Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1999-12-13
2001-02-27
Mis, David (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S175000, C331S17700V, C331S179000, C331S185000, C455S075000, C455S256000
Reexamination Certificate
active
06194975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to voltage controlled oscillators (VCO's), and more particularly to reduction of phase noise in the frequency spectrum around a VCO signal frequency.
2. Description of the Prior Art
As is well known in the art, a dual band VCO selects between oscillator output frequencies by switching resonant circuit elements in the active circuit. For each output frequency selected, the oscillator produces a single frequency signal and additional energy in the form of phase noise. This additional sideband noise produced by modulation of the single frequency, is produced by the active device in the oscillator as flicker noise, or the noise figure of the active device under large signal conditions and the filtering effect or the resonant circuit. Achievement of good phase noise performance in each of the switched bands, by use of separate VCO's, has the disadvantage of requiring more components, weight, and cost. Phase noise, as it is known and understood by those skilled in the art, is discussed in 7.5
Noise and Performance Analyses Using CAD Tools
, Communications Receivers. 2nd Edition, Ulrich L. Rohde, Jerry Whitaker and T. T. N. Bucher, which is published by McGraw Hill Company. However, changing the resonant elements without more, does not achieve optimum phase noise performance in a dual band VCO.
Therefore, there exists a need to resolve the problems with the prior art to significantly improve phase noise performance of a dual band VCO as discussed above.
SUMMARY OF THE INVENTION
The invention disclosed according to its inventive principles, achieves optimum phase noise performance in each of the VCO's switched bands output signal frequencies by controlling the bias applied to the active device in the oscillator circuit to a level producing the best phase noise performance for the respective output signal frequency. As shown in the preferred embodiment, the resonant circuit components are selectively switched to change the output signal frequency to a desired band. For each of the bands, the active device, such as a semiconductor device, for example a bipolar transistor, will have an optimum bias point where the phase noise is minimum. Altering the bias to the active device in the oscillator circuit is accomplished at the time the resonant circuit elements are selectively switched so the bias applied to the active device is the optimum bias for optimum phase noise performance and minimum phase noise at the respective output signal frequency selected. Phase noise may be from the flicker noise contribution of the active element in the oscillator circuit or the noise figure of the active device under large signal conditions or from the filtering effect of the resonant circuits or may be phase noise produced, for example, by oscillator amplitude variations producing sidebands on each side of the signal frequency.
Cellular telephones and radios operating in several modes are typically required to switch between receiving frequencies and transmitting frequencies. As would be apparent to one skilled in the art, the use of dual band VCO's for generating the signals needed for modulation and transmission of the base band signal or for demodulation and detection of the baseband signal is more complex than using separate VCO's to generate the same four frequencies, for example. One challenge presented when using a switchable dual band VCO is the achievement of optimum phase noise performance at each of the selected bands. Phase noise, for example, may be measured over frequency and time by measuring the spectral energy at frequencies about the signal frequency and over a set period of time. In this way the spectrum of undesired energy at in a frequency spectrum about the signal frequency may be determined.
A VCO which may be used according to the disclosed inventive principles, for example in a remote cellular phone or duplex radio, uses the same active circuit to generate the desired frequencies or bands. The variation of the bands is achieved, in the preferred embodiment, by selectively switching the resonant circuit components as needed to change the output frequency. For each of the output frequencies, there is a bias level for the active device, such as a semiconductor, for example, where the phase noise's performance is optimum. Accordingly, when the VCO components are switched, changing the resonant elements in the oscillator circuit to generate a separate distinct output signal frequency, the bias level is applied to the active device for which the phase noise performance is optimum for that separate respective output signal frequency. According to the principles of the invention, for the switchable VCO, there is a separate respective bias level for the oscillator active element, at which the phase noise performance is optimum for each respective output signal frequency.
Switching of the resonant elements to achieve a change in the VCO output signal frequency, may be by a diode or micro electronic switch, or semiconductor such as a bipolar transistor or field effect transistor (FET) or other solid state device or micro electromechanical switch or by an electromechanical switch or by other switch means as would be known and understood to those skilled in the art.
This invention is able to achieve those optimum bias levels matched to each of the output signal frequencies for optimum phase noise performance, as disclosed in the description of the preferred embodiment below, by applying a bias level for each output signal frequency at which the phase noise performance is optimum.
What is shown and described is a variable output frequency oscillator with the output signal frequency controlled in response to a band switch signal, having a variable output signal frequency oscillator with an active element and respective output signal frequencies, the active element having bias levels for optimum phase noise performance at the respective output signal frequencies; the variable output signal frequency oscillator including switched elements for producing the respective output signal frequencies and a switch coupled to said switched elements for switching said switched elements to produce the respective output signal frequencies in response to a band change signal, and a terminal coupled to said active element for applying respective bias level signals to the active element for optimum phase noise performance of the variable output signal frequency oscillator at the respective output signal frequencies.
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Leizerovich Gustavo D.
Yeh Peter J.
Doutre Barbara R.
Mis David
Motorola Inc.
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