though SC cut variants exist as well. BVA resonators are often used in spacecraft applications. In the 1930s to 1950s, it was fairly common for people to adjust the frequency of the crystals by manual grinding. The crystals were ground using a fine abrasive slurry, or even a toothpaste, to increase their frequency. A slight decrease by 1–2 kHz when the crystal was overground was possible by marking the crystal face with a pencil lead, at the cost of a lowered Q. The frequency of the crystal is slightly adjustable ("pullable") by modifying the attached capacitances. A varactor, a diode with capacitance depending on applied voltage, is often used in voltage-controlled crystal oscillators, VCXO. The crystal cuts are usually AT or rarely SC, and operate in fundamental mode; the amount of available frequency deviation is inversely proportional to the square of the overtone number, so a third overtone has only one-ninth of the pullability of the fundamental mode. SC cuts, while more stable, are significantly less pullable. Circuit notations and abbreviations On electrical schematic diagrams, crystals are designated with the class letter Y (Y1, Y2, etc.). Oscillators, whether they are crystal oscillators or others, are designated with the class letter G (G1, G2, etc.). Crystals may also be designated on a schematic with X or XTAL, or a crystal oscillator with XO. Crystal oscillator types and their abbreviations: 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 TSXO — Temperature-sensing crystal oscillator, an adaptation of the TCXO VCTCXO — Voltage-controlled temperature-compensated crystal oscillator VCXO — Voltage-controlled crystal oscillator See also Clock generator Clock drift – Clock drift measurements of crystal oscillators can be used to build random number generators. Crystal filter Erhard Kietz work on electronic tuning forks and with quartz crystals for precise signal frequencies Issac Koga – inventor of the temperature-stable R1 Koga cut Pierce oscillator Thin-film thickness monitor Variable-frequency oscillator (VFO) References Further