Oscillators – Electromechanical resonator – Crystal
Utility Patent
1999-07-14
2001-01-02
Pascal, Robert (Department: 2817)
Oscillators
Electromechanical resonator
Crystal
C331S173000
Utility Patent
active
06169462
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to oscillators, generally, and to oscillators with start stop control circuitry, in particular.
BACKGROUND OF THE INVENTION
The uses of oscillators are well known. A known form of oscillator comprises an inverting amplifier having a DC feedback path for biasing the amplifier to a linear operating range and an AC feedback path for causing oscillations to occur.
In certain applications, such as in television apparatus, a need may arise for preventing undesired oscillator interference effects. For example, in television receivers, monitors VCR's or the like designed for multi-mode operation (e.g., NTSC and ATSC reception modes), different clock signal frequencies may be required in different operating modes. In general, it is common practice to provide separate oscillators for the respective operating modes rather than changing the frequency of a common oscillator. When separate oscillators are provided, however, signal radiation from an un-used oscillator when receiving signals in one operating mode may be picked up by the video processing circuitry and cause interference effects in displayed images (e.g., “beat” patterns).
Various methods are known for reducing such oscillator interference effects. One method would be to allow the un-used oscillator to continue to run but gate its output. This approach, however, might require shielding the continuously running oscillator which may be both costly and complex. A better approach, which does not require shielding, is to turn off the oscillator that is not being used. This requires an oscillator with start/stop capability.
Two possible methods one might consider for stopping an oscillator are (I) interrupt the oscillator power or (ii) add a load for “swamping” the oscillations (i.e., reducing the loop gain below unity thus not satisfying one of the Barkhausen criteria for oscillations). Such methods, however, are generally unsatisfactory. For example, removing power can create problems with start-up since the oscillator may be required to turn on and off frequently during “channel surfing” and such an approach would also require cut-off circuitry with a very low series drop to keep from reducing the supply voltage (Vcc) of the oscillator down too low. Switching in a load for detuning or loop gain reduction may create new problems necessitating some special circuitry for the oscillator to restore a proper tuning range and loop gain when oscillations are again desired.
SUMMARY OF THE INVENTION
The present invention is directed to meeting the need for an oscillator having start stop capabilities and which overcomes the problems discussed above.
An oscillator, in accordance with the invention, comprises an amplifier having a DC feedback path for biasing the amplifer to a linear operating range and having an AC feedback path for causing oscillations to occur. A controlled current source is coupled to an input of the amplifer, for supplying a current in a first sense thereto for enabling the oscillations and for supplying a current in a second sense thereto for inhibiting the oscillations.
In accordance with a feature of the invention, the controlled current source includes a logic level translator coupled in series with an output current limiting impedance.
REFERENCES:
patent: 4016508 (1977-04-01), Kobori
patent: 4043109 (1977-08-01), Numabe
patent: 4370625 (1983-01-01), Someshwar
patent: 5909152 (1999-06-01), Li et al.
patent: 5982246 (1999-11-01), Hofhine et al.
Choe Henry
Pascal Robert
Shedd Robert D.
Thomson Licensing S.A.
Tripoli Joseph S.
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