DP8464B datasheet(17/26 Pages) NSC

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Application Information (Continued)

The simplest operation is for systems operating entirely in

Region 1 that is no amplitude reduction between the high-

est and the lowest frequency at the inner track The inner

track is specified because the pulse interaction is most se-

vere on the inner track For Region 1 operation only the

Time Channel filter is required so the Gate Channel Input is

connected to the Time Channel Input Since no external

time delay is required to align the time and gate channels

the Time Pulse Out is connected directly to the Time Pulse

In The Region 1 connection is shown in

Figure 2 The inter-

nal timing for this operation is shown in

Figure 13

If there is significant amplitude reduction at the highest fre-

quency the peak detection becomes more complex If the

worst case waveform is like the fourth waveform on

Figure

14 then the Region 1 connection might still work satisfacto-

rily However if the input begins to approach the fifth wave-

form this system configuration will completely fail One

problem is that the AGC will respond to the frequency de-

pendent amplitude modulation and distort the waveform

Figure 16 illustrates this problem which is encountered in

systems operating in Region 2 If the input digital pattern

suddenly shifts from a high frequency to a low frequency

the bit density may shift from the 70% level on the BPI

curve of

Figure 1 to a point at 90% on the BPI curve As

shown the AGC loop is correcting for this frequency-in-

duced change in amplitude by quickly decreasing the ampli-

fier gain The situation gets worse if the input digital pattern

shifts back to a high frequency The AGC loop now cannot

quickly increase the amplifier gain so the output waveform

will very slowly increase The AGC response to frequency

related amplitude change is not desirable since the AGC is

now distorting the input waveform This can be prevented by

inserting a lead network between the Gain Controlled Ampli-

fier’s output and the AGC input as shown in

Figure 17 This

will increase the amplitude of the higher frequency into the

AGC thereby preventing the AGC from changing gain

Another problem encountered in Region 2 operation is that

the amplitude of the highest frequency may be so low that it

may not trip the hysteresis level If this happens these

peaks would not be gated on to the output This problem

can also be corrected by placing a separate filter to the

gating channel which will make the amplitude of the highest

frequency equal the amplitude of the lowest frequency This

is illustrated in the following example

Consider a disk system which uses the 27 code and has an

input at the inner track which looks like the fifth waveform in

Figure 15 Since the flux density on the outer track is 117

times the flux density of the inner track the outer track

waveform will look like the third waveform One filter cannot

perfectly compensate both these extremes so we design to

compensate a waveform between these two The track

which is

of the way in towards the inner track is a good

compromise The filter in this example is a single zero

placed such that the lowest frequency followed by the high-

est frequency have the same amplitude on the track

the way in

Figure 18 shows the operation of the inner track

of this example While the gating channel filter has made

the amplitudes of the two frequencies nearly the same the

time relationship to the Time Channel Input has not been

preserved The proper operation is to have the positive

edge of the signal at the Time Pulse In pin which corre-

sponds to a peak be the first positive edge after the output

of the comparator has changed states This can be accom-

plished either of two ways One way is to insert an external

delay between the Time Pulse Out and the Time Pulse In as

shown in

Figure 18 The required delay can be determined

by comparing the Time Pulse Out to the Channel Alignment

Output with both external filters in the circuit Another way is

to design the Time Channel Filter with more group delay

This will probably require additional poles

Figure 19 shows the outer track operation of our example

Notice how the system has taken care of the shoulder-in-

duced-noise on the Time Pulse Out The external delay has

shifted the Time Pulse In so the noise is not clocking in new

data to the flip-flop It is important to select this delay such

that the positive edge corresponding to a signal peak is al-

ways the first positive edge after the output of the compara-

tor has changed states

While the gating filter has equalized the amplitudes between

the highest and the lowest frequency the amplitude be-

tween the inner and the outer track has not been held con-

stant This can be seen by comparing the Gate Channel

Input between

Figure 18 and Figure 19 In order to avoid

saturating the Gain Controlled Amplifier the voltage on the

VREF pin must be set so that the voltage out of the Gain

Controlled Amplifier is 4 Vpp or less for all tracks The low

frequency signal on the inner track contains far more funda-

mental frequency than the low frequency signal on the outer

track Consequently the low frequency inner track signal

will experience more attenuation than the low frequency

outer track signal in passing through the gating channel fil-

ter which for this example has been optimized to pass

higher frequencies The AGC tends to hold the input to the

gating channel constant for a fixed VREF level Therefore

the largest output from the Gain Controlled Amplifier is for

the low frequency inner track signal The voltage on VREF

should be adjusted so that the differential output swing of

the Gain Controlled Amplifier is 4 Vpp maximum for this

signal This means that the output voltage on the outer track

will be less than 4 Vpp

TLF5283 – 22

FIGURE 16 Improper AGC Response to Region 2 Signal

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