Could some electronics guru please clarify the following. A quartz crystal, as used in a crustal oscillator, has two characteristic properties -- series resonance frequency and parallel resonance frequency. How are these two frequencies related to the crystal frequency, i.e., the frequency as advertised for that crystal. Thanks in advance for your help.
In a nutshell : Every crystal can be excited in serial resonance (lowest impedance) or parallel resonance (highest impedance). The serial resonance frequency is always slightly lower than the parallel resonance one, and is roughly independant from the external capacitances. At the contrary, the parallel resonance frequency is directly influenced by the packaging plus external parasitic capacitors. Therefore a given crystal is either specified :
1/ for frequency X MHz, serial resonance
2/ or for frequency Y MHz, parallel resonance, with n pF capacitance between its terminals.
These crystal are exactly the same but adjusted differently. If you use one in the other configuration then its oscillating frequency will be slightly wrong.
For more information you can try to find my old article "let's be crystal clear" published in Circuit Cellar some years ago.
Crystals will be advertised as series resonant or parallel resonant.
If it's sold as a series resonant crystal, then the advertised frequency should be the series resonant frequency.
If it's sold as a parallel resonant frequency then it'll include a rated parallel capacitance, and the advertised frequency will be the parallel resonant frequency when it's in parallel with that capacitance.
I've always found crystals to be odd devices they way they are used. The series resonance would appear to be a fixed value and so easier to get right, but nearly all circuits use parallel resonance. I guess the bottom line is that stray capacitance will impact the frequency of either and the parallel circuit seems to be easier to use.
Stray parallel impedances won't impact a series-resonant circuit, much. Amplifier phase shift and series impedances (R, L or C) will.
I think it's mostly the fact that parallel circuits are at a sweet spot between ease of application and "good enough".
AFAIK, the "gold standard" super-accurate crystal oscillator circuit is the Butler oscillator, which has the crystal in series-resonant. By the time you're using a Butler oscillator you're probably also putting your crystal into an oven, and possibly even in a holder that keeps the thing in a vacuum for higher Q.
It's not so easy to find, at least a good source of info isn't. There are a bazillion sources that parrot the same limited info. There are rather few sources that do a good job of explaining the real nature of crystals. I found an app note from HP, AN 200-2, "Fundamentals of Quartz Oscillators". That was very informative.
Another good text is by John R. Vig, a reprint from "Ultrasonic Instruments and Device". Not copyrighted because it was done as part of official duties for the government. This document is a scanned PDF which must have been converted to text as the copy I have is full of extra characters which make it a little hard to read.
I have read about the Shortt clock which has a pair of pendulums and a minimal amount of electrics to provide better than 1 second accuracy over a year, the best clocks until fully electric clocks were made. But in reality quartz crystal clocks are still regulated by mechanical vibrations of the crystal while all-electronic (LC) oscillators have rather poor frequency stability.
The 524B 10MHz counter cost as much as a couple of Chevrolets.
When I was a youngster, I saw a frequency counter setup at Offshore Navigation, a Raydist outfit (sort of like Loran) that provided navigation for oil rigs in the Gulf. It was a wall full of gear: reference oscillators, WWV receivers, interpolation oscillators, lissajous scopes. Accurately measuring an arbitrary frequency was a serious feat. So was finding a wellhead 200 miles out in the Gulf.
BTW, we have a nice writeup of quartz crystals in the new 3rd edition of AoE. I concentrated on exploring the crystal's high-Q LCR impedances. Includes bench measurements and exploration with SPICE. Interesting material you won't find anywhere else. Check it out.
Used DuckDuckGo with query exactly like: John R. Vig "Ultrasonic Instruments and Device" gave a number of "hits",the second one seen:
[PDF] Frequency Control Devices Reprinted from Ultrasonic Instruments and Devices 01999, ... Frequency Control Device Fundamentals. ... 638 John R. Vig and Arthur Ballato [Search domain
resulted in a very readable document, printing would seem to be done by a dod-matrix printer that missed use of a pin during print of some text lines; a thin band of white is seen (which leads me to the dot matrix conclusion). I think this versoin maybe much better than the one you have.