Kevin Weddle Posted December 11, 2006 Report Share Posted December 11, 2006 Corroboration on a few things would be appreciated.A transmission line can appear to the source as a parallel resonant circuit, a series resonant circuit, a capacitor, or an inductor.When constructed correctly, the source only sees a very high resistive impedance. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 11, 2006 Report Share Posted December 11, 2006 My cable TV is fed from a 75 ohms coax cable. It isn't a very high resistance. Some coax transmission lines are 50 ohms. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 11, 2006 Author Report Share Posted December 11, 2006 What about the parallel resonance and things? Quote Link to comment Share on other sites More sharing options...
AN920 Posted December 11, 2006 Report Share Posted December 11, 2006 There are a few conditions of importance. Example:1) for lines < 1/4 wavelength50Ohm source into 50 ohm (Zo) line, OC at the end appears (capacitive)50Ohm source into 50 ohm line, SC at the end appears (inductive)50Ohm source into 50 ohm line, Rl at the end (Rl> Zo)appears (cap//resistance)50Ohm source into 50 ohm line, Rl at the end (Rl< Zo)appears (ind + resistance)2) for lines = 1/4 wavelength50Ohm source into 50 ohm (Zo) line, OC at the end appears (SC)50Ohm source into 50 ohm (Zo) line, SC at the end appears (OC)50Ohm source into 50 ohm (Zo) line, Rl at the end appears (Zo*Zo/Rl)3) for lines > 1/4 wl and < 1/2 wl50Ohm source into 50 ohm (Zo) line, OC at the end appears (inductive)50Ohm source into 50 ohm (Zo) line, SC at the end appears (capacitive)50Ohm source into 50 ohm (Zo) line, Rl at the end (Rl>Zo) appears (ind + res)50Ohm source into 50 ohm (Zo) line, Rl at the end (Rl<Zo)appears (cap//res)where OC :-open circuitSC :-short circuitRl :-Rloadcap//res :-cap and resistor in parallelind + res :-ind and resistor in series Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 11, 2006 Author Report Share Posted December 11, 2006 You know, I thought a tuning stub was to be shorted, instead I've seen it open. Quote Link to comment Share on other sites More sharing options...
AN920 Posted December 11, 2006 Report Share Posted December 11, 2006 It all depends what you want to do. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 11, 2006 Report Share Posted December 11, 2006 Coax cable has series inductance and capacitance between conductors. If it is terminated with an incorrect resistance like a short or an open, then it resonates as a series resonanlt circuit with a very low resistance or as a parallel resonant circuit with a high impedance. You tune the resonant frequency by changing its length which might be 1/4 wavelength.In "the good old days" we used to notch out interference from a very strong local TV station with a piece of coax cable hanging at the antenna terminals. Quote Link to comment Share on other sites More sharing options...
AN920 Posted December 11, 2006 Report Share Posted December 11, 2006 The resonance conditions in the coax will happen only at a 1/4 wavelength (when the coax cable is 1/4 wl long) Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 12, 2006 Author Report Share Posted December 12, 2006 Isn't a 50 ohm termination much like a short? Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 12, 2006 Report Share Posted December 12, 2006 Coax cable is 50 ohms or 75 ohms. A short is zero ohms, a big difference. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 13, 2006 Author Report Share Posted December 13, 2006 Audioguru,We'll set the line, load, and source impedances properly for argument sake. As far as I know, the source sees parallel resonance, series resonance, a capacitor, or inductor. And since these conditions are determined by the load value and length of the line, these four conditions are all it sees.Why not parallel resonance? Or series resonance? Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 13, 2006 Report Share Posted December 13, 2006 A 75 ohm source connected to a 75 ohm coaxial cable that is terminated by a 75 ohm resistor does not resonate, even though the transmission line has inductance and capacitance. The circuit is purely resistive.The source and cable's output will be messed up if the termination resistance doesn't match. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 13, 2006 Author Report Share Posted December 13, 2006 A series resonant circuit looks only to be the resistance of the coil. Outside the resonant frequency, it looks to be a capacitor or inductor, whichever takes precedence. It would seem to be the same with a cable. Add any reactive element, they call it a resonant line. A non resonant line would be a resistor circuit, or a cap/ind operated at the resonant frequency. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 17, 2006 Author Report Share Posted December 17, 2006 If you took a 50 ohm source, a 50 ohm cable, and a 50 ohm load and did not operate on the conditions of resonance, you would have a VERY LOW impedance as seen by the signal source. They say 200 KHz is the leveling frequency, but you would have to have a cable a mile long to not see a 50ohm source,line,load combination. You HAVE to resonate. It would be the same with high frequency, but luckily the cable length is reasonable. If it were not, you would be driving a virtual short just like at 200 KHz. Quote Link to comment Share on other sites More sharing options...
