Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2000-10-25
2003-11-18
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S167000, C331S057000, C331S03600C
Reexamination Certificate
active
06650194
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of voltage controlled oscillators, and more particularly, the invention is directed to an apparatus and a method for controlling the phase shift of a voltage controlled oscillator independent of changing the oscillation frequency.
BACKGROUND OF THE INVENTION
A voltage controlled oscillator (VCO) is simply a circuit that generates an oscillating signal at a frequency proportional to an externally supplied voltage. VCOs are basic building blocks of many electronic systems especially phase-locked loops and may be found in computer disk drives, wireless electronic equipment such as cellular telephones, and other systems in which oscillation frequency is controlled by an applied tuning voltage. There are three basic designs for VCOs: an inductor/capacitor (LC) oscillator; a ring oscillator; and a relaxation or multivibrator oscillator.
In computer disk drive applications, ring oscillators have been used. Ring oscillators use voltage-controlled variable delay cells connected into a ring. The circuit oscillates at a frequency equal to the voltage controlled cell delay multiplied by twice the number of delay stages. Although high frequencies can be obtained with ring oscillators, they are susceptible to noise and jitter.
The multivibrator or relaxation oscillator oscillates by continuously charging and discharging a capacitor between two voltage levels using a voltage-controlled current source that supplies the current for charging and discharging the capacitor. These multivibrators are relatively simple, require few devices, and are capable of high speeds up to 7.4 GHz. The oscillation frequency, however, is partially dependent upon the capacitor value and is rather sensitive to thermal effects. Because of the sensitivity of the ring oscillators to noise and jitter and because of the sensitivity of the multivibrator or relaxation oscillator to thermal effects, experts in the industry use an oscillator based on inductors and capacitors, the LC oscillator, which is less susceptible to noise and jitter than the previously mentioned oscillator types.
One type of LC oscillator uses a field effect transistor (FET) as a varactor, i.e., a variable capacitance device, by electrically connecting the source and drain to provide a first terminal and the gate of the transistor then provides the other terminal. The capacitance of the modified FET varies significantly with the applied voltage. The capacitance is not only a function of the thickness of the gate oxide of the FET but also its size and whether the FET is in the depleted mode or is in the inverted mode. Transitioning the variable FET capacitor between inverted and depleted mode can yield a capacitance ratio of four or five to one. The capacitance and subsequently the frequency can be finely tuned by varying the voltage difference between the source/drain and the gate of the FET. Pairs of binary weighted FETs may be connected in parallel with a fixed capacitor to yield a voltage controlled tuning circuit. The effective capacitance of selected pairs of FETs can be varied and even switched into or out of a desired capacitive state to yield broadband performance. For example, by the inclusion of four such pairs, a four bit input provides a selection of up to sixteen different bands. Thus, the voltage controlled oscillator circuit having the FETs arranged as above into variable capacitive pairs may be switched in and out of different frequencies exhibiting operational characteristics that are relatively immune to parasitic effects.
Such a voltage controlled oscillator is particularly described in Mourant et al., “A Low Phase Noise Monolithic VCO in SiGe BiCMOS”
IEEE Radio Frequency Integrated Circuits Symposium
65-68 (2000), also in U.S. patent application Ser. No. 09/483,368 filed Jan. 14, 2000, now abandoned, entitled “A Band-Switched Integrated Voltage Controlled Oscillator” by Mourant et al., which is commonly owned by the assignee herein and hereby incorporated by reference in its entirety. The voltage controlled oscillator of Mourant et al. takes advantage of the phenomena described above that a MOSFET becomes a variable capacitor when its source and drain are connected together. The gate to source/drain capacitance varies to a much greater degree with the applied voltage than the alternative reverse bias P/N diode resulting in a VCO circuit having a wide tuning range. Because switching into capacitive mode induced by the applied voltage is abrupt, the modified FET circuit of Mourant et al. is ideal in digital switching circuits, particularly in high frequency operations such as cellular telephones and/or in certain computer disk drive circuits. For precise frequency applications, however, the use of the binary-weighted FETS can only get so close; there must be an analog capacitor provided by a pair of diodes.
With respect to
FIG. 1
, a conventional phase-locked loop (PLL)
10
is shown. The PLL comprises conventional elements such as a charge pump
20
and a loop filter
22
. Loop filter
22
comprises the capacitor
24
and a resistor
26
in series with the capacitor
24
to achieve rapid lock-in of the appropriate frequency. The voltage across the loop filter
22
is provided to a varactor driver
28
which provides a voltage to the oscillator
30
to generate a signal having a frequency proportional to the input voltage from the driver
28
. The frequency is prescaled
32
to become a clock signal
34
of a selected frequency which is input to the timing detector
36
. The data
38
is also input to the timing detector
36
which detects the difference, if any, between the frequency and the phase of the data
38
and the clock
34
. If there is a difference between the frequencies and/or phase of the signals, then a timing correction signal
40
is fedback to the charge pump
20
to adjust the charge on the capacitor
24
which in turn adjusts the clock frequency
34
by another iteration of the process described above. There is, its however, a charge-up time of the timing loop capacitor
24
so that after its capacitor
24
charges, the clock frequency
34
changes slightly.
In disk drive and other applications where data and the oscillator are at or very near the same frequencies there is little or no need to adjust the frequencies by multiple iterations as above. There is, however, a need in the industry to align the phase of the clock with the data without changing the frequency in an LC oscillator.
SUMMARY OF THE INVENTION
These needs and other are met by an embodiment of the present invention, herein disclosed as an LC sinusoidal oscillator comprising an inductive element connected to a supply voltage and a capacitive element connected to an applied voltage which generate an oscillation frequency proportional to the applied voltage, and a negative resistance structure connected to the inductive element and capacitive element; the capacitive element further comprises a fixed capacitor circuit, an incremental capacitive element and a decremental capacitive element connected in parallel to the fixed capacitor circuit. The incremental capacitive element may further comprise at least one field effect transistor whose gate is electrically connected to the capacitive element and whose source and drain are electrically connected to an incremental voltage source to change the state of the at least one field effect transistor to a depleted mode thereby decreasing the capacitance of the capacitive element and providing an instantaneous incremental frequency for phase alignment of the oscillation frequency. The decremental capacitive element may further comprise at least one field effect transistor whose gate is electrically connected to the capacitive element and whose source and drain are electrically connected to a decremental voltage source to change the state of the at least one field effect transistor to an inverted mode thereby increasing the capacitance on capacitive element and providing an instantaneous decremental frequency for phase alignment of
Kertis Robert Andrew
Windler Peter John
International Business Machines - Corporation
Kinkead Arnold
Ojanen Karuna
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