Defect detector circuit with a signal synthesizer and magnet disk certifier using the same defect detector circuit

A defect detector circuit for writing a predetermined test data on a magnetic disk according to a test signal having a predetermined frequency and detecting an error on a basis of a read-out signal having a frequency following the frequency of the test signal and obtained by reading out the test data from said magnetic disk, comprising:

• a low-pass filter for extracting a low frequency component of the read-out signal, the low frequency component containing a signal component of the error;
• a signal generator circuit for generating a signal having a predetermined frequency lower than the frequency of the read-out signal;
• a synthesizer circuit for synthesizing an output signal of said low-pass filter and the signal outputted from said signal generator circuit;
• and
• a comparator for generating an error signal by comparing a synthesized signal from said synthesizer circuit with a predetermined reference level.