High-specification AT-cut crystals

EDN Europe, 28 Oct 2011

IQD Frequency Products has launched a range of high specification crystals for specialist applications such as military systems and radar. IQD manufactures the components at its FOQ division in Germany, in a variety of package styles including UM1, HC49, HC43CW (TO8) and HC45CW. For the highest possible ‘Q’ factor, customers should specify the Cold Weld (CW) packages. You can use the crystals to generate frequencies from 10 MHz to 175 MHz; 10 to 42MHz at fundamental mode, 10 to 125MHz at 3rd Overtone and 70 to 175MHz at 5th Overtone. The latter represents an upper limit for AT-cut crystals (AT-cut refers to the geometry and the piezoelectric resonance mode in the crystal) as, above this figure, the crystal blank itself becomes unworkably thin.

The new range offers low frequency drift with time (aging), down to 0.1ppm per year, and tight frequency tolerance at 25 °C, as low as to ±3ppm. This compares to standard crystals where typical figures would be 3ppm per year aging and ±10ppm frequency tolerance. Temperature stabilities – variation of frequency with temperature – of as low as 4ppm over 0 to 70 °C, or 12ppm over -40 to 85 °C are available. IQD also notes phase noise as a parameter where these crystals perform well, including at high frequencies up to 100MHz at 5th Overtone.
‘Q’ factor is a measurement of the loss of energy within the quartz, and determines the sharpness of the resonant peak; it is affected by the quality of the crystal blank, the surface finish produces by lapping, mounting technique and package sealing. The higher the ‘Q’ factor, the better the crystal, and the higher the frequency of operation, the greater the significance of Q to the designer. The maximum attainable short term stability of a crystal also depends on the ‘Q’ value.
IQD offer an express manufacturing service – to a particular frequency and depending on the specification required – of as little as 3 days.