The phase, frequency, and amplitude of singlephase voltages are the most important and basic information required for single-phase grid connected applications. This paper proposes a new “D-estimation method” for instantly estimating the phase, frequency, and amplitude of frequency-varying single-phase signals in such applications. The D-estimation method has the following attractive features: 1) it is a new filtering method using the D-filters; 2) it involves the use of only filters and can be easily designed; 3) even in hostile envelopments caused by a frequency variation, a phase jump, amplitude sag/swell, harmonic distortion, and/or contaminated noise, good instant estimates of the phase, frequency, and amplitude can be obtained; 4) it is simple, but can exhibit high performance similar to the “robust PLL method.” This paper presents the D-estimation method together with the design rules in detail; the usefulness of the method is verified by performing extensive numerical experiments.