Oscillators – Solid state active element oscillator – Transistors
Utility Patent
1999-09-15
2001-01-02
Grimm, Siegfried H. (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S159000
Utility Patent
active
06169460
ABSTRACT:
BACKGROUND OF THE PREFERRED EMBODIMENT(S)
1. Field of the Preferred Embodiment(s)
This invention generally relates to ovenized crystal oscillators which provide a stable reference source or frequency in computers or other electronic equipment. Specifically, there is a circuit that is capable of suppressing unwanted oscillation frequencies and allowing oscillation in the selected frequency band. The circuit has less sensitivity to aging of components causing a shift in frequency.
2. Description of the Related Art
Various devices are well known for providing a reference frequency or source such devices are called oscillators. The oscillator typically has a quartz crystal or other resonator and also has electronic compensation circuitry to stabilize the output frequency. Ovenized oscillators heat the temperature sensitive portions of the oscillator which is isolated from the ambient to a uniform temperature to obtain a more stable output.
A resonator designed for one mode will have various other unwanted modes, which are near harmonic, near sub-harmonic or inharmonic. Any of these modes which have series resistance values near or below that of the desired mode could cause the oscillator to run at an undesired mode frequency instead of the desired mode frequency. These unwanted modes must be suppressed, so that no build up to any level in the oscillator will occur. For example, using a doubly rotated crystal, the desired frequency is the third harmonic or overtone. It is desired to suppress or trap out the unwanted modes above and below the third overtone. Unfortunately, there are unwanted modes approximately 10% above, 30% below and 67% below the desired third mode. To prevent or separate the undesired frequencies, various mode suppression circuits have been developed. Some of these are resonant traps to suppress each unwanted frequency, low pass traps to suppress frequencies below the desired frequency, high pass traps to suppress frequencies above the desired frequency. Each of these approaches have various problems. The resonant trap approach for each unwanted frequency requires a separate trap for each unwanted frequency which needs a large number of parts and increases the circuit cost. The low pass and high pass suppression traps, while needing fewer parts, require more precise tolerance values on the capacitors, inductors and resistors in the trap circuit to maintain a precision oscillator frequency. Often the desired modes are very close to the undesirable modes (<10%). This close proximity causes the oscillator to have high frequency sensitivity to changes in the mode suppression circuit component values. This high sensitivity can result in poor frequency aging of the oscillator, especially due to inductor changes over time.
In oscillators of Pierce or Colpitts design, it is known that the circuit of the feedback path should be such that the phase of the feedback signal is −180 degrees. When the resonator has strong undesired modes nearby this −180 degrees should occur only for a band of frequencies that includes the desired mode. This approach allows unwanted oscillation modes above and below the desired oscillation frequency to be suppressed.
Despite the advantages of the prior art oscillators, there is a need for an oscillator mode suppression circuit that uses fewer components and that has fewer problems due to components aging over time.
Description of Incorporated Art
Examples of patents related to the present invention are as follows, wherein each patent is herein incorporated by reference for related and supporting teachings:
U.S. Pat. No. 4,378,532, is an oscillator mode suppression apparatus having bandpass effect.
U.S. Pat. No. 5,053,726, is a circuit for preventing vcxo mode jumping.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicants' acknowledged duty of candor in disclosing information that may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, applicants' claimed invention.
SUMMARY OF THE PREFERRED EMBODIMENT(S)
It is a feature of the invention to provide an oscillator circuit that is capable of suppressing unwanted oscillation frequencies and allowing oscillation in the selected frequency band. The circuit has less sensitivity to aging of components causing a shift in frequency.
A further feature of the invention is to provide an oscillator operable to oscillate on a desired range of frequencies. The oscillator includes a first and second inverting amplifier that each have an input terminal and an output terminal. The first inverting amplifier has a high input and low output impedance, while the second inverting amplifier has high input and output impedances. A crystal resonator in a pi-network is connected between the input terminal of the first amplifier and the output terminal of the second amplifier. A 2 port LC suppression trap is connected between the first and second amplifier for suppressing oscillation outside the desired range of frequencies. The trap includes a parallel tank circuit that is connected between the output terminal of the first amplifier and the input terminal of the second amplifier. A capacitor is connected between the input terminal of the second amplifier and a ground.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Further, the abstract is neither intended to define the invention of the application, which is measured by the claims, neither is it intended to be limiting as to the scope of the invention in any way.
REFERENCES:
patent: 3581236 (1971-05-01), Berman
patent: 3916344 (1975-10-01), Enderby
patent: 4378532 (1983-03-01), Burgoon
patent: 5053726 (1991-10-01), Christopher et al.
patent: 5361045 (1994-11-01), Beaussier et al.
Borgman Mark W.
Bourgeois Mark P.
CTS Corporation
Grimm Siegfried H.
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