Radio Frequency (RF) & Wireless Design

There are many types of oscillators and although most people are familiar with quartz crystals that serve as an oscillator, most people are not aware of the distinction between a crystal and an oscillator.

First off is that an oscillator does NOT need a crystal to function. There are many resistor (R) and capacitor (C) that form astable multivibrators wherein the period is based upon the R*C time constant as Tau (t). The simplest form is using a couple of transistors that act in a push-pull circuit configuration :

https://en.wikipedia.org/wiki/Multivibrator

This is very similar to the transistors used for logic to form a flip-flop oscillator :

https://en.wikipedia.org/wiki/Flip-flop_(electronics)

Flip Flops can go into more complex logic using latches and registers and form the basis of the 555 timer that everyone is familiar with :

https://en.wikipedia.org/wiki/555_timer_IC

However the 555 timer is NOT a good oscillator since its duty cycle CANNOT be made to an exact 50%/50% ratio. Moreover some of the best logic designs are not edge triggered oscillators but rather sine wave oscillators since edges may have jitter as a consequence of load imbalances and other signal integrity issues.

There are other issues of drift, stability, accuracy, component tolerances, non-linearity, and noise that can affect the timing accuracy.

This is where crystal (primary quartz, but also sapphire and other elements) eliminate most of the issues, but not all, and are the go to device for most timing and RF functions.

https://en.wikipedia.org/wiki/Crystal_oscillator

There are many types of crystal oscillators that are based upon how the crystal is cut and what angle the cuts are. In most cases common crystals are fabricated (grown) rather than cut and defined by their shape and size.

There are many ways to increase the stability and accuracy for crystals and these are some of them :

• size (cut)
• packaging
• temperature
• voltage
• aging
• environment (ovens)
• materials (e.g. Rubidium, Sapphire)

Some of the nomenclature for crystal oscillators are :

• ATCXO — Analog temperature controlled crystal oscillator
• CDXO — Calibrated dual crystal oscillator
• DTCXO — Digital temperature compensated crystal oscillator
• EMXO — Evacuated miniature crystal oscillator
• GPSDO — Global positioning system disciplined oscillator
• MCXO — Microcomputer-compensated crystal oscillator
• OCVCXO — oven-controlled voltage-controlled crystal oscillator
• OCXO — Oven-controlled crystal oscillator
• RbXO — Rubidium crystal oscillators (RbXO), a crystal oscillator (can be an MCXO) synchronized with a built-in rubidium standard which is run only occasionally to save power
• TCVCXO — Temperature-compensated voltage-controlled crystal oscillator
• TCXO — Temperature-compensated crystal oscillator
• TMXO – Tactical miniature crystal oscillator[69]
• TSXO — Temperature-sensing crystal oscillator, an adaptation of the TCXO
• VCTCXO — Voltage-controlled temperature-compensated crystal oscillator
• VCXO — Voltage-controlled crystal oscillator

https://en.wikipedia.org/wiki/Crystal_oscillator#Circuit_notations_and_abbreviations

Although most of the low cost microcontrollers use just the crystal and some external capacitors for the MCU oscillator, most digital engineers get lost in designing a very solid and reliable crystal oscillator circuit. In fact most semiconductor manufacturers have created very thick and detailed application notes to guide the digital engineer (and some analog engineers) to make sure that the oscillator starts up properly over temperature, is not over loaded with too much capacitance, and has a symmetrical waveform.

So its a good idea to avoid or eliminate most crystal oscillator design problems for MCUs and just use a 'canned' oscillator rather than design around a crystal circuit. Only in cases where cost savings is paramount are the benefits outweighed by the reliability and simplicity of a canned oscillator.