Oscillators – Automatic frequency stabilization using a phase or frequency... – Particular frequency control means
Reexamination Certificate
1997-04-15
2001-08-07
Mis, David (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Particular frequency control means
C331S010000, C331S016000, C331S1170FE, C331S17700V, C331S185000, C455S260000, C455S262000
Reexamination Certificate
active
06271731
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to control circuits for programmable frequency synthesizers. More specifically, the present invention relates to a control circuit for a programmable frequency synthesizer utilizing a voltage controlled oscillator circuit.
2. Description of Related Art
Voltage controlled oscillators are widely used in communication devices such as cellular telephones to generate oscillatory signals. In particular, a voltage controlled oscillator is one of the components of a phase-locked loop or PLL, which is an electronic circuit used to generate a stable oscillatory signal having a desired frequency of oscillation. Such circuits are commonly referred to as frequency synthesizers. The signal output by a PLL frequency synthesizer may be used, for example, as a carrier signal in a transmitter or as a local oscillator signal in a receiver.
In order to tune a transmitter or receiver to a particular frequency, it is necessary to alter the frequency of oscillation of the carrier or local oscillator signal in a controlled manner. This can be achieved by synthesizing the carrier and local oscillator signals using a programmable frequency synthesizer including a VCO. A VCO is tuned by varying an input DC control voltage level.
Most portable electronic devices use a five-cell or four-cell battery to generate DC power. Devices having five-cell and four-cell batteries can generate a regulated DC power supply voltage level of about 4.8 volts and 3.8 volts, respectively. In the case of a five-cell battery, the DC voltage control signal VC used as a control input to the VCO has a range of between about 0.7 and 4.3 volts, or approximately 3.6 volts. In the case of a four-cell battery, VC has a range of between about 0.6 and 3.0 volts, or 2.4 volts. The measure of the frequency responsiveness of a VCO with respect to input voltage VC is known as the gain of the VCO, and is measured in megahertz per volt (MHz/V). Thus, if the input control voltage has a lower range, the VCO must have a greater gain in order to be tunable over the same frequency range.
To be useful in an RF communication device such as a dual band cellular telephone, a voltage controlled oscillator must be tunable over a range of approximately 90 MHz for some applications. Thus, a VCO for use in a device having a five-cell supply must have a gain of about 26 MHz/Volt. A VCO for use in a device having a four-cell supply must have a gain of about 40 MHz/Volt.
One drawback to a VCO is the fact that it produces a certain amount of phase noise, which can degrade the performance of a communication system. A VCO must be designed such that phase noise is kept within certain predetermined limits. In a voltage controlled oscillator, phase noise is directly proportional to the gain of the oscillator. This is due to the fact that the varactor diode in a VCO exhibits a higher internal series resistance when a low control voltage is applied across its terminals. This resistance decreases the quality factor of the oscillator's resonant circuit, leading to increased phase noise.
There is currently a trend in the communication industry to design and produce electronic devices capable of operating with a three-cell battery. A three-cell battery provides a control voltage range of from about 0.5 to 2.2 volts, or 1.7 volts. A conventional VCO having a gain corresponding to such a control voltage range would exhibit unacceptable phase noise characteristics. Thus, there is a need for a voltage controlled oscillator and related control circuitry which exhibits acceptable phase noise performance in a device having a low supply voltage level relative to that normally used for the operation of a voltage controlled oscillator in a battery powered RF communications device, for example.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a voltage controlled oscillator having improved phase noise performance when operated using a low supply voltage.
The foregoing objects are achieved in a voltage controlled oscillator including a voltage controlled capacitor having a first terminal and a second terminal. A positive control voltage is applied to the first terminal of the voltage controlled capacitor and a negative control voltage is applied to the second terminal of the voltage controlled capacitor.
In another aspect of the present invention, circuit for generating a negative control voltage is provided in a phase-locked loop circuit. The circuit includes a negative DC generator for generating a negative DC voltage from an-AC signal or a transitioning logic signal, and a programmable variable attenuator for selectably attenuating the negative control voltage.
These and other objects of the invention, together with features and advantages thereof will become apparent from the following detailed specification when read with the accompanying drawings in which like reference numerals refer to like elements.
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NUD7070 VHF Transceiver board Component Location Diagrams and Schematic Diagram.
Burns Doane Swecker & Mathis L.L.P.
Ericsson Inc.
Mis David
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