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Described below and shown in the drawings is a frequency generator which is based upon a phase-locked loop which provides its own reference signal, and does not depend upon a separate reference Oscillator.
English (United States)
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Auto Reference Phase Locked Loop Clock Generator
Described below and shown in the drawings is a frequency generator which
is based upon a phase-locked loop which provides its own reference signal, and
does not depend upon a separate reference Oscillator.
The conventional phase-locked-loop frequency generator is illustrated in
block diagram in Fig. 1. A reference oscillator signal f(ref) is applied to one input
of a phase detector or comparator, and a feedback signal derived from a voltage-
controlled oscillator is applied to the other signal input. The output from the
phase detector or comparator is filtered and used to control the voltage-
controlled oscillator. The voltage-controlled oscillator output is divided by M (a
programmed count wherein M is a programmable divider), and the result
compared to the f(ref) frequency by the phase comparator. The phase
comparator or detector output is arranged to increase the voltage-controlled
oscillator's frequency if the angle by which the feedback signal lags the reference
frequency is increasing, and to decrease the voltage-controlled oscillator
frequency in the opposite case.
Fig. 2 is a functionally identical circuit in which the
reference oscillator is eliminated by including a high Q mechanical
resonator (the crystal) within the phase-locked-loop system.
Referring now to Fig. 2, the voltage-controlled oscillator (VCO)
center of frequency is selected to be a useful sub-multiple of the
crystal frequency; for example, a 7.5 MHz crystal may be required for
a system clock, and the output frequency Kf(O) would be 7.5 MHz. If
the system also required a lower frequency, say, 300 KHz, f(O) could
furnish this signal. With f(O) set to 300 KHz (VCO center
frequency), the crystal thus represents the 25th harmonic of f(O).
Since the usual commercial phase-locked loop has a square wave as the
output of the VCO, the crystal may be directly excited by this
harmonic-rich signal. K is thus seen to be one divided by 25 in this
example. The buffer element, which may be a comparator or level