1. Field of the Invention
The present invention relates generally to multiphase oscillators and more specifically to active devices that maintain the oscillation of such oscillators.
2. Description of the Related Art
Most electronic systems encounter noise from several sources, such as shot noise, thermal noise, and flicker noise, which usually arise from currents flowing in the electronic system.
Most electronic systems include one or more oscillators, which provide the clocks that pace or operate the system. Because oscillators are linear, time-varying (LTV) systems, electronic noise has unusual effects on oscillators. One way to characterize these effects is to determine the LTV unit impulse response hφ for the oscillator. This function hφ includes a key function called the impulse sensitivity function (ISF) Γ(ω0τ), which captures the essential information about the oscillator and allows one to compute the oscillator's response to a phase impulse. The ISF for an oscillator shows that the various noise sources mentioned above give rise to phase φ(t) modulation cos [ω0t+φ(t)] of the oscillator frequency ω0, which appears as a spectral “skirt” about the oscillator frequency ω0. This spectrum, actually a power spectral density (Δω), is usually separated into three regions. The region nearest the oscillator frequency is the 1/f3 region. At some point ω1/f (the 1/f noise corner), this region breaks into a 1/f2 region, which continues until it reaches the noise floor of the system, which is a flat region at frequencies far distant from the oscillator frequency.
Using the ISF, the power spectral density (PSD) for an oscillator in the 1/f2 region can be shown to be
is the power spectral density of the noise, Γrms is rms value of the ISF, and Δω is the offset frequency from the oscillator nominal frequency. The PSD for an oscillator in the 1/f3 region is
where now the ISF is included in the c0 term. These formulae for the power of the sidebands about the oscillator frequency indicate that there are both up and down frequency translations of circuit noise currents in into noise near the oscillator frequency. The 1/f2 region results from down-converted white noise at integer multiples of the oscillator frequency and the 1/f3 region results from up-converted 1/f circuit noise. The 1/f noise is particularly troublesome for some applications, but the presence of the c0 term (the dc term) in the PSD function suggests that to minimize this noise, one should make the term as small as possible. One way of doing this is to arrange the circuitry so that disturbances during the rising edge in the oscillation are the same as those during the falling edge, thereby making the net disturbances over the cycle almost cancel out.
Not only does the oscillator itself have a time-varying characteristic, but the oscillator can be subject to time-varying noise sources that vary in a periodic fashion. In fact, in a full analysis, another function called a noise modulating function (NMF) α(ω0t) modifies the ISF to create an effective ISF (ISFe) that accounts for periodic time-varying noise. As with the other noise sources, one should attempt to find circuit topologies that minimize the effect of time-varying noise. One way to minimize the noise is to arrange circuitry so that time-varying noise sources have their maximum power at the minimum sensitivity point in the oscillator waveform.
Rotary wave oscillators are a particular type of oscillator, operating by means of a wave that travels in a closed loop that reverses the phase of the wave on every transit of the loop.
In summary, it is desirable to have an oscillator with low phase noise to make it useful in a great variety of applications and it is desirable, in the case of a rotary wave oscillator, to control the direction of travel of the wave.
An embodiment of the present invention is an amplifier for generating and maintaining a traveling wave on a RTWO. The amplifier is connected to the phases of the RTWO such that it turns off at the critical time when noise would otherwise be injected into the oscillator.
One of the advantages of the present invention is that it eliminates crowbar current. In particular, the dwell time of the amplifier eliminates crowbar current, which flows from supply to ground, thereby contributing noise power but not energy to the oscillator.
Another advantage is that it enforces the wave direction of oscillator of a rotary traveling wave oscillator.
Yet another advantage is that, with proper location of the amplifier phase taps, it reduces the sensitivity of an RTWO due to process variations.
The present invention thus reduces phase noise and provides a means for controlling the direction of the traveling wave in the oscillator.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
Any negative resistance device that operates to add energy to the traveling wave will meet the requirements of a regeneration device for the rotary oscillator.
Operation of
When a positive-going wave front V(Phj) arrives the Phj point 82 on the RTWO, input 2 of the amplifier 40 goes positive first. This causes one of the p-channel transistors (42 in
When the positive-going wave front V(Phk) arrives at the Phk point 84 on the RTWO, now input 1 of the amplifier goes positive. This has the effect of turning ON both n-channel transistors (46, 48 in
Conversely, if we assume that the output of the amplifier is at a relative low voltage, then both n-channel transistors (46, 48 in
The size of the delay or dwell time τpch (Phj−Phk) is adjustable, as shown in
Operation of
The amplifiers in
Direction Control
To encourage a wave front to travel in a preferred direction, an additional constraint must be imposed on the amplifiers' input connections. In particular, if the input connections are subjected to a constraint that input 2 is reached before input 1 and |Phj−Ph0|>|Phk−Ph0|, then the wave front propagation from Phj to Phk in
Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.
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