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How is the frequency of a low pass filter determined?
In an electronic low-pass RC filter for voltage signals, high frequencies in the input signal are attenuated, but the filter has little attenuation below the cutoff frequency determined by its RC time constant.
When does the low pass filter transition to significant attenuation?
The spectrum certainly changes relative to 50% duty cycle, but one thing doesn’t change: the first spike is at the carrier frequency. So regardless of the duty cycle, we have a fairly large frequency band—in this case, from DC to 100 kHz—in which the low-pass filter can transition from no attenuation to significant attenuation.
Why do we use a low pass filter in a PWM DAC?
At this point you can probably see why we use a low-pass filter in a PWM DAC: the filter retains the DC component while suppressing everything else.
Why does a RC filter lower the cutoff frequency?
This occurs because the higher resistance in the RC filter not only lowers the cutoff frequency but also increases the time constant—more resistance means less current flowing to the capacitor, and thus the capacitor charges up more slowly. The next plot helps to convey the limitation that this imposes on a DAC:
How is the frequency response of a filter represented?
There are many different types of filter circuits, with different responses to changing frequency. The frequency response of a filter is generally represented using a Bode plot, and the filter is characterized by its cutoff frequency and rate of frequency rolloff.
What is the frequency response of a Butterworth filter?
On any Butterworth filter, if one extends the horizontal line to the right and the diagonal line to the upper-left (the asymptotes of the function), they intersect at exactly the cutoff frequency. The frequency response at the cutoff frequency in a first-order filter is 3 dB below the horizontal line.
Why are filters tuned below the desired harmonic?
The main reason for tuning the filter below the desired harmonic is due to the nature of filter interaction with the system. That is, the filter itself can shift the parallel resonant frequency very close to the harmonic.