Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
2001-09-13
2003-04-29
Lam, Tuan T. (Department: 2816)
Oscillators
Solid state active element oscillator
Transistors
C331S016000
Reexamination Certificate
active
06556093
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a voltage controlled oscillator including an oscillator circuit with a control input, an output terminal and a variable capacitor, said oscillator circuit being adapted to provide, at said output terminal, an output signal having a frequency that is a function of the value of said variable capacitor, said value being controlled by a control voltage applied to said control input.
Such a voltage controlled oscillator is already known in the art. It generally forms part of a Phase Locked Loop [PLL] arrangement wherein the frequency of the output signal has to be selected within a relatively large frequency range. This is for instance the case when the voltage controlled oscillator is used in mobile telecommunication systems and if different transmission protocols, such as the known Global System for Mobile communications [GSM] at 900 MHz and Digital Enhanced Cordless Telecommunications [DECT] or Digital Crossconnect System [DCS] in the 1,800 MHz frequency band, have to be covered by a same circuit. Since the frequency is dependent of the control voltage, the latter must then also be able to vary within a relatively large voltage range.
Furthermore, due to variations in the processing technology, the frequency of the output signal may shift, for a same control or tuning voltage, from the desired one by a large amount. Here again, a solution of using a large tuning range requires a high supply voltage and/or an abrupt variable capacitor or “varicap”. A large tuning range results in a large sensitivity of the VCO, which severely influences the surrounding parts of the PLL arrangement.
The trend is now however to reduce the supply voltage in order to reduce accordingly the production cost. Low supply voltage means a reduction of the number of batteries and allows using “deep submicron technology” that permits to reduce the production cost but that can not accept high supply voltages because of risks of break-down.
A problem rising with the reduction of the supply voltage is that a single voltage controlled oscillator may then no longer reach the limits of the requested frequency range.
A possible solution should be to integrate two VCOs in a same circuit. However, such a solution is avoided because of the relatively high cost of this component.
Another possible solution is to design and fabricate several VCOs with “center” frequencies shifted by a certain amount. A center frequency is the frequency at which the VCO should normally operate and that preferably corresponds to a control voltage having an average value, i.e. in the middle of the supply voltage range. Only the VCOs having the correct frequency, under the actual technology characteristics, should then be selected and activated. This also results in an expensive and non-efficient process.
It is finally also possible to perform a digital calibration of the center frequency with a factory trimming. However, digital factory calibration is costly and does not cover variations over the products life time.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a voltage controlled oscillator of the known type but adapted to operate with a relatively low power supply, while allowing a large operational frequency range and keeping the production cost low.
According to the invention, this object is achieved due to the fact that said voltage control oscillator further includes calibration means and switchable capacitor means which comprises switches and capacitors, said calibration means comprising comparator means coupled to a detection circuit, that said comparator means has a first input to which said control voltage is applied, at least a second input to which a predetermined reference voltage is applied, and an output connected to a level input of said detection circuit, that said detection circuit has a control output coupled to switch control inputs of the switches of said switchable capacitor means, said switches being adapted to connect/disconnect, in parallel to said variable capacitor, a capacitor of said switchable capacitor means, that said detection circuit is adapted to activate a control signal if a signal received at said level input indicates that said control voltage has reached said predetermined reference voltage, and that said detection circuit is further adapted to provide said activated control signal at said control output in order to control the switches of said switchable capacitor means.
In this way, when the control voltage reaches the predetermined reference voltage, the fact of adding or removing a capacitor in parallel with the variable capacitor of the oscillator circuit makes the frequency to jump towards another control voltage versus frequency characteristic. As a result, in function of the number of capacitors branched in parallel with the variable capacitor of the oscillator circuit, different frequencies may be obtained for a same control voltage. This allows to increasing the frequency range of a single VCO at relatively low cost and is especially useful in low power supply applications.
Another characterizing embodiment of the present invention is that said voltage control oscillator further includes a feedback circuit having a feedback input to which said output terminal is connected and a feedback output at which said feedback circuit is adapted to provide said control voltage, that said feedback circuit further has a feedback control input to which a reset output of said detection circuit is connected, said detection circuit being adapted to apply a reset pulse to said reset output when said control signal is activated, and that said feedback circuit is adapted to provide, at said feedback output, said control voltage as a function of the frequency of a signal at said feedback input when no reset pulse is received from said detection circuit, and to provide said control voltage equal to a predetermined middle reference voltage when a reset pulse is received from said detection circuit.
When the control voltage changes, say increases, the frequency of the output signal also increases according to a control voltage versus frequency characteristic of the VCO. When the control voltage reaches the predetermined reference voltage, the control signal is activated by the detection circuit and causes the frequency to jump to another control voltage versus frequency characteristic, i.e. to another frequency value. This new frequency value is then generally higher than the requested frequency. This is for instance the case where the control voltage is used for fine-tuning purposes. As a consequence, the control voltage, being then anyway to high and not allowed increasing anymore, needs to be decreased to obtain the correct frequency value. The detection circuit applying a reset pulse to the reset output when the control signal is activated performs this. This forces the feedback circuit to provide a control voltage equal to the predetermined middle reference voltage. This new control voltage is lower than the previous one so that it may again increase thereby increasing the frequency of the output signal.
Preferably, said predetermined middle reference voltage has an intermediate value between said predetermined low reference voltage and said predetermined high reference voltage.
The predetermined middle reference voltage is thus generally a voltage in the middle of the supply voltage range. This allows the frequency to be changed in both directions, i.e. up or down, starting from an average value.
Also another characterizing embodiment of the present invention is that said voltage control oscillator further includes selection means having a first selection input to which said control voltage is applied, a second selection input to which said predetermined middle reference voltage is applied, a selection output connected to the control input of said oscillator circuit, and a selection control input to which the reset output of said detection circuit is connected, and that said selec
Craninckx Jan Frans Lucien
Ingels Mark Maria Albert
Op't Eynde Frank Nico Lieven
Sevenhans Joannes Mathilda Josephus
Alcatel
Lam Tuan T.
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