RL Bandpass Filter: Impedance & Voltage Signal

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Discussion Overview

The discussion revolves around the functioning of RL low pass and high pass filters, specifically addressing the relationship between circuit impedance and source impedance, as well as measurement techniques for evaluating filter performance. Participants explore theoretical and practical aspects of filter design and measurement methods.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions whether the RL low pass and high pass filters need to have the same impedance as the source impedance for proper operation, noting a lack of filtering in their setup.
  • Another participant suggests that while the source impedance does play a role, it is not strictly necessary for the filters to function.
  • A participant mentions calculating a transfer function of 0.002 but is corrected by another who states that a transfer function should be a function of frequency rather than a single number.
  • Concerns are raised about the appropriateness of using multimeters for measuring transfer functions, with suggestions that oscilloscopes and signal generators are more suitable tools.
  • One participant expresses confusion about why multimeters may not adequately measure voltage, frequency, and current in this context, prompting a discussion about the limitations of multimeters at higher frequencies.
  • Another participant shares their experience measuring a 60Hz signal with a multimeter capable of reading up to 1 MHz, noting discrepancies between expected and actual output current.
  • Questions arise regarding the voltage source used and whether measurements are being taken directly from AC mains, with a request for a circuit sketch to clarify the setup.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of matching impedances for filter operation and the appropriate methods for measuring filter characteristics. The discussion remains unresolved regarding the effectiveness of the filters and the accuracy of the measurement techniques employed.

Contextual Notes

Participants reference specific measurements and calculations, but there are unresolved questions about the assumptions underlying these measurements and the definitions of terms like transfer function. The discussion also highlights potential limitations of measurement tools in the context of filter analysis.

Who May Find This Useful

Individuals interested in circuit design, filter theory, and measurement techniques in electronics may find this discussion relevant.

Idea04
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Does an RL low pass and high pass filter need to have the same impedance in the circuit as to the source impedance to work properly. I put an inductor in series with the source with a resistor to ground as the low pass, and a resistor in series with the source with an inductor to ground as a high pass. and there is no filtering of the voltage signal.
 
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Idea04 said:
Does an RL low pass and high pass filter need to have the same impedance in the circuit as to the source impedance to work properly. I put an inductor in series with the source with a resistor to ground as the low pass, and a resistor in series with the source with an inductor to ground as a high pass. and there is no filtering of the voltage signal.

It doesn't need to, but the source impedance certainly comes into play. What transfer functions do you calculate for the HPF and LPF configuratoins? How are you measuring the filter characteristic?
 
I calculated a transfer function of .002. I measured the signal by having two different multi meters, one for ground and one for output. The voltage gradually increased, but I didn't get any current amplification.
 
Idea04 said:
I calculated a transfer function of .002. I measured the signal by having two different multi meters, one for ground and one for output. The voltage gradually increased, but I didn't get any current amplification.

A transfer function is a function of frequency, not a number. You will get a polynominal function that varies with the frequency of the excitation.

http://en.wikipedia.org/wiki/Transfer_function

You don't measure the transfer function with multimeters. You can measure it with a signal generator and a dual-channel oscilloscope, though.
 
I'm a bit confused here. To measure the output voltage, frequency and current I have to use an oscilloscope. Why won't a multimeter measure the signals voltage, frequency, and current properly.
 
Idea04 said:
I'm a bit confused here. To measure the output voltage, frequency and current I have to use an oscilloscope. Why won't a multimeter measure the signals voltage, frequency, and current properly.

Multimeters are usually used to measure DC values and AC values up through AC Mains power distribution frequencies (50/60Hz). You may find some that can go a bit higher in frequency on the Vac setting, but not very far. Check the specifications for your multimeter -- what is the max frequency that it says the Vac setting can handle?

What frequency range do you want to test the filter transfer function over?
 
The frequency I am trying to filter is 60Hz, My meter can read to 1 MHz. The meter measures the signal voltage and it gradually rises, but with the components I used I should have a higher current through the output. The calculations show an output current of 1.7 amps but I don't get any output current. So the filter must not be working properly.
 
Idea04 said:
The frequency I am trying to filter is 60Hz, My meter can read to 1 MHz. The meter measures the signal voltage and it gradually rises, but with the components I used I should have a higher current through the output. The calculations show an output current of 1.7 amps but I don't get any output current. So the filter must not be working properly.

A 1MHz bandwidth -- that's an impressive multimeter!

BTW, you aren't making measurements connected directly to the AC Mains line, are you? What is your voltage source? Those are larger currents than you get with most signal generators.

Can you post a sketch of your circuit and how you are making the measurements? If you can make a PDF or JPG file, you can add it as an attachment.
 

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