Can coax cause distortion in signals?

In summary: This is the loss.The conversation discusses the properties of lossless transmission lines and whether or not they distort signals. The participants also mention the equations that express the attenuation and phase constants in terms of the transmission line's parameters, and how these parameters are functions of frequency. They also mention the use of coaxial cables in signal transmission and how the velocity and attenuation of the signal can be affected by the length and quality of the cable. In summary, the conversation concludes that while theoretically lossless transmission lines do not distort signals, in practice there is always some level of distortion due to factors such as frequency-dependent dielectric properties and the length and quality of the cable.
  • #1
seang
184
0
Is it true that lossless transmission lines don't distort signals?

to me, they seem like low pass filters.

i had this class a while ago, but I guess i forgot this, or never understood it

thanks
 
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  • #2
I was under the impression they don't really exist...hmm

you can probably get very close at least
 
  • #3
Are speaking things like a 50 ohm coaxial cables, or the ideal model in terms of the L-C ladder network in the limit as the ladder is more finely divided?
 
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  • #4
seang said:
Is it true that lossless transmission lines don't distort signals?

no. losslessness is an independent property of distortion free propagation.

i don't remember the equations that express [itex]\alpha[/itex] and [itex]\beta[/itex] (expressed in terms of nepers per unit length and radians per unit length) in terms of R, L, G, C of the transmission line. all of these parameters are functions of frequency [itex]\omega[/itex]. if [itex]\alpha[/itex] and [itex]\frac{\beta}{\omega}[/itex] are both constant with frequency, then you have a distortionless line. if [itex]\alpha=0[/itex], then you have a lossless line.
 
  • #5
P=alpha+j * beta
where P is the propogation constant
alpha = (R/(2*z))
where alpha is the attenuation constant
beta = (2*pi/lambda)
where beta is the phase constant
z= characteristic impedence (usually z0)
For a lossless line alpha is 0
 
  • #6
seang said:
Is it true that lossless transmission lines don't distort signals?

to me, they seem like low pass filters.

i had this class a while ago, but I guess i forgot this, or never understood it

thanks

Theoractically yes!

But there is not lossless Tx lines. Just how much. They are not exactly low past. For lossy Tx line, different frequencies have slightly different propagation velocities. So for a long line, signals don't arrive at the same time. Of cause the higher the frequency, the more attenuation on the line. Because the attenuation constant is proportional to frequency.
 
  • #7
I can't see that there are any ways that coax can distort a signal.

The velocity is more or less constant with frequency.

There is no frequency cut-off as such except when the diameter is half a wavelength or more and waveguide modes appear. This limits the maximum diameter of coax at a given freq.
 
  • #8
Pumblechook said:
I can't see that there are any ways that coax can distort a signal.

The velocity is more or less constant with frequency.

There is no frequency cut-off as such except when the diameter is half a wavelength or more and waveguide modes appear. This limits the maximum diameter of coax at a given freq.

Not true, depend on how long the coax is. I have experience with this very issue. I worked for LeCroy before that design digital scope and transient recorder in the 80s. Those days, you can't buy 8bits 100MHz ADC. We had to design subrange configuration using two 4 bits ADC of 100MHz. We have to do summing of two signals and delay using coax as delay lines. The wave form get distorted and I have to design compensation networks for different frequencies to line the signal up again. We started out with RG-175, that has so much distortion that no networks can even compensate it back. I end up using big RG-58 and 4 compensating network in order to make it work.

For single frequency application in typical RF application, you don't see this problem because the frequencies are close together. You see attenuation only and it is easy to get the signal back. For broadband, it is very very obvious! We were using about 12 feet and it really showed.

Check out "group velocity" in EM books, they talk about the exact issue. There are no lossless dielectric, just how much. Not even taflon or any of the fancy dielectrics from Rogers or 3M.

Velocity=[tex]\frac{1}{\sqrt{\epsilon\mu}}[/tex] Where [tex]\epsilon[/tex] is frequency dependent. The imaginary part is [tex]\sigma[/tex]/ [tex]\omega[/tex].
 
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1. What is a transmission line?

A transmission line is a specialized type of electrical circuit that is used to transfer electrical energy from one point to another. It consists of two parallel conductors that are separated by a dielectric material, such as air or a non-conductive material.

2. What is the purpose of a transmission line?

The main purpose of a transmission line is to transport electrical energy efficiently and effectively over long distances. This is achieved by minimizing the losses of energy due to resistance, capacitance, and inductance that occur during the transfer of electrical energy.

3. What are the types of transmission lines?

There are two main types of transmission lines: overhead lines and underground lines. Overhead lines use tall poles or towers to support the conductors, while underground lines are buried beneath the ground. There are also different types of transmission lines based on the voltage level, such as high voltage and low voltage lines.

4. What factors affect the performance of a transmission line?

The performance of a transmission line is affected by several factors, including the type of conductor used, the distance between the conductors, the type of dielectric material, and the frequency of the electrical signals. Other factors that can impact performance include weather conditions, temperature, and the presence of any obstacles or interference.

5. What are the advantages of using a transmission line?

One of the main advantages of using a transmission line is that it allows for the efficient and reliable transfer of electrical energy over long distances. This is especially important for power grids that need to transport electricity from power plants to homes and businesses. Additionally, transmission lines can also help reduce the costs associated with energy distribution, as they minimize energy losses during transmission.

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