TABLE OF CONTENTS Few circuit problems are less welcomed than frequency drift. This problem is characterized by the radio transmitter or receiver changing frequency not under the influence of the operator. Actually, two different problems are seemingly related: drift and shift. Drift is a gradual change of frequency, usually as a function of temperature. Shift, on the other hand, is an abrupt change of frequency. The causes of these two related phenomena are different, but they are often confused with each other.
There is also a difference between problems with new projects and drift on old equipment that once worked well. On a newly constructed project, or new equipment from less reputable sources, the problem might well be an inherent error in the design (some easily corrected, others not). On equipment that once worked well, however, you have a problem of a failed component.
Resonant circuits in modern electronic equipment might be LC-tuned by a combination of inductance and capacitance or tuned by a piezoelectric resonator element (crystal). In either event, the cause of a sudden unwanted shift of operating frequency is usually some form of mechanical trauma somewhere in the circuit. In other words, some component is either broken or has an intermittent connection.
Figure 25-1 shows a partial circuit of an oscillator. The resonance of this circuit is determined by the combination of C 1/ C 2/ C 3/ L 1. If any of these components fail, change value, or become disconnected then the...
