clock accuracy for the price of a receiver. It is what makes amateur microwave and precision measurement practical.
Check yourself
- An oscillator’s feedback comes through a tapped coil. Which type?
- You need 145.000 MHz with the stability of a 10 MHz crystal. PLL or DDS, and what makes each work?
- Your VFO drifts as the radio warms up. Which component type helps?
Answers
- Hartley — a tapped inductor. Colpitts uses a capacitive divider; Pierce uses a crystal.
- Either. A PLL divides the VCO down and locks it to the reference; a DDS builds the waveform numerically from the reference. The PLL reaches higher frequencies more easily; the DDS gives finer resolution and discrete spurs.
- NP0 (C0G) capacitors, which have a near-zero temperature coefficient.