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Your given link very helpful, thank you, but let me know that in the SPWM, the PWM signal is actually a PWM or this is as that only ON time change and OFF time constant.
like 1ms ON 5ms OFF then 2ms ON 5ms OFF........ and so on for one cycle.
The most common and popular technique of digital pure-sine wave generation is pulse-width-modulation (PWM). The PWM technique involves generation of a digital waveform, for which the duty-cycle is modulated such that the average voltage of the waveform corresponds to a pure sine wave. The simplest way of producing the PWM signal is through comparison of a low-power reference sine wave with a triangle wave . Using these two signals as input to a comparator, the output will be a 2-level PWM signal . This PWM signal can then be used to control switches connected to a high-voltage bus, which will replicate this signal at the appropriate voltage. Put through an LC filter, this PWM signal will clean up into a close approximation of a sine wave . Though this technique produces a much cleaner source of AC power than either the square or modified sine waves, there is a relatively high amount of higher level harmonics in the signal .
In order to create a signal which is closer to a true sine wave, a 3 level PWM signal can be generated with high, low, and zero voltage levels. For the resulting 3-level PWM signal to correspond to a sine wave, the signal comparison stage must also be 3-level . A triangle wave is used as it is in the 2-level PWM comparison, but it half the amplitude and summed with a square wave to compare one half of the sine reference signal at a time. The resulting PWM signal is used to control one half of an H-bridge , which controls how long the bus voltage is allowed through to the load. The other half of the H-bridge controls the polarity of the voltage across the load, and is controlled by a simple square wave of the same frequency and in phase with the sine signal. Generally, this square wave can simply be created in a stage of the sine wave generation circuit. The resulting 3-level high-voltage PWM signal can be filtered into a very close approximation of the desired sine wave using lc filter.
In order to create a PWM signal which more closely follows the desired sine wave output, the design described for the 3-level PWM technique can be expanded to 5-, 7- and 9+ level PWM. Each additional 2 levels added on top of the 3-level design adds an H-bridge (added in series), a comparator, and a summer. The added accuracy of the signal due to increasing the level therefore brings with it the addition of components, and the space, cooling, and power they require. Control signals must be created separately for each H-bridge , each of which correspond to one layer of the sine voltage . Higher level PWM also requires multiple isolated voltage buses. . The buses must be isolated, as they need to be connected in series. In the 5-level PWM circuit, one half of each H-bridge is controlled by the square wave from the 3-level circuit, and simply controls polarity across the bridge. The other half of each bridge is controlled by the PWM output of each respective comparator. One of the advantages of higher level PWM generation is that there is less of a voltage swing from the minimum and maximum of each step, which results in less power loss due to the slope up and down for each step (known as dV/dt losses).
The Bubba Oscillator is a circuit that provides a filtered sine wave of any frequency
based upon the configuration of resistors and capacitors in the circuit. The circuit completes this task
with four operational amplifiers that either buffer or amplify the signal. This oscillator is a phase shift
oscillator, but unlike other phase shift varieties that require phase shifts of 90 degrees or more, the bubba
oscillator only requires a 45 degree shift in order to function. This is because of the four op amps, that
when placed in series, produce a total 180° shift.
The bubba oscillator offers a few features that other oscillators cannot, the biggest factor is that the
frequency stability holds while still giving a low distortion output. The reason for this involves the four
filters that the signal passes through, providing a clear and stable signal at point P5, as shown in attached figure .