Fujitsu MHN2150AT Manual/User Guide - Page 65

4.6 Read/write Circuit 4.6.3 Read circuit The head read signal from the PreAMP is regulated by the automatic gain control (AGC) circuit. Then the output is converted into the sampled read data pulse by the programmable filter circuit and the flash digitizer circuit. This clock signal is converted into the NRZ data by the 16/17 GCR decoder circuit based on the read data maximum-likelihood-detected by the Viterbi detection circuit, then is sent to the HDC. (1) AGC circuit The AGC circuit automatically regulates the output amplitude to a constant value even when the input amplitude level fluctuates. The AGC amplifier output is maintained at a constant level even when the head output fluctuates due to the head characteristics or outer/inner head positions. (2) Programmable filter circuit The programmable filter circuit has a low-pass filter function that eliminates unnecessary high frequency noise component and a high frequency boost-up function that equalizes the waveform of the read signal. Cut-off frequency of the low-pass filter and boost-up gain are controlled from the register in read channel by an instruction of the serial data signal from MPU (M5). The MPU optimizes the cut-off frequency and boost-up gain according to the transfer frequency of each zone. Figure 4.6 shows the frequency characteristic sample of the programmable filter. -3 dB Figure 4.6 Frequency characteristic of programmable filter C141-E120-02EN 4-13