Crystal packages - Page 1
Forum section: electronics / beginners

author: Maxlor
author_rank: Frequent Contributor
timestamp: November 23, 2013, 07:18:03 pm
content: While gathering some components, I noticed that many crystals are available in different sized packages, for example look at theABL-16.000MHZ-B2vs. theAB-16.000MHZ-B2:I was wondering what the reason might be for choosing the AB package over the smaller ABL package. The only difference I can see in the data sheets is that the ABs are available down to 1.8MHz, whereas the ABLs only go down to 3.6MHz. Otherwise their performance seems to be identical. So if you needed, say, a 16MHz crystal, is there any reason for choosing the bigger package?

author: madires
author_rank: Super Contributor
timestamp: November 23, 2013, 11:06:03 pm
content: I don't know of any performance differences between HC49S and HC49U packages, besides what you already mentioned about the available frequencies. So there's no reason to use the larger package if you can get the smaller one with the same frequency.

author: Udayan Sinha
author_rank: Regular Contributor
timestamp: November 25, 2013, 12:24:46 pm
content: Refer the datasheet for that particular crystal when you are buying. As long it fulfills your requirements, you shouldn't have a problem.

author: ludzinc
author_rank: Unknown
timestamp: November 25, 2013, 12:50:29 pm
content: Some crystals are optimized for series resonant mode, others series resonant.I forget which is which, just that one type is larger than the other, hence the bigger case.(And iirc some crystals are both, xMHz in one mode, yMHz the other.

author: JackOfVA
author_rank: Unknown
timestamp: November 25, 2013, 11:13:48 pm
content: Quote from: ludzinc on November 25, 2013, 12:50:29 pmAll crystals are inherently series and parallel resonant. A crystal can be modeled as a series RLC circuit in parallel with a C. The series elements are the motional parameters of the crystal, i.e., they are fictitious electrical representations of the mechanical resonance of the crystal element. This is a series resonant circuit. The RLC motional "elements" are in parallel with a C formed by the plates of the filter and the plate to holder capacitance. These form a parallel resonant circuit.In addition, most crystals, particularly inexpensive ones without great attention to edge finishing, will have spurious resonances from multiple mechanical reflections. The spurious resonances are usually quite close to the fundamental resonance. In addition, the crystal will exhibit odd order "overtone" resonance at roughly the 3rd, 5th, etc. harmonic of the fundamental resonance frequency. The reason the overtones are not an exact integer multiple of the fundamental relate to imperfections in the crystal structure, edges and other considerations.When purchasing a crystal that is resonant at say 8 MHz, the data sheet should be consulted to determine whether this is the series or parallel resonant frequency. If parallel resonance, the data sheet will usually specify the total capacitance of the oscillator circuit needed for this particular crystal to meet the specified frequency, e.g., 20 pF or 32 pF. By slightly varying this capacitance with a trimmer, the exact parallel resonance frequency can be "tweaked" to the desired value. In a series resonant design, the trimming capacitor would be in series with the crystal.

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: November 26, 2013, 11:53:07 am
content: Crystals in the larger package may be better for VXO applications where maximum frequency swing is desirable (eg FM transmitter or HF transceiver with VXO).The old HC6 types swing further than HC18. However 2 or 3 crystals can be wired in parallel to increase the pulling range by 200 - 300%.