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Digitally Sampled, Non-Proportional Control Voltage-Frequency Oscilla- Tor

IP.com Disclosure Number: IPCOM000038921D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 4 page(s) / 59K

Publishing Venue

IBM

Related People

McDonald, BS: AUTHOR [+2]

Abstract

Fig. 1 illustrates in block diagram form an accurate, inexpensive voltage-frequency oscillator (VFO) operating at multiple frequencies. Computing systems generally operate on non-return to zero (NRZ) data. This format is inefficient and/or impractical in data bus and magnetic media storage systems. The NRZ data is encoded into formats where the information is contained in pulse transitions. Bus systems frequently use MANCHESTER or DIFFERENTIAL MANCHESTER coding. Magnetic media systems use "run-length-limited" codes - frequency modulated (FM), modified FM (MFM), and 2/7 code. (Image Omitted) A common feature of these encoding schemes is that they require a phase-locked loop (PLL) to decode them back into NRZ data.

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Digitally Sampled, Non-Proportional Control Voltage-Frequency Oscilla- Tor

Fig. 1 illustrates in block diagram form an accurate, inexpensive voltage-frequency oscillator (VFO) operating at multiple frequencies. Computing systems generally operate on non-return to zero (NRZ) data. This format is inefficient and/or impractical in data bus and magnetic media storage systems. The NRZ data is encoded into formats where the information is contained in pulse transitions.

Bus systems frequently use MANCHESTER or DIFFERENTIAL MANCHESTER coding. Magnetic media systems use "run-length-limited" codes - frequency modulated (FM), modified FM (MFM), and 2/7 code.

(Image Omitted)

A common feature of these encoding schemes is that they require a phase-locked loop (PLL) to decode them back into NRZ data. A conflict arises between the need for fast acquisition of lock (wideband) and tight tracking to minimize the effects of bit jitter and intersymbol distortion (narrow band). Band switching is obviously desirable but this is difficult in a pure analog system.

This system, shown in the figure, offers an accurate, flexible, and inexpensive PLL implementation for these applications. It features digital techniques to implement band switching in the analog portion of the circuitry. Its effectiveness allows multi-frequency operation from a single voltage-controlled oscillator (VCO). Most PLL circuits attempt to acquire phase and frequency lock simultaneously. Varying initial conditions make the lock mechanism difficult to understand and, therefore, optimize. This system sets the initial phase difference to zero. The VCO is then allowed to run through one period of the incoming data. Its phase is then sampled to determine if it is running faster or slower than the data rate. An appropriate correction is applied to the VCO. This procedure is per formed four times in a wideband mode, three times in reduced band mode. Acquisition is complete, and the loop is switched to narrow band in the tracking mode. This portion of the band switching is accomplished by changing the magnitude of the current supplied to the filter and VCO. Frequency switching is programmable via three control lines. This activates counter outputs that are used in the phase compare previously described. Further bandwidth control is implemented by altering the pulse width of the current supplied to the filter and VCO. Below is description of each function of the blocks shown in the figure. SEQ CTR - SEQUENCE COUNTER is a five-bit polynomial counter

locked by the phase-encoded data, DATA

IN. Counting begins when READ ENABLE is

activated. SEQ DECODE - SEQUENCE DECODE processes the five counter outputs

to generate a pulse at selected times in

1

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the count sequence. Note the pulse

duration is equal to the period of the

DATA IN signal. Counts 1, 4, 7, 10, 13,

18, and 24 are logically ORed so a

series of pulses occur at output BLOCK.

Similarly, counts of 2, 5, 8, 11, 16,

19, and 29 occur...