Line impedance versus shunt capacitance

In summary, line impedance and shunt capacitance are two important factors that affect the performance of electrical systems. Line impedance refers to the resistance and reactance of the transmission line, which can impact the quality of power flow and cause losses. On the other hand, shunt capacitance is the ability of a material to store electric charge, and excessive levels can lead to voltage instability and power factor issues. Balancing line impedance and shunt capacitance is crucial in maintaining a stable and efficient electrical system.
  • #1
Landru
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Hello, I'm having the damnedest time wrapping my head around a concept of audio electronics:

http://whirlwindusa.com/support/tech-articles/high-and-low-impedance-signals/

High impedance lines are more adversely affected by the inherent capacitance that is present in the cable itself. This capacitance combines with the impedances of the source and destination to set up a filter. As the impedance increases and/or the capacitance per foot increases, the active frequency at which the filter comes into play gets lower.

How exactly does a higher impedance bring down the frequency at which capacitance dominates? My current understanding is that the frequency at which capacitance will dominate is dependent on the capacitance value. Through what mechanism of physics does the a increase in source or input impedance decrease the frequency at which capacitance dominates?

Thanks!
 
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  • #2
Well, if there was no impedance at all, there would only be a fixed load on the signal - no low pass filter at all.
At low frequencies, the resistance dominates. At high frequencies, the capacitor acts as a low resistance element compared to the actual resistors.
Here is a link that will allow you to experiment:
https://www.circuitlab.com/editor/#?id=7xn74dzye4qa

Try a frequency sweep from 1Hz to 1MHz.
 
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  • #3
Neat website. So R1 in the simulation is meant to be the internal impedance of the source, right? Increasing the load impedance, R2, has little effect on the simulation as I would expect.
 
  • #4
  • #5
Paul Colby said:
Neat website. So R1 in the simulation is meant to be the internal impedance of the source, right? Increasing the load impedance, R2, has little effect on the simulation as I would expect.
For the case of short cables at low frequencies:-
The low frequency attenuation is determined by source and load resistances in series, forming a potential divider.
The turn over frequency, where the slope commences, is determined by source and load resistances in parallel, so they have equal effect. At the -3dB point, the two resistances in parallel equal the capacitive reactance of the cable.
 
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  • #6
.Scott said:
Well, if there was no impedance at all, there would only be a fixed load on the signal - no low pass filter at all.
At low frequencies, the resistance dominates. At high frequencies, the capacitor acts as a low resistance element compared to the actual resistors.
Here is a link that will allow you to experiment:
https://www.circuitlab.com/editor/#?id=7xn74dzye4qa

Try a frequency sweep from 1Hz to 1MHz.

Thanks for making that demo. Between your response and and tech99's post it makes good sense now. So the issue is that a high impedance causes the capacitance to become a "path of least resistance" as frequency rises, where as a low impedance deprives the capacitance of that opportunity to become a path of least resistance.

I recreated it in LTSpice so I could see multiple resistances in a single plot. The green line is the lowest resistance value, and each line below it corresponds to higher values for the resistors.

upload_2018-1-24_19-33-39.png
 

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  • #7
Landru said:
Thanks for making that demo.
You're welcome - but I did not need to create that demo. It was already there.
 

1. What is line impedance and how does it relate to shunt capacitance?

Line impedance refers to the total opposition to current flow in an electrical circuit. It is affected by various factors, including the type of conductor used, the length of the line, and the frequency of the current. Shunt capacitance, on the other hand, is the measure of capacitance between two points in a circuit. Line impedance and shunt capacitance are related because shunt capacitance can affect the overall impedance of a line.

2. How does shunt capacitance affect line impedance?

Shunt capacitance can affect line impedance in two ways. First, it can increase the overall capacitance of the line, which can decrease the line impedance. Second, it can cause reactive power to flow, resulting in an increase in the line impedance. The exact effect of shunt capacitance on line impedance depends on the specific characteristics of the circuit.

3. Why is it important to consider line impedance and shunt capacitance in circuit design?

Line impedance and shunt capacitance are important factors to consider in circuit design because they can affect the overall performance and efficiency of the circuit. If the line impedance is too high, it can lead to power losses and voltage drops. Similarly, excessive shunt capacitance can cause problems such as harmonic distortion and noise interference. Therefore, it is essential to carefully consider these factors to ensure optimal circuit operation.

4. How can line impedance and shunt capacitance be calculated?

Line impedance and shunt capacitance can be calculated using various formulas and equations, depending on the specific circuit parameters and configurations. In general, line impedance can be calculated by dividing the voltage by the current in a given circuit. Shunt capacitance can be calculated using the formula C = Q/V, where C is the capacitance, Q is the charge, and V is the voltage.

5. What are some ways to mitigate the effects of line impedance and shunt capacitance?

There are several methods to mitigate the effects of line impedance and shunt capacitance. One way is to use conductors with a lower resistance, which can reduce the overall line impedance. Another method is to add inductors or resistors in parallel to the circuit to mitigate the effects of shunt capacitance. Additionally, proper circuit layout and design can also help minimize the impact of these factors on circuit performance.

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