DP8464B datasheet(16/26 Pages) NSC

16 / 26 page

Application Information (Continued)

PULSE DETECTOR OPERATION IN REGIONS 1 AND 2

Figure 14 shows the input waveform for the lowest frequen-

cy followed by the highest frequency for an MFM code In

MFM the highest frequency is twice the lowest frequency

The outer track has the least flux changes per inch (FCI)

and is illustrated in the waveforms at the top There is so

much room between the pulses that the signal returns to the

baseline for the lowest frequency while there is shouldering

at the highest frequency As you go towards the inner track

the pulses become more crowded and bit interaction oc-

curs At the third curve down (N c 17 FCI) there is shoul-

dering at the lowest frequency while the highest frequency

is almost sinusoidal At higher bit densities the lowest fre-

quency looks sinusoidal while the highest frequency is de-

creasing in amplitude In

Figure 14 the first three wave-

forms are examples of Region 1 operation (very little

change in amplitude with frequency) The last two wave-

forms are examples of Region 2 operation

In a disk system the bit density changes about a factor of

17 between the inner and the outer track For instance if

the input waveform for the F-2F signal on the inner track of

a system looks similar to waveform

4in

Figure 14 (N c

22 FCI) then the outer track will have a bit density that is

approximately N c 2217 or N c 13 FCI This is shown in

the second waveform Tracks half the way in will have a bit

density of the average between the inner and outer tracks

in this case N c 17 FCI which is illustrated in the third

waveform Note that the analog waveforms change consid-

erably with track location Self-gating circuits (‘‘desnakers’’)

can be used in MFM systems which operate in these last

three curves (from N c 17 FCI to N c 29 FCI) If the FCI

becomes much less the shouldering on the lowest frequen-

cy will let in too much noise If the FCI is increased the peak

resolution gets very poor Now we can compare these

waveforms to longer run length limited codes

Figure 15 shows the analog waveform for the lowest fre-

quency followed by the highest frequency for a 27 code In

the 27 code the frequency range is from F to 266 c F

Unlike the MFM code there is no region where the self-gat-

ing ‘‘desnaker’’ will work on both the inner and outer tracks

TLF5283 – 20

FIGURE 14 MFM F-2F Pulse Waveforms

for Various Flux Changes per Inch

TLF5283 – 21

FIGURE 15 F-266 c F Pulse Waveforms

for Various Flux Changes per Inch

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