Qume DataTrak 8 Manual descargar pdf (Pagina 24)

SECTION 2 HARDWARE DESCRIPTION

and provides enhanced data recovery margins in both single and

double density. The loop is constructed of both digital and

analog circuitry.

Figure 2-14. Phase Detector and Filter

The Voltage Controlled Oscillator (VCO) is a 741S123 with

both sections configured to trigger each other. The control

voltage is applied to the resistors R3 and R4/5. An increase in

voltage corresponds to an increase in frequency. IC 2A, a

section of a 741S123, also uses VX to provide this one-shot with

a period which proportionally tracks the period of the VCO. See

Figures 2-14 and 2-15.

A set of timing signals are generated from the VCO which are

used by the Digital Phase Detector. IC's 1C and 3A are 741S1138

and are used to generate these signals. They are all clocked

simultaneously to eliminated any skew in the generated signals.

In the following discussion 8 inch operation is assumed (With 5

inch operation all timings are doubled). In double density

(MFM), the incoming data stream contains bits which are

separated by 2, 3, or 4 microseconds (wishful thinking!). Read

Clock makes a transition every 1us. The Phase-Locked Loop

adjusts the VCO such that each data pulse is centered in the

middle of a half-cycle of Read Clock (Window). More precisely,

due to bit-shift, very few individual data pu1ses are centered.

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Qume DataTrak 8 Manual Pagina 24