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| Does cable impedance add to source or load impedance? |
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| ggchab:
Isn't impedance the sum of resistance and reactance ? So, I believed resistance is a special case of impedance where reactance is close to 0 ? Am I wrong ? |
| fonograph:
--- Quote from: Richard Crowley on February 15, 2019, 10:39:56 am ---There are several resources online that explain cable impedance. For example: http://www.epanorama.net/documents/wiring/cable_impedance.html Down at audio frequencies, there is no practical IMPEDANCE in cables. There is only RESISTANCE (series resistance, to be precise). It is only up at radio frequencies (MHz, GHz, etc.) that cable has impedance. If you only stick to audio frequencies and use the word "resistance" when talking about cable, you will be OK and others will understand what you are talking about. But if you start playing with RF, then at least you will need to understand that they are different things. --- End quote --- I understand your point, I will avoid using the audio frequency example in future. But just to make it clear, I DONT care about the amount of impedance average audio cable has in audio frequencues. You say there is no practical impedance, I say label it as you want there is still impedance and its amount is completly irrelevant in the context of this question. I am not aware of any real world conductor that has only resistance, sure you can name billion examples where its so small that nobody gives a funk about it but we are not solving real world problem here, we are trying to make me understand how impedance counts. I understand that you meant good by telling me audio cables dont care about reactance, I just want to concentrate about how the impedance adds up and not if this is real world problem. We can change my example to 140 GHz phased array radar antenna and cryogenic HEMT based amplifier if you prefer. |
| vealmike:
Richard is correct. Impedance is relevant only when you are dealing with a transmission line. The usual test for a whether or not you have a transmission line is whether the propagation delay of the line is more than 1/3 the 10% to 90% time of your signal. It's a rule of thumb, but a very good one. For a 20KHz audio signal, let's approximate that rise time to 1/2 the period. 1/20KHz is 50us, half that is 25us. Lets assume a very rough 200ps/inch propagation velocity in your cable. 25us/200ps = 125,000 inches. 1/3 of that is 3472 feet. Below a length of 3472 feet, you can ignore the effects of cable impedance at 20KHz. If you have a little over half a mile of speaker cable, sure, you have a transmission line. You will need to start worrying about what we normally term "high speed" effects. You'll also find that the CD will have finished playing by the time you've walked from the hi-fi to the speaker. If you move to rf, then a shorter cable becomes a transmission line. A mismatch of impedances in the channel will cause a reflection. The signal bounces off the mismatch, some of the signal does an about face and heads back to the transmitter. This means less of the signal gets to the receiver. But if you keep everything even, a transmitter with a 50Ohm impedance, a 50Ohm transmission line and a 50 Ohm load then 100% of your transmitted energy gets to the load. This is different to the way that cable series resistance adds to the load in exactly the way you'd expect at dc. This doesn't stop happening in a transmission line, its still there and we still have to take it into account. The two are totally different things, with confusingly similar terminology. I didn't get it when I was a student either, it wasn't until I started working with high speed stuff and playing with a TDR that the penny dropped. I suggest you look online for articles written by Dr. Howerd Johnson. He's really good at explaining this stuff. |
| Doctorandus_P:
It seems that different kinds of impedance are getting intermixed here. A loudspeaker definately has an impedance because it has a resistive and an inductive component, which combine to form the impedance vector. Impedance of a transmission line is a whole other beast, and Richard keeps hammering on it that it is not relevant. But why then bring it up? The dog with it's paw's on the keyboad does not even know what it is. |
| Zero999:
--- Quote from: ggchab on February 15, 2019, 11:07:04 am ---Isn't impedance the sum of resistance and reactance ? So, I believed resistance is a special case of impedance where reactance is close to 0 ? Am I wrong ? --- End quote --- It's the vector sum. Z = √(X2+R2) Yes, when X is near zero, then we might as well just use R. --- Quote from: fonograph on February 15, 2019, 11:12:19 am --- --- Quote from: Richard Crowley on February 15, 2019, 10:39:56 am ---There are several resources online that explain cable impedance. For example: http://www.epanorama.net/documents/wiring/cable_impedance.html Down at audio frequencies, there is no practical IMPEDANCE in cables. There is only RESISTANCE (series resistance, to be precise). It is only up at radio frequencies (MHz, GHz, etc.) that cable has impedance. If you only stick to audio frequencies and use the word "resistance" when talking about cable, you will be OK and others will understand what you are talking about. But if you start playing with RF, then at least you will need to understand that they are different things. --- End quote --- I understand your point, I will avoid using the audio frequency example in future. But just to make it clear, I DONT care about the amount of impedance average audio cable has in audio frequencues. You say there is no practical impedance, I say label it as you want there is still impedance and its amount is completly irrelevant in the context of this question. I am not aware of any real world conductor that has only resistance, sure you can name billion examples where its so small that nobody gives a funk about it but we are not solving real world problem here, we are trying to make me understand how impedance counts. I understand that you meant good by telling me audio cables dont care about reactance, I just want to concentrate about how the impedance adds up and not if this is real world problem. We can change my example to 140 GHz phased array radar antenna and cryogenic HEMT based amplifier if you prefer. --- End quote --- I don't see what all the fuss is about. At low frequencies and DC, then impedance and resistance are often used interchangeably. True, this may not always be strictly correct, but who cares? We all know what people mean when they talk about the impedance of a battery or audio amplifier. |
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