Oscillators – Automatic frequency stabilization using a phase or frequency... – Particular error voltage control
Reexamination Certificate
1999-02-26
2002-06-11
Kinkead, Arnold (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Particular error voltage control
C331S00100A, C331S175000
Reexamination Certificate
active
06404292
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to voltage controlled oscillators and their use in phase locking oscillators. More particularly, the present invention pertains to voltage controlled oscillators with reduced incidental frequency modulation and their use in phase locking oscillators.
2. Description of the Related Art
As is well known, frequencies in electronic circuits have a tendency to drift. Therefore, phase locking oscillators are used widely to provide electronic circuits that are highly immune to frequency drift. The output frequency is phase locked to a reference frequency that is commonly crystal controlled.
In phase locking oscillators, an edge-triggered phase detector compares a frequency that is fed back from a voltage controlled oscillator (VCO) with a reference frequency, and delivers voltage pulses that are a function of the phase difference between the reference frequency and the feedback frequency to an integrator, or filter.
The integrator integrates the voltage pulses received from the phase detector and then delivers the integrated output to the VCO, driving the VCO away from its free-running frequency and toward phase lock with the reference oscillator. Although it may take several or many cycles, eventually the VCO will lock onto the reference frequency, which is commonly crystal controlled.
While some phase locking oscillators are used to provide a carrier that is not modulated, the output frequencies of phase locking oscillators may be frequency modulated on an AC basis at any frequency that is higher than the natural loop frequency by applying a modulating voltage to the VCO. Alternately, the output frequency of the phase locking oscillator may be DC modulated, or both AC and DC modulated, as taught by Lautzenhiser in U.S. Pat. No. 5,091,706, issued Feb. 25, 1992; U.S. Pat. No. 5,097,230, issued Mar. 17, 1992; and U.S. Pat. No. 5,311,152, issued May 10, 1994.
When a VCO is used in a high performance electronic circuit such as a phase locking oscillator, incidental frequency modulation of the output frequency of the VCO is a critical problem, as will become apparent as this discussion continues.
A basic principle of electronic and electrical design is that any wire conductor or any trace on a circuit board that vibrates through an electrical field will induce a voltage in that wire or trace. And, of course, it is inherent in electronic circuits that there are various electrical fields.
Induced voltages, or noise, or voltage spikes that are superimposed on a voltage driving a voltage controlled oscillator, will cause unwanted modulation of output frequency. This unwanted modulation is called incidental frequency modulation. Incidental frequency modulation (IFM) is a serious problem in high performance electronic circuits.
For instance, if a given voltage controlled oscillator (VCO) has a sensitivity of 20 MHz per volt, an induced voltage of only 1.0 millivolt will cause incidental frequency modulation of 20 kHz. Since 20 kHz is the maximum allowable incidental frequency modulation for many military devices, vibration of a wire conductor or trace leading to a VCO can consume the entire allowable incidental frequency modulation bandwidth.
Of course, every attempt is made to keep wire conductors and traces short, thereby minimizing the problem of incidental frequency modulation. However, often an electrical connection must be made from some relatively distant portion of a circuit board, or even from a separate board.
Voltage controlled oscillators have many uses in addition to being a part of a phase locking loop, and whenever voltage controlled oscillators are used in high performance electronic circuits, incidental frequency modulation is potentially a serious problem.
Reducing the sensitivity (MHz per volt) of the VCO by a factor of ten would reduce incidental frequency modulation caused by induced voltages by the same factor of ten. However, the maximum frequency locking range, or capture range, that could be achieved would also be reduced by a factor of ten, in many applications preventing a phase locking oscillator from phase locking.
In summary, while prior art crystal-controlled phase locking oscillators provide improved frequency stability by minimizing both short term and long term frequency drift, voltage spikes and other electrical noise, whether emanating from the integrator, the lead connecting the integrator to the VCO, or from some other induced voltage, widen the required bandwidth, and make it difficult to manufacture electronic apparatus within military specifications.
BRIEF SUMMARY OF THE INVENTION
Voltage controlled oscillators (VCOs) are provided in which incidental frequency modulation is reduced by a factor up to 10.0 or more. Therefore, phase locking oscillators using VCOs of the present invention also enjoy reduced incidental frequency modulation.
In all embodiments of the present invention, reduction in incidental frequency modulation is reduced by reducing a frequency-deviation sensitivity of the VCO. And, in all embodiments, the VCO includes means for restoring a maximum frequency range and a capture range of the phase locking oscillator.
In several embodiments, the frequency-deviation sensitivity is reduced when a frequency of a frequency-control voltage applied to a VCO is above a predetermined roll-off frequency. And the frequency-deviation sensitivity is increased, or restored, when the VCO is subjected to a frequency-control voltage whose frequency is below the predetermined roll-off frequency. In like manner, the VCO responds at the higher frequency-deviation sensitivity if the frequency-control voltage is a constant DC voltage.
The result is that, when used as a part of a phase locking oscillator, the voltage controlled oscillator responds at the reduced frequency-deviation sensitivity to loop frequencies which are above the predetermined roll-off frequency, and at full frequency-deviation sensitivity to loop frequencies, including DC, which are below the predetermined roll-off frequency.
Therefore, voltage spikes and other electrical noise are attenuated in direct proportion to the reduction in frequency-deviation sensitivity of the VCO, and undesirable incidental frequency modulation caused by the voltage spikes and other electrical noise is attenuated by the same ratio.
While the frequency deviation sensitivity of the voltage controlled oscillator is reduced in response to frequencies above the predetermined roll-off frequency, both the capture range and the frequency response of the phase locking loop remains unaffected.
In two embodiments of the VCOs of the present invention, incidental frequency modulation also is attenuated by reducing a frequency-deviation sensitivity thereof, but a maximum frequency range of the VCO is maintained by means, included in the VCO, for augmenting a frequency-control voltage applied thereto.
Therefore, all of the VCOs of the present invention include means for reducing the frequency-deviation sensitivity, whereby a maximum frequency range is also reduced, and all of the VCOs include means for restoring the maximum frequency range. When used in a phase locking oscillator the means for restoring the maximum frequency range also provides means for restoring a capture range of the phase locking oscillator that was reduced by reducing the frequency-deviation sensitivity of the VCOs.
All components of the VCOs, both for reducing the frequency-deviation sensitivity and for restoring the maximum frequency range thereof, are contained in a shielded can, thereby isolating all of their components from stray voltage fields.
By using VCOs of the present invention in phase locking oscillators, voltage spikes, other electrical noises, AC modulation signals, and incidental frequency modulation are attenuated. This is true for voltage spikes and other electrical noises developed within the phase locked loop, or introduced to the VCO from an external source.
In a first aspect of the present invention, a method is provided for reducing incidenta
Emhiser Research Ltd.
Kinkead Arnold
Miller Wendell E.
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