Oscillators – With parasitic oscillation control or prevention means
Reexamination Certificate
1998-02-27
2001-04-10
Kinkead, Arnold (Department: 2817)
Oscillators
With parasitic oscillation control or prevention means
C331S10800D, C331S1170FE, C331S17700V, C331S1170FE
Reexamination Certificate
active
06215367
ABSTRACT:
The invention is based on a circuit for avoiding parasitic oscillation modes in an oscillator resonant circuit.
Oscillator circuits can be subdivided into 2 classes, multivibrators and harmonic oscillators.
Multivibrators oscillate by virtue of a capacitance being charged and discharged between 2 internal switching thresholds. The circuit switches back and forth from one state to the other each time one of the 2 switching thresholds is reached. The charging and discharging operation of the capacitance determines the time constant for the switching back and forth. Multivibrators are highly suitable as integrated oscillators since they require no external components, in particular no inductance and no variable-capacitance diode. In addition, they can be set well and reliably in a wide frequency range. They do have limitations, however. Their radio-frequency capability is limited by the switching times and their spectral purity is poor because the switching thresholds and the charging currents are subject to random variations which cause phase jitter. RC relaxation oscillators and emitter-coupled oscillators belong to the multivibrator class. Ring oscillators can also be integrated well and can even operate in the GHz range, but have a similarly poor phase noise to that of multivibrators.
Harmonic oscillators normally require an LC resonant circuit or a quartz crystal as frequency-determining component and are correspondingly poorly integrable. Their phase noise, on the other hand, is good and given by the frequency-selective nature (high quality factor) of the LC resonant circuit. For broadband applications, however, it is necessary, under certain circumstances, to make the compromise of accepting a poorer quality factor and corresponding degradation of the phase noise in order to enlarge the frequency setting range.
The phase noise is an important parameter of a receiver. Stringent phase noise requirements are imposed particularly for the reception of modern digital modulation methods, such as, for example, QPSK or QAM for satellite and cable television transmission or GMSK and GFSK for GSM and DECT mobile telephones.
Many of these services have settled in the low GHz range. 0.95 . . . 2.15 GHz for digital satellite reception, 0.9 and/or 1.8 GHz for GSM and DECT. The current trend shows that further promising applications will use frequency bands in this low 1 . . . 6 GHz range. In this respect, it is an important task to design receivers and, in particular, oscillators which have good radio-frequency properties.
A typical harmonic oscillator thus comprises an integrated amplifier and an external LC resonant circuit; in between is a part of the housing, which, under certain circumstances, forms a parasitic resonant circuit, as a result of which a parasitic oscillation mode can then also be caused. The gain factor and the phase shift in the oscillator loop govern whether the desired oscillation mode or the parasitic oscillation mode is established. If it is desired to achieve high oscillation frequencies—for example above 1 GHz—, the problem is aggravated since bonding wire inductance and pin capacitance may be of the order of magnitude of the useful elements of the external resonant circuit and then have a greater effect, that is to say are even more interfering than otherwise. If it were additionally desired to cover a wide frequency range, it is no longer possible to rely on the high frequency selectivity or the high quality factor of the external LC resonant circuit and the suppression of radio-frequency parasitic oscillation modes becomes even more difficult.
In such an oscillatory having an external LC resonant circuit, the radio-frequency parasitic oscillation mode caused by the housing may possibly be attenuated by integrated resistors, so that only the desired oscillation mode occurs. However, this measure impairs the circuit quality factor and the phase noise of the oscillators. In addition, it is an empirical and possibly lengthy method until the attenuating resistors are optimized, because it is difficult to model a housing with the required accuracy and this is ultimately achieved only by trial and error.
The invention is based on the object of reducing the interfering influences in a harmonic oscillator having an LC resonant circuit. This object is achieved by means of the features of the invention which are specified in the claims. Advantageous developments of the invention are specified in the subclaims.
In the circuit according to the invention, the reduction of interfering influences in an LC resonant circuit with an integrated circuit is effected by including the interfering elements of the housing in the resonant circuit. The interfering elements are included by forming a series resonant circuit via 2 pins, where the inductance of the housing (bonding wire and lead frame) and of the external connections on the printed circuit board are added to the useful inductance of the LC resonant circuit. These values can thus be incorporated into the resonant circuit calculation, with the result that the factors that otherwise have an interfering effect are already taken into account from the outset. The series LC resonant circuit thus comprises, in principle, an integrated capacitance and an inductance which is composed of the housing inductance via 2 pins and of the external—parasitic and/or useful—inductance on the printed circuit board.
A—possibly more than one—variable-capacitance diode is used for the frequency tuning. A variable-capacitance diode with good tuning capabilities can be integrated only with difficulty. It should preferably be arranged externally for broad band applications.
This variable-capacitance diode is included in the series resonant circuit and may be regarded as a shortening capacitor which reduces the total series capacitance. This enables high frequencies to be set. With a low capacitance, that is to say a small value “C”, the oscillation frequency f=½&pgr;{square root over (LC)} becomes high, and a good phase noise is maintained even after these high frequencies.
In principle, the phase noise deteriorates with frequency. However, it is also proportional to the circuit quality factor Q=(1/R
series
)*{square root over (L/C)}, which in this case improves with a low capacitance, that is to say a small value “C”, that is to say at higher frequencies.
The oscillator according to the invention is preferably intended to be constructed as a balanced Colpitts oscillator. If an external variable-capacitance diode is required, at least one connecting pin is necessary. In order to offer a return path to the radio-frequency currents occurring in the external part of the LC resonant circuit, it is advantageous to make the LC resonant circuit balanced, in other words to use 2 connecting pins. This balanced arrangement additionally makes the oscillator more robust with regard to interference, and reduces the risk of the oscillator itself interfering with other circuit blocks.
Typical, preferably balanced, oscillators require 4 connecting pins for the external LC resonant circuit. The inventive circuit manages with 2 connecting pins and the resonant circuit is now partially integrated. This reduces the number of external components and reduces the space requirement on the printed circuit board. This higher integration level is particularly advantageous for portable applications. However, it is also possible either to completely omit the 2 pins that have become free and select a smaller, less expensive housing, or to use the said pins only for improving the IC function, for example for better heat dissipation or for a double bond on a critical radio-frequency pin.
Compared with multivibrators, harmonic oscillators intrinsically have a good phase noise. It is critically determined by the quality factor of the LC resonant circuit.
In the case of the oscillator arrangement that has been invented, a high quality factor can be achieved if the parasitic serial resistance of the LC resonant circuit is minimized. The variable-capacitance diode
Blaud Phillippe
Rieger Martin
Schemmann Heinrich
Akiyama Kuniyuki
Deutsche Thomson-Brandt GmbH
Kinkead Arnold
Tripoli Joseph S.
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