Low pass filter

May 27 • Notes • 1400 Views • 2 Comments on Low pass filter

Low pass filtering

The basic low-pass filter have the following 2 components.

1. A capacitor installed between the signal line and GND line.

As the frequency become higher  and impedance of the

capacitor becomes lower. Thus noise is forced to go

through bypass capacitors to GND.)

2. An inductor (coil) installed in series with the signal line.

As from the increase in frequency , the impedance of the inductor

increases which can prevents noise from flowing into the signal

line. In the frequency band where EMI noise problems occur, the

loss of insertion of  filters increases  20 dB in every time the frequency is multiplied by ten. When the constant of filters (capacitor’s capacitance or inductor’s inductance) is increased, the  loss of insertionof filters increases by 20 dB every time constant is multiplied by 10. To increase the angle of the loss of  insertion , filters are used in combination. The angle of insertion loss increases by 20 dB/decade every time one filter element is added. If the filter constant was increased by 10 times, the insertion loss angle does not change. However, the insertion loss is increased by 20 dB across the entire frequency As mentioned earlier, the insertion loss is measured with input and output impedances of 50 W . However, actual circuit

impedances are not 50 W. Actual filter effects vary depending on

the impedances of that circuit where  filter is installed.

Generally, a capacitor is more effective in suppressing noise in

high impedance circuits, while an inductor is more effective in

low impedance circuits. If the output is taken off  capacitor, it would respond as a low-pass filterloss of   insertion,filters are used in combination. The angle of insertion loss increases by 20 dB/decade every time one filter element is added. If the filter constant was increased by 10

times, the insertion loss angle does not change. However, the insertion loss is increased by 20 dB across the entire frequency

As mentioned earlier, the insertion loss is measured with input

and output impedances of 50 W . However, actual circuit

impedances are not 50 W. Actual filter effects vary depending on

the impedance of that circuit at that the filter is installed.

Generally, a capacitor is more effective in suppressing noise in high impedance circuits, while an inductor is more effective in low impedance circuits. If the output is taken off  capacitor, it will respond as a low-pass filter.and the open-circuit equivalent may be substituted for the capacitor, resulting in Vo = Vi.

Normalization is a process whereby a quantity  as current, voltage, or impedanc is divided through a quantity of the same unit of measure to establish a level of dimensionless  of a specific value . A plot in the filter domain may be obtained by dividing the plotted quantity such as Vo with the applied voltage Vi for the frequency range of the interest. as the maximum value of Vo for the low-pass filter is Vi, each level of is divided by this level  Vi. The result is the plot of Av = Vo/Vi. Note that the maximum value is 1 and the cutoff frequency is defined at the 0.707 level.  At any intermediate frequency, the output voltage Vo  may be determined using the voltage divider rule.The magnitude of the ratio Vo/Vi is therefore determined and the phase angle is determined by which defines the cutoff or critical   frequency this extends beyond its simplicity. For any low-pass filter, the application of any frequency less than fc will result in an output voltage Vo that is at least 70.7% of the maximum value. For any of the frequency above the value of fc, the output is less than 70.7% of the applied signal.

Q1: low pass filter have how many components?

Ans: The basic low-pass filter have the following 2 components.

Q2: What are those?

Ans: 1. A capacitor installed between the signal line and GND line.

As the frequency become higher  and impedance of the

capacitor becomes lower. Thus noise is forced to go

through bypass capacitors to GND.)

2. An inductor (coil) installed in series with the signal line.

As from the increase in frequency , the impedance of the inductor

increases which can prevents noise from flowing into the signal

line.

Q3: loss of insertion of  filters increases how much   in every time?

Ans:  loss of insertion of  filters increases  20 dB in every time.

Q4: Generally,which componentr is more effective in suppressing noise in high impedance circuits.

Ans: Generally, a capacitor is more effective in suppressing noise in high impedance circuits.

Q5:  The open-circuit equivalent may be substituted for the capacitorTo what equation?

Ans: resulting in Vo = Vi.

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