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The principle and design of the trigger circuit of a signal-seeking radio using the "difference-voltage" of an IF transformer is described in this paper. The "difference-voltage" is first expressed in a mathematical form, from which the ''crossover points" can be found. Constant anticipation can be obtained if this "difference-voltage" is used as a trigger pulse to stop the seeking action. The amount of anticipation is related with the mixing index n and parameters of the IF transformer and it can be expressed in a simple form. The effect of the avc delay voltage in the described circuit does not affect the amount of anticipation. It only affects the proper trigger level. "Error-asymmetry" and the utilization of the property of detuning the output IF transformer in unidirectional seeker are discussed. Besides the error due to the improper trigger level, there are errors due to the nonlinearity of the tuner. This results in the socalled "error spread." Formulas for the amount of anticipation desired in terms of the tuner sweep characteristics and the stopping time are given. The design of the IF transformer with respect to errors due to components tolerances is fully discussed. Approximate formulas are derived for the error variations.