Oscillators – Polyphase output
Patent
1992-04-03
1993-08-03
Grimm, Siegfried H.
Oscillators
Polyphase output
331 55, 331113R, 331143, H03K 3282, H03K 406
Patent
active
052333159
DESCRIPTION:
BRIEF SUMMARY
The invention relates to an oscillator circuit of the regenerative type, comprising a first oscillator at least including embodying a first feedback amplifier and a first integrator in which the first oscillator has an output signal having two stable levels alternating in one oscillation cycle and an unstable or regenerative switching state between each of them, in which the loop gain is greater than unity, and an input signal which originates from the integrator and which varies time-dependently in a positive or negative sense, a separate excitation signal being applied to the oscillator in order to fix the instant of switching.
Such an oscillator circuit is known from the article entitled "A new model for regenerative circuits" by C. J. M. Verhoeven in 30th Midwest Symposium on Circuits and Systems, Syracuse, N.Y., August 1987.
The simple oscillator of the regenerative type known from said article essentially comprises a feedback amplifier and an integrator. The output signal of such a regenerative oscillator has two extreme stable levels in the oscillation cycle thereof and an unstable switching state for a short time between each of them. In said unstable switching state, the loop gain is greater than unity and the amplifier operates with positive feedback or direct feedback. The amplifier configuration is such that in this case the output signal is in phase with the input signal. Said switching state involves regenerative operation and at the end of it, at least one of the components of the amplifier is in the blocked or saturated state. In this case, the loop gain is less than unity, with the result that the state can remain stable until the next switching instant. Such circuits are known in practice by different names, of which Schmitt trigger and flip-flop are the most well known. Characteristic of these circuits is that they exhibit a hysteresis and therefore have a memory action. The output signal is always in one of the two extreme states except during the very short switching time.
The integrator is used in the regenerative oscillator to generate precisely from a constant signal a time-dependent signal which varies in a positive or negative sense. In the case of an integrable regenerative oscillator, the integrator is usually a capacitor which integrates a constant current presented by the said amplifier. The amplifier has two threshold levels for the input or excitation signal originating from the integrator. If the input signal exceeds a threshold level associated with the present level of the output signal, the output signal will switch to the other stable signal level. The said current will change sign and the capacitor voltage will change in the opposite sense until the other threshold level is reached.
The switching of the current by the regenerative amplifier, in other words the instant at which the integrator voltage reaches the threshold level, will exhibit a certain uncertainty because both threshold level and integrator voltage have noise components. In addition, the oscillator will not change over immediately when the threshold level is reached since a certain time is necessary for changing over. The length of this time interval is again strongly affected by noise. As a consequence of said uncertainty, phase jitter occurs, and this results in undesirable energy components in the spectrum of the oscillator around the desired frequency. The relatively slowly changing integrator or capacitor voltage plays an unfavourable role in this connection since the excitation of the regenerative oscillator proves to be minimal as a result of this slow change.
In the said article, to reduce said phase jitter, it is proposed to increase the excitation, for example by injecting an additional excitation signal into the regenerative oscillator. Such an increase in excitation will improve the noise behaviour of the oscillator if the injected signal is always generated at the correct instant, i.e. with as little uncertainty in time as possible, and rapidly at full strength.
The object of the invention i
REFERENCES:
patent: 4468636 (1984-08-01), Monticelli
patent: 4785262 (1988-11-01), Ryu et al.
"One-chip oscillator generates in -quadrature waveforms", Electronics, vol. 51, No. 24, Nov. 23, 1978, by J. R. Pimentel, p. 147.
Grimm Siegfried H.
Telefonaktiebolaget LM Ericsson
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