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| Please explain scope delay lines? |
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| ebastler:
Going by the drawing in the Tek 465 service manual, I think the delay line is a spool of coax cable. Just skimming through the manual quickly, I couldn't find more details. But if the drawing, including the indicated wire thickness, is roughly to scale, I'd say 24 m of cable is not an implausible length. (Say 60 cm circumference, 40 turns?) |
| TimFox:
Yes, that appears to be a spool of cable in the 465. Solid polyethylene coax has a delay of about 5 ns/m = 1.5 ns/foot. Higher-impedance and slower cables were made using a helical center conductor to increase the inductance per unit length. I remember the delay lines in vacuum-tube Tektronix scopes, where the impedance level was higher than practicable for coax. Anyway, so long as the time required from the trigger input to the start of the horizontal sweep is less than 120 ns, the CRT is able to display the signal shortly before the trigger event actually happened. |
| kloetpatra:
yes the length is about right here are some pictures of the 466 delay line which should be nearly the same |
| TimFox:
Yes, that is a balanced transmission line, apparently with solid polyethylene insulation. |
| CatalinaWOW:
The how it works part has been well described here. It is worth thinking about why you want to see signals before the trigger level has been achieved. The simple answer, implied above is to see the beginning of the pulse you are triggering on. Does it rise uniformly or are there steps and bumps in it. But you may also want to see other signals. Triggering on the output of a logic gate you would never see the signals that caused the state change without a delay line long enough to account for the gate delays in the logic gate. Modern DSOs with deep memory can make this ability to see the precursors to a triggering event seem nearly infinite. Before the DSO became practical the delay line gave a window into this world, with the length of the delay line being a compromise between many factors including signal attenuation, signal fidelity loss due to dispersion in the delay line, the physical volume occupied by the delay line, cost and other factors. |
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