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Distance to Fault
On Nov 10, 7:44*pm, "Barry" wrote:
Jim, I think you need to go back and read Spamhog's original question. *He was trying to determine whether the center conductor of a piece of coax had migrated away from center. *He knew where this might have happened - 10 feet from the end, and the migration would have occurred over less than two inches. *So the question of locating where the problem might be is moot. *What is needed is a measurement of the cable impedance in this region. First, let us get an estimate of what the impedance of the damaged section might be. *Spamhog was using RG-6 cable with a foamed polyethylene dielectric. *Its velocity factor is 0.85 making its relative permittivity 1.384. *The center conductor is 1 mm, and the normal diameter of the center insulator is 4.7 mm. *The thickness of the insulator is 1.85 mm. *We need to know the impedance if the center conductor had migrated 0.925 mm toward the jacket. For a quick estimate, use the formula for off-center coax (http://www.microwaves101.com/encyclo...offcenter.cfm). *This gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in the non-distorted coax. *A TDR displays the reflection coefficient from -1 (short) to +1 (open). *Here the reflection coefficient is -0.06.. So the TDR trace will drop from the center line by 6% for 200 picoseconds. If your 100 MHz scope has a typical Gaussian response, its rise time is at least 3.5 nanoseconds. *Do you really think that your oscilloscope trace will clearly show the 200 picosecond dip? *Even with the wide-screen magnifier that KB7QHC suggested, I think you will have great difficulty seeing this. * * 73, Barry *WA4VZQ It may be that a 100 MHz scope has better than a 3.5 nsec risetime, given that it is sped'ed for response flatness to that limit and its response actually extends beyond !00 MHz. In retirement, I no longer have access to test equipment that would support my point. |
#2
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Distance to Fault
On Sun, 14 Nov 2010 18:22:54 -0800 (PST), "Sal M. Onella"
wrote: It may be that a 100 MHz scope has better than a 3.5 nsec risetime, given that it is sped'ed for response flatness to that limit and its response actually extends beyond !00 MHz. The simple correlation between risetime and bandwidth is roughly: BW = 1/(3·t) Unfortunately, peaking bandwidth can degrade risetime, and vice-versa. O'scopes have a lot of conflicting adjustments within them. The TEK545 had something like an 8 to 12 hour tune-up procedure for the average bench tech (a calibration specialist could do it in 3 to 4 hours). 73's Richard Clark, KB7QHC |
#3
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Distance to Fault
"Sal M. Onella" wrote in message
... It may be that a 100 MHz scope has better than a 3.5 nsec risetime, given that it is sped'ed for response flatness to that limit and its response actually extends beyond !00 MHz. In retirement, I no longer have access to test equipment that would support my point. See the following article: http://www.eetimes.com/design/microw...ight-Bandwidth There are two paragraphs in the article of importance he "All oscilloscopes exhibit a low-pass frequency response that rolls-off at higher frequencies, as shown in Figure 1. Most scopes with bandwidth specifications of 1GHz and below typically have what is called a Gaussian response, which exhibits a slow roll-off characteristic beginning at approximately one-third the -3dB frequency. Oscilloscopes with bandwidth specifications greater than 1GHz typically have a maximally-flat frequency response, as shown in Figure 2. This type of response usually exhibits a flatter in-band response with a sharper roll-off characteristic near the -3dB frequency. "Closely related to an oscilloscope's bandwidth specification is its rise time specification. Scopes with a Gaussian-type response will have an approximate rise time of 0.35/f(sub)BW based on a 10- to 90-percent criterion. Scopes with a maximally-flat response typically have rise time specifications in the range of 0.4/f(sub)BW depending on the sharpness of the frequency roll-off characteristic. But it is important to remember that a scope's rise time is not the fastest edge speed that the oscilloscope can accurately measure. It is the fastest edge speed the scope can possibly produce if the input signal has a theoretical infinitely fast rise time (0 ps). Although this theoretical specification is impossible to test (since pulse generators don't have infinitely fast edges) from a practical perspective, you can test your oscilloscope's rise time by inputting a pulse that has edge speeds that are 3 to 5 times faster than the scope's rise time specification." 73, Dr. Barry L. Ornitz WA4VZQ |
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