Oscillators – Automatic frequency stabilization using a phase or frequency... – Plural a.f.s. for a single oscillator
Reexamination Certificate
1999-11-12
2001-07-10
Grimm, Siegfried H. (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Plural a.f.s. for a single oscillator
C331S017000, C331S034000, C331S057000
Reexamination Certificate
active
06259327
ABSTRACT:
This invention relates to phase locked loops (PLL) and, more particularly, to phase locked loops arranged to operate according to a plurality of characteristics.
A PLL of this kind is described in European patent application EP-A 0 915 568, in the name of the applicant.
The phase locked loop of the above cited application comprises a comparator capable of generating, starting from an input signal and from a feedback signal, a deviation signal, as well as a loop filter fed with said deviation signal. Further, an oscillator is provided for generating, starting from at least one driving signal derived from the deviation signal, filtered by the loop filter, an output signal locked with the input signal. The oscillator can operate according to a plurality of characteristics that relate the driving signal with the output signal. Control means are provided within the circuit to originate said plurality of characteristics and to control automatically and selectively the operation of the oscillator, according to a characteristic that is selectively determined among the above mentioned plurality.
This invention has been conceived with the main purpose of improving the characteristics of flexibility and reliability of operation of such a circuit, in particular as far as the possibility is concerned of making the oscillator (and therefore the phase locked loop as a whole) operate not only at a predetermined frequency, but in a sufficiently wide frequency range. To explain the basic ideas, reference can be made, by way of an example, to a range between 100 MHz and 2 GHz. It is however evident that the reference to said values does in no way limit the scope of the invention.
To meet such a requirement, it is however necessary to take various factors into account.
First, the oscillator, usually a VCO (voltage-controlled oscillator), is a device with a wide output (oscillation frequency) range associated to a small input (control voltage) range.
Having a wide range of tunability is an advantage in terms of flexibility of use and increased possibility of application, but it is a critical and destabilising factor if the small input range is taken into account. As a matter of fact, the control voltage range cannot exceed the power supply voltage of the device (for example 2 V), what, in the event of an oscillator capable of covering for example the above cited frequency range, requires a sensitivity higher than 1 GHz/V. Such a sensitivity of the device makes the implementation of the device extremely critical, since any disturbance, albeit small, causes a considerable disturbance in the output frequency. Further it must be noted that under standard operating conditions, the system locks to the frequency of interest and makes use of the control voltage to keep the locking. The extent of the relating corrections is usually small with regard to the voltage that sets the nominal frequency. Therefore there is the risk of having a device which exploits a considerable part of its input range for determining the nominal frequency, while making instead use of a minimum part of such range for the standard tracking function.
The need of operating selectively at frequencies within a wide variation range makes then the circuit intrinsically more sensitive to the variations of the operating characteristics of the oscillator, in particular variations depending on the fabrication process of the circuit, which typically will be manufactured as an integrated circuit.
At the same time there is the aim of keeping low the sensitivity or gain of the oscillator so as to reduce consequently the cut off frequency of the transfer function of the phase of the PLL (with a consequent reduction of the noise effects): indeed it is impossible—due to the well known phenomena relating to the integration process of the circuit—to reach particularly high values for the capacity corresponding to the behaviour of the loop filter.
Lastly, in trying to meet the various needs described above by increasing the number of possible operating characteristics of the oscillator, the following issues must be taken into account:
the convergence process of the oscillator (thus of the phase locked loop circuit) towards the optimum operating point must be sufficiently fast and, at the same time,
the circuit must exhibit a certain immunity against phenomena leading it to depart from the operating point attained through the initial convergence process.
According to the invention, said aim is achieved by virtue of a circuit that has the characteristics recalled in the following claims.
A tuneable PLL capable of operating according to a plurality of characteristics is disclosed in U.S. Pat. No. 5,075,640. There, the different characteristics correspond to different operating frequencies and are obtained by varying the length of the delay line of the oscillator. No provision is made for choosing an operating characteristic among a plurality of characteristics while keeping the operating frequency constant.
Obviously, even if the introductory part of this description and the subsequent detailed description of an exemplary embodiment of the invention make reference to a PLL destined to be manufactured in the form of an integrated circuit, the scope of the invention must in no way be considered limited to such specific context. In practice, the invention finds application in all the situations in which the previously mentioned problems occur either individually or in combination. This holds true in particular as far as the manufacture of the oscillator is concerned.
REFERENCES:
patent: 5075640 (1991-12-01), Miyazawa
patent: 6127896 (2000-10-01), Burzio
patent: 0 915 568-A2 (1999-05-01), None
Balistreri Emanuele
Burzio Marco
CSELT - Centro Studi e Laboratori Telecomunicazioni S.P.A.
Dubno Herbert
Grimm Siegfried H.
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