Output Noise and Filtering

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

The noise in the output signal is that present in the bandwidth (1/2r) of the band-pass filter. If the power noise spectrum at the input is white (flat) and given by wn(f)9 the lock-in output noise will be simply ... 

One frame-based implementation of this time-varying linear filtering is to filter windowed blocks of white noise and overlap and add the outputs over consecutive frames. A time-varying impulse response of a linear system can be associated with... 

To evaluate the proposed adaptive control performance, opening of production choke and flow rate of injected gas at the well-head are random pulses as command signals, as shown in figure 5. The opening value of the production choke Upc also is the manipulated variable of the control strategy. Figure 6 shows noisy measurements and filtering outputs of closed-loop system, where variable variances of measurement noises are apparent. Note that Wpc illustrated the stabilized behavior of closed-loop system. 

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