AN920 Posted December 17, 2006 Report Share Posted December 17, 2006 Specify if you are talking about ideal cases or practical cases. If we know what you are talking about then we can give answers, otherwise it becomes confusing! At the moment I can't follow your reasoning. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 18, 2006 Report Share Posted December 18, 2006 My cable TV company has many miles of 75 ohm coax around my city and has signals from 50MHz to 900MHz in it. It is 75 ohms everywhere and nothing resonates. In my home the 75 ohm coax cable from outside is connected to a 75 ohm splitter which feeds 75 ohm coax to my 75 ohm cable-modem and feeds 75 ohm coax cable to my 75 ohm TV.It is a 75 ohm transmission line!The signal source has a 75 ohm impedance, is perfectly matched to the 75 ohm transmission line which is perfectly matched to the 75 ohm load. Nothing resonates. Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted December 18, 2006 Author Report Share Posted December 18, 2006 My cable TV has a wide bandwidth too. Because the frequencies are realtively high, I could get the transmission line length perfect for one station if a wanted to. I could add a reasonable tuning stub, or even cut the line to the exact length.I don't think the TV is a good indicator of the quality of cable line signal transmission anyway. Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 18, 2006 Report Share Posted December 18, 2006 Because the source has a 75 ohm impedance and it matches the 75 ohm impedance of the transmission line and the 75 ohm impedance of the TV also matches the impedance of the transmission line, the response is pretty flat on a cable TV system from 50Mhz to 900MHz. The length of the line doesn't matter because the impedances match. Quote Link to comment Share on other sites More sharing options...
ante Posted December 26, 2006 Report Share Posted December 26, 2006 Coax cable is 50 ohms or 75 ohms. A short is zero ohms, a big difference.Hi AG,At which frequency is it a short (zero ohms)? How many meters (or feet) is 50/75Ohms? Are you talking resistance or impedance here and what frequency? Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 26, 2006 Report Share Posted December 26, 2006 Hi Ante,A short is zero ohms resistance to DC and is zero ohms impedance to AC.If the impedance of a transmission line matches its source and termination resistances then its length doesn't affect the impedance.A transmission line is many series inductors with many distributed bypass capacitors along its lenth, forming a lowpass filter that has a very high cutoff frequency. Quote Link to comment Share on other sites More sharing options...
ante Posted December 26, 2006 Report Share Posted December 26, 2006 OK, and at which frequency does the 50 and the 75Ohm wire put up zero Ohms impedance? Quote Link to comment Share on other sites More sharing options...
audioguru Posted December 26, 2006 Report Share Posted December 26, 2006 OK, and at which frequency does the 50 and the 75Ohm wire put up zero Ohms impedance?If the transmission line coax cable is 1/4 wavelemgth long then if its far end is open its close end will appear as a short. Quote Link to comment Share on other sites More sharing options...
ante Posted December 26, 2006 Report Share Posted December 26, 2006 That didn’t exactly answer my questions. :-\ Quote Link to comment Share on other sites More sharing options...
AN920 Posted December 27, 2006 Report Share Posted December 27, 2006 That you can calculate from the well known formula wl=speed of wave/frequencywl = full wavelength in mspeed = 3 X 10 ^8 m/sfrequency of operation in Hz Quote Link to comment Share on other sites More sharing options...
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