[hpsdr] Crystal Packages

[Chris Bartram]

Amateurs - and a lot of professionals - tend not to specify crystals properly, 
and manufacturers will only produce what's specified!

The HC18/U package was a solder seal package and is now effectively obsolete 
apart, maybe from when it is used for the lowest quality clock crystals. 
(Using solder to seal a crystal can is an invitation to excessive ageing...) 
The package has been replaced by similar packages such as HC49/U using either 
resistance welding or (better)... cold welding to seal the envelope. 

The most common packages for VHF crystals are now UM-1 and UM5 or their US 
military equivalents. These are quite a lot smaller than the old HC18/U 
package and are are available with cold weld and resistance weld seals. 
Crystals using in a TO-5 package are also quite common in high-end 
applications, as the blank can be supported at three points, which makes them 
a lot less susceptible to vibration-induced problems.

There are also ceramic-metal surface-mount packages, but they don't seem to be 
used for high quality small quantity custom crystals. However, they are quite 
popular for TCXOs, and a 0.5ppm over -20 to +60C temperature range unit in a 
3 x 5mm package is the sort of part I'm currently designing into commercial 
products. However, the phase noise of these, while OK in the world of ETSI 
and FCC type approval specs., isn't really good enough for the sort of things 
that we're trying to do.

Vy 73

Chris
GW4DGU