Crystal oscillators in communication receivers

The application of crystal oscillators to communications receivers is considered and the problems encountered are enumerated and separately analyzed. In regard to crystal oscillator operation, two typical circuits are cited for illustration and are analyzed in regard to the type of feedback used to produce oscillation. The analysis concludes that for maximum oscillator reliability in obtaining the inherent stability a crystal is capable of; regenerative feed-back which is present irrespective of the crystal impedance should be avoided and that these effects are largely determined by the type of crystal oscillator circuit used in a particular application. Over the temperature range normally encountered in communication receiver applications the reversals in the frequency vs. temperature characteristics for a typical close tolerance un-heated crystal is shown in Fig. #1. This data shows that for this type of crystal the total drift for temperature extremes is less rather than maximum due to the curvature of the frequency vs. temperature characteristic. Typical spurious responses of crystals normally used for oscillator purposes are also shown and the possibility of operation on spurious of the crystal is considered from a circuit viewpoint. Preferred methods for crystal oscillator injection and frequency trim are also discussed.