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#5 Passive Filters [Post Homework Here]

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Re: #5 Passive Filters [Post Homework Here]
Q3) Draw an RL highpass filter circuit.
Note that the inductor is a low impedance path for low frequency signals (essentially a short circuit) and a high impedance path for high frequencies (acts like a high frequency open circuit).
Herb
Last edited by wr9h on Sun Nov 17, 2013 11:13 pm, edited 1 time in total.
Re: #5 Passive Filters [Post Homework Here]
Used the wrong code for last upload
This is an RL highpass filter. Note that the inductor is a low impedance path to ground for low frequencies (a short circuit) an a high impedance path for high frequencies (an open circuit). Essentially, the high frequencies have no where else to go but out to the output!!!
Herb
Re: #5 Passive Filters [Post Homework Here]
Lesson 5 Passive RC and RL Filters
Q1) What is the cut off frequency, fc, of a filter and why is it important?
Short Answers:
fc is the frequency at which filter input frequencies will start undergoing alteration/attenuation (3dB)
Importance: RC and RL filter design is predicated on the value of fc!!
Long Answers:
In a lowpass filter any frequencies below the fc, or cutoff frequency, are passed to the filter output without alteration or modification. Frequencies greater than the fc are altered and filtered out (passed to ground in an RC filter and "blocked" by the high frequency impedance of the inductor in an RL filter).
"Only frequencies less than the cutoff frequency will pass through the filter unchanged..."
In a highpass filter any frequencies higher than fc will pass through the filter unchanged while those frequencies below fc will be altered and "filtered out".
Only those frequencies above the cutoff frequency will pass through the filter unchanged....
Herb
Q1) What is the cut off frequency, fc, of a filter and why is it important?
Short Answers:
fc is the frequency at which filter input frequencies will start undergoing alteration/attenuation (3dB)
Importance: RC and RL filter design is predicated on the value of fc!!
Long Answers:
In a lowpass filter any frequencies below the fc, or cutoff frequency, are passed to the filter output without alteration or modification. Frequencies greater than the fc are altered and filtered out (passed to ground in an RC filter and "blocked" by the high frequency impedance of the inductor in an RL filter).
"Only frequencies less than the cutoff frequency will pass through the filter unchanged..."
In a highpass filter any frequencies higher than fc will pass through the filter unchanged while those frequencies below fc will be altered and "filtered out".
Only those frequencies above the cutoff frequency will pass through the filter unchanged....
Herb
Re: #5 Passive Filters [Post Homework Here]
Lesson 5 RC and RL Passive Filters
Q2) Can a resistor+capacitor (RC) be used in both a lowpass AND a highpass filter?
YES. The fc for an RC circuit is fc=1/2Pi(RC). This formula works for both low and highpass RC filters?
How can this be? It's all in the resistor and capacitor arrangement and in how the capacitor behaves with applied AC frequencies. If the circuit is setup with the capacitor across both the input and output of the circuit and there is a series resistor in the input circuit the filter is a lowpass. It works like this: the capacitor acts as a very high impedance to low AC frequencies so they cannot pass through the capacitor to ground (the common grounded lead of both the filters input and output). One might say that the capacitor "appears" as an open circuit to low frequencies. High frequencies pass through the capacitor and are "shorted" to ground thus "eliminated" from the filters output.
If the resistor is placed in the shunt position and the capacitor is placed directly in series between the filters input and output the filter is a highpass filter. How does this work? The high frequencies are easily passed through the capacitor and the ease is directly related to the capacitor values; higher value capacitor pass higher frequencies (capacitors act as high frequency 'shorts"). Low frequencies see the capacitors as open circuits and are blocked.
Our experiment illustrated the above nicely:
With increasing capacitance the lowpass filter removed the high frequencies (passed them to ground).
With increasing capacitance in the highpass filter low frequencies were blocked and high frequencies passed.
Very cool and fun experiment!
Herb
Q2) Can a resistor+capacitor (RC) be used in both a lowpass AND a highpass filter?
YES. The fc for an RC circuit is fc=1/2Pi(RC). This formula works for both low and highpass RC filters?
How can this be? It's all in the resistor and capacitor arrangement and in how the capacitor behaves with applied AC frequencies. If the circuit is setup with the capacitor across both the input and output of the circuit and there is a series resistor in the input circuit the filter is a lowpass. It works like this: the capacitor acts as a very high impedance to low AC frequencies so they cannot pass through the capacitor to ground (the common grounded lead of both the filters input and output). One might say that the capacitor "appears" as an open circuit to low frequencies. High frequencies pass through the capacitor and are "shorted" to ground thus "eliminated" from the filters output.
If the resistor is placed in the shunt position and the capacitor is placed directly in series between the filters input and output the filter is a highpass filter. How does this work? The high frequencies are easily passed through the capacitor and the ease is directly related to the capacitor values; higher value capacitor pass higher frequencies (capacitors act as high frequency 'shorts"). Low frequencies see the capacitors as open circuits and are blocked.
Our experiment illustrated the above nicely:
With increasing capacitance the lowpass filter removed the high frequencies (passed them to ground).
With increasing capacitance in the highpass filter low frequencies were blocked and high frequencies passed.
Very cool and fun experiment!
Herb
Re: #5 Passive Filters [Post Homework Here]
Lesson 5 Passive RC and RL Filters.
Lowpass RC circuit consisting of R=10 ohms and C=10uf, what is fc?
fc= 1/2Pi(RC)
fc= 1/(6.2831)(10)(10x10^6)
fc= 1592Hz or 1.592kHz
Herb
Lowpass RC circuit consisting of R=10 ohms and C=10uf, what is fc?
fc= 1/2Pi(RC)
fc= 1/(6.2831)(10)(10x10^6)
fc= 1592Hz or 1.592kHz
Herb

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 Posts: 1181
 Joined: Mon Nov 12, 2007 9:24 pm
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 Contact:
Re: #5 Passive Filters [Post Homework Here]
wr9h wrote:
Very cool and fun experiment!
Glad to hear this.
The active filter experiment is even better in my opinion, keep at it!
Good homework answers as well.
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