Oscillators – Amplitude control or stabilization – Automatic
Reexamination Certificate
1999-03-31
2001-08-07
Pascal, Robert (Department: 2817)
Oscillators
Amplitude control or stabilization
Automatic
C331S185000, C331S186000, C331S17700V, C455S260000
Reexamination Certificate
active
06271737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a communications device, particularly but not exclusively, to controlling the frequency of a generating source in response to perturbations in the supply voltage due to significant levels of noise. The present invention also relates to a frequency generating circuit and to an integrated circuit including the frequency generating circuit.
2. Description of the Related Art
The communications device may be a one-way pager having a receiver stage including a local oscillator source or a two-way device such as a two-way pager, a cellular telephone and a cordless telephone equipped with a transceiver with a frequency generator for generating the local oscillator frequency for the receiver and the master oscillator frequency for the transmitter. These devices include continuously running circuits which introduce significant levels of noise on the dc supply voltage. This problem is particularly relevant for battery powered devices operating at voltages of the order of 1 volt.
U.S. Pat. No. 5,101,178 discloses a crystal oscillator for a battery powered portable telephone. In order to compensate for changes in frequency in response to changes in the dc source voltage, the source voltage is compared against a reference voltage and a control signal is produced on the basis of whether the source voltage is above or below the value of the reference voltage, a capacitor is added
ot added to compensate or not for any change in oscillator frequency which is intended to be a fixed frequency. A drawback to this known circuit is that the frequency compensation is not continuous because the addition of, or disconnection of, the capacitor produces a step-like change in the oscillator frequency. Also it is not intended to deal with the effects of perturbations on the dc voltage source due to noise generated by other circuits within the device.
OBJECT AND SUMMARY OF THE INVENTION
An object of the present invention is to improve on the stability of an oscillator frequency in response to changes in dc source voltage.
According to one aspect of the present invention there is provided a communications device comprising frequency conversion means having a first input for a signal to be frequency converted and a second input for a converting signal generated by a frequency generating means, the frequency generating means comprising a voltage controlled oscillator (VCO) including an active element coupled to a dc voltage source, frequency determining reactive elements coupled to the active element for determining the frequency to be generated, at least one of the reactive elements being adjustable in response to an applied voltage and means for phase modulating the oscillator in an opposite sense to that caused by voltage perturbations in the dc voltage of the dc voltage source.
According to another aspect of the present invention there is provided a frequency generating circuit comprising a voltage controlled oscillator (VCO) including an active element coupled to a dc voltage source, frequency determining reactive elements coupled to the active element for determining the frequency to be generated, at least one of the reactive elements being adjustable in response to an applied voltage and means for phase modulating the oscillator in an opposite sense to that caused by voltage perturbations in the dc voltage of the dc voltage source.
According to a further aspect of the present invention there is provided an integrated circuit comprising the frequency generating circuit in accordance with the another aspect of the present invention.
By altering the reactance of the adjustable reactive element, for example a varactor diode, in the opposite sense to perturbations in the dc voltage source, the effects of noise on the active element are reduced if not cancelled.
In an embodiment of the present invention the phase modulating means comprises another adjustable reactive element to which the dc voltage is applied. More particularly a capacitance is coupled in series with the another adjustable reactive element. The means for applying the dc voltage comprises a rf blocking inductance coupled to a junction of the capacitance and the another adjustable reactive element.
A high impedance rf filter may be coupled in the dc path to the rf blocking inductance in order to match the phase modulation/modulating frequency characteristics of the dc supply feed to the another adjustable reactive element to the modulation caused by the active element.
The dc voltage may be derived from the dc voltage source or from a suitable point in the oscillator circuit.
The VCO may be embodied in a phase locked loop frequency synthesizer which has a loop filter for applying a control voltage to the adjustable reactive element.
REFERENCES:
patent: 4074209 (1978-02-01), Lysobey
patent: 4442415 (1984-04-01), Ashida
patent: 5101178 (1992-03-01), Komoda
Glenn Kimberly E
Pascal Robert
Slobod Jack D.
U.S. Philips Corporation
